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Applied Radiological Anatomy Second Edition

Applied Radiological Anatomy Second Edition Edited by:

Paul Butler Consultant Neuroradiologist, The Royal London Hospital, London, UK

Adam W. M. Mitchell Consultant Radiologist, Chelsea and Westminster Hospital, London; Honorary Senior Lecturer at Imperial College London, UK

Jeremiah C. Healy Consultant Radiologist, Chelsea and Westminster Hospital, London; Honorary Senior Lecturer at Imperial College London, UK

cam b rid ge un iversit y press Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, São Paulo, Delhi, Tokyo, Mexico City Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.cambridge.org Information on this title: www.cambridge.org/9780521766661 First edition © Cambridge University Press 1999 Second edition © Cambridge University Press 2012 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First edition published 1999 Second edition published 2012 Printed in the United Kingdom at the University Press, Cambridge A catalogue record for this publication is available from the British Library Library of Congress Cataloguing in Publication data Applied radiological anatomy / edited by Paul Butler, Adam Mitchell, Jeremiah C. Healy. – 2nd ed. p. cm. Includes bibliographical references and index. ISBN 978-0-521-76666-1 (hardback) 1. Radiography, Medical. 2. Human anatomy. 3. Human anatomy–Atlases. I. Butler, Paul, 1954 June 4– II. Mitchell, Adam W. M. III. Ellis, Harold, 1926– IV. Title. RC78.A675 2011 616.07'572–dc22 2011007348 ISBN 978-0-521-76666-1 Hardback Cambridge University Press has no responsibility for the persistence or accuracy of URLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Every effort has been made in preparing this book to provide accurate and up-to-date information which is in accord with accepted standards and practice at the time of publication. Although case histories are drawn from actual cases, every effort has been made to disguise the identities of the individuals involved. Nevertheless, the authors, editors and publishers can make no warranties that the information contained herein is totally free from error, not least because clinical standards are constantly changing through research and regulation. The authors, editors and publishers therefore disclaim all liability for direct or consequential damages resulting from the use of material contained in this book. Readers are strongly advised to pay careful attention to information provided by the manufacturer of any drugs or equipment that they plan to use.

Contents List of contributors

ix

Section 1 – Central Nervous System

Section 3 – Upper and Lower Limb

1

The skull and brain 1 Kieran M. Hogarth, Jozef Jarosz and Paul Butler

15 The upper limb 278 Gajan Rajeswaran and Justin Lee

2

The orbit and visual pathway 35 Indran Davagnanam and Jonathan L. Hart

3

The petrous temporal bone Tim Beale and Simon Morley

16 The lower limb 319 Gonzalo Ansede, Adam W. M. Mitchell and Jeremiah C. Healy

4

The extracranial head and neck 56 Tim Beale

5

47

The vertebral column and spinal cord 75 Asif Saifuddin

Section 2 – Thorax, abdomen and Pelvis 6

The chest 91 Sheila Rankin

7

The heart and great vessels 109 Simon Padley and Narayan Karunanithy

8

The Breast 126 Steven D. Allen

9

The anterior abdominal wall and peritoneum 134 Nishat Bharwani and Rodney H. Reznek

Section 4 – Obstetrics and Neonatology 17 Obstetrical imaging Ian Suchet

Index

366

383

10 The abdomen and retroperitoneum 150 Navin Ramachandran and Aslam Sohaib 11 The gastrointestinal tract 181 Nasir Khan 12 The kidney and adrenal gland 213 Uday Patel and Hema Verma 13 The male pelvis 230 Nevin T. Wijesekera, Michael Gonsalves and Uday Patel 14 The female pelvis Catriona L. Davies

247

v

Contributors

Steven D. Allen, BSc, MBBS, MRCS, FRCR Consultant Radiologist, Royal Marsden Hospital, Sutton, Surrey, UK Gonzalo Ansede Specialist Registrar in Radiology, Royal Brompton Hospital, London, UK Tim Beale Consultant Radiologist, University College London Hospitals and Royal National Throat, Nose and Ear Hospital, London, UK Nishat Bharwani, BSc, MBBS, MRCP, FRCR Consultant Radiologist, Imperial College Healthcare NHS Trust, London, UK Paul Butler, MRCP, FRCR Consultant Neuroradiologist, The Royal London Hospital, London, UK Indran Davagnanam, MB, BCh, BAO, BMedSci, FRCR Neuroradiology Specialist Registrar, National Hospital for Neurology and Neurosurgery, Queen Square, London, UK Catriona L. Davies, MBBS, MRCP, FRCR Consultant Radiologist, Chelsea and Westminster Hospital, London, UK Michael Gonsalves Radiology Registrar, St George’s Hospital, London, UK Jonathan L. Hart, MA (Oxon), BMBCh, MRCS, FRCR Specialist Registrar, Neuroradiology, National Hospital for Neurology and Neurosurgery, Queen Square, London, UK Jeremiah C. Healy Consultant Radiologist, Chelsea and Westminster Hospital, London; Honorary Senior Lecturer, at Imperial College London, UK

Kieran M. Hogarth, BSC, MBBS, FRCR Consultant Neuroradiologist, John Radcliffe Hospital, Oxford, UK Josef Jarosz Consultant Neuroradiologist, King’s College Hospital, London, UK Narayan Karunanithy, MRCS, FRCR Consultant Radiologist, Guy’s and St Thomas’ NHS Foundation Trust, and Honorary Clinical Lecturer King’s College, London, UK Nasir Khan, MBBS, MRCP, FRCR Consultant Radiologist, Chelsea and Westminster Hospital, London, UK Justin Lee Consultant Radiologist, Chelsea and Westminster Hospital, London, UK Adam W. M. Mitchel Consultant Radiologist, Chelsea, and Westminster Hospital, London; Honorary Senior Lecturer, at Imperial College London, UK Simon Morley Consultant Radiologist, University College London Hospitals, London, UK Simon Padley, BSc, MBBS, FRCP, FRCR Consultant Radiologist, Chelsea and Westminster and Royal Brompton Hospitals and Honorary Senior Lecturer, Imperial College London, UK Uday Patel Consultant Radiologist, St George’s Hospital, London, UK Gajan Rajeswaran, FRCR Consultant Radiologist, Chelsea and Westminster Hospital, London, UK

vii

Contributors

Navin Ramachandran, BSc, MBBS, MRCP, FRCR Consultant Radiologist, University College London Hospitals, London, UK

Aslam Sohaib, MRCP, FRCR Consultant Radiologist, Royal Marsden Hospital, London, UK

Sheila Rankin Radiology Department, Guy’s Hospital, London, UK

Ian Suchet Department of Medical Imaging, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Rodney H. Rezrek, MA, FRANZCR (hon), FFR RCSI (hon), FRCP, FRCR Emeritus Professor of Diagnostic Imaging, St. Bartholomew’s Cancer Institute, St. Bartholomew’s and the London School of Medicine and Dentistry, Queen Mary University of London, UK Asif Saifuddin Consultant Musculoskeletal Radiologist, The Royal National Orthopaedic Hospital NHS Trust, Stanmore, Middlesex, UK

viii

Hema Verma, MRCP, FRCR Specialist Registrar in Radiology, St George’s Hospital, London, UK Nevin T. Wijesekera Specialist Registrar in Radiology, Royal Brompton Hospital, London, UK

Section 1 Chapter

1

Central Nervous System

The skull and brain Kieran M. Hogarth, Jozef Jarosz and Paul Butler

Introduction and imaging methods Computed tomography (CT) and magnetic resonance imaging (MRI) are the mainstays of cerebral imaging. Skull radiography now plays very little part in diagnosis, being largely replaced by multislice CT. Non- or minimally invasive angiography performed using CT (CT angiography) or MRI (magnetic resonance angiography) has resulted in invasive catheter angiography being reserved for a few special diagnostic indications or as part of an interventional, (therapeutic), procedure. Anatomical detail is far better displayed by MRI than by CT, although both are valuable in clinical practice. With T1-weighted (T1W) MR images, grey matter is of lower signal intensity (darker) than white matter (Fig.1.1). On T2-weighted (T2W) images, including T2-FLAIR sequences, the reverse is true (Fig. 1.2). With CT, somewhat paradoxically, white matter is depicted as darker grey than grey matter (Fig. 1.3). The explanation is

Frontal grey matter

Frontal white matter

Insula

Superior sagittal sinus

Frontal grey matter Frontal white matter

Head of the caudate nucleus Lentiform nucleus Thalamus

Splenium of the corpus callosum

Superior sagittal sinus

Fig. 1.1 T1W MRI. ‘Mid-axial’ section of the brain.

Fig. 1.2 T2W MRI. ‘Mid-axial’ section of the brain. Note the signal void due to blood flowing rapidly.

that CT is an X-ray investigation. White matter contains lipid as part of myelin, which is relatively radiolucent. The appearance of myelinated tracts on MRI is rather more variable and will be influenced by the pulse sequence used. In perhaps its simplest form, the lipid in subcutaneous fat is typically high signal (white) on both T1 and T2 MR sequences. Conversely, lipid is extremely radiolucent and appears black on CT. Dense bone contains few free protons on which MRI is based and therefore appears as a signal void (black) on MR. On CT, bone, which is radio-opaque, appears white. Air in the paranasal sinuses appears black on both CT andMRI. Besides compact bone and air, hypointensity on MRI occurs also with iron deposition in the globus pallidus and substantia nigra and as a feature of rapid blood or CSF flow (see below).

Applied Radiological Anatomy, 2nd Edition ed. Paul Butler, Adam W.M. Mitchell and Jeremiah C. Healy. Published by Cambridge University Press. © Cambridge University Press 2011.

1

Section 1: Central Nervous System

Frontal grey matter Frontal white matter

Lentiform nucleus Internal capsule (posterior limb) Thalamus

Superior sagittal sinus

Fig. 1.3 Cranial CT. ‘Mid-axial’ section of the brain.

The intravenous contrast agents used in CT and MRI do not cause significant cerebral parenchymal enhancement, when the blood–brain barrier is intact. Iodinated contrast agents administered intravenously for CT enhance blood within the cranial arteries and veins and dural venous sinuses (Fig.1.4). Enhancement is seen also in the highly vascular choroid plexuses and in those structures outside the blood–brain barrier such as the pituitary gland and infundibulum. With MRI the mechanism of contrast enhancement with intravenous gadolinium DTPA is quite different from CT, but

nevertheless, on T1W images, those structures which enhance become hyperintense (whiter) in much the same way as with CT. One notable difference, however, is in the depiction of rapidly flowing blood with MRI, which appears as a ‘signal void’ (black) and does not enhance (Fig. 1.2). This principle applies also to CSF, which can flow rapidly through the cerebral aqueduct, causing a signal void seen particularly on T2W axial images.

Osteology of the skull The brain is supported by the skull base and enclosed in the vault or calvarium. The skull base develops in cartilage, the vault in membrane. The central skull base consists of the occipital, sphenoid and temporal bones. The frontal and ethmoidal bones complete the five bones of the skull base. Skull sutures are located between bones formed by membranous ossification and consist of dense connective tissue. In the neonate they are smooth, but through childhood interdigitations develop, followed by perisutural sclerosis, prior to fusion in the third or fourth decades or even later (Fig. 1.5). The anterior fontanelle or bregma is located between frontal and parietal bones atthejunction of sagittal and coronal sutures. It closes in the second year. The posterior fontanelle or lambda is closed by the second month after birth. The skull vault consists of inner and outer tables or diploe separated by a diploic space. This space contains marrow and large valveless, thin-walled diploic veins, which contribute to a rich cranial-cerebral anastomosis to provide both a route for the spread of infection across the vault and collateral pathways in the event of venous sinus occlusion.

Metopic suture

Coronal suture

Internal carotid a. Lateral wall of the cavernous sinus

Sagittal suture

Basilar a.

Fig. 1.4 Contrast-enhanced cranial CT.

2

Fig. 1.5 Cranial CT, bone algorithm. The cranial sutures at the vertex of the skull. There is a persistent metopic suture. Note the interdigitation and perisutural sclerosis of the sagittal suture.

Chapter 1: The skull and brain

Lesser wing of sphenoid

Pseudo-erosive change due to arachnoid granulations

Fig. 1.6 Cranial CT, bone algorithm. Pseudo-erosive changes due to the arachnoid granulations.

Optic foramen

Foramen spinosum

Anterior clinoid process

Middle clinoid process

Internal carotid a.

Tuberculum sellae

Posterior clinoid process Foramen rotundum

Greater wing of sphenoid Superior orbital fissure

Foramen lacerum

Dorsum sellae

Foramen ovale

Fig. 1.7 The bony anatomy of the sellar region.

Venous lacunae are found mainly in the parietal bone, near to the midline adjacent to the superior sagittal sinus. They receive some of the cerebral venous return and are invaginated by arachnoid granulations, which are the sites of reabsorption of cerebral spinal fluid into the venous system. Lacunae cause localized thinning of the inner table (Fig.1.6). The frontal bone forms in two halves, which normally fuse at five years. The intervening suture is known as the metopic suture. Occasionally, the halves remain separate and the suture may persist wholly or in part into adult life in 5–10% of individuals(Fig.1.5). The orbital plates of the frontal bone contribute most of the anterior fossa floor with a cribriform plate of the ethmoid bone interposed between them in the midline. The crista galli, to which the falx is attached, ascends vertically from the cribriform plate and may appear hyperintense on T1W images due to contained fatty marrow. The two parietal bones are separated from each other by the sagittal suture and from the frontal bone by the coronal suture (Fig. 1.5). Posteriorly, each parietal bone articulates with the occipital bone. Anteriorly, it articulates with the frontal bone and the greater wing of the sphenoid bone and inferiorly with the temporal bone. The frontal, sphenoid parietal and temporal bones meet at the pterion, which normally closes at 3–4 months. The sphenoid bone consists of a body, greater and lesser wings and the pterygoid plates. The body encloses the sphenoid air sinuses, which are paired and usually asymmetrical. The pituitary fossa and posterior clinoid processes are borne on the superior surface. The planum sphenoidale articulates with the cribriform plate. The anterior clinoid processes are part of the

lesser wing and the tuberculum sellae dips anteriorly between them into the optic groove. The lesser wing forms the posterior part of the floor of the anterior cranial fossa and its posterior border constitutes the sphenoid ridge. Meningiomas of the skull base may arise from any of these sphenoid locations, hence the detail given (Fig.1.7). The greater wing of the sphenoid bone forms the floor of the middle cranial fossa, which extends posteriorly to the petrous ridge and dorsum sellae. The dorsum sella is the posterior boundary of the pituitary fossa and merges laterally with posterior clinoid processes. The greater wing also separates the temporal lobe of the brain from the infratemporal fossa below. The medial and lateral pterygoid plates of the sphenoid bone pass inferiorly behind the maxilla. The foramina ovale rotundum and spinosa are within the greater wing of the sphenoid bone (Fig. 1.8). The foramina ovale and spinosum are often asymmetrical, the foramen rotundum rarely so. The foramen rotundum travels from Meckel’s cave to the pterygopalatine fossa and transmits the maxillary division of the trigeminal nerve. On coronal CT the foramina are demonstrated inferior to the anterior clinoid processes. The foramen ovale transmits the mandibular division of the trigeminal nerve and the accessory meningeal arteries. It runs anterolaterally from Meckel’s cave to emerge near to the lateral pterygoid plate. The foramina may be identified on coronal CT scan inferolateral to the posterior clinoid processes. The foramen spinosum is situated posterolateral to the larger foramen ovale and transmits the middle meningeal artery and vein between the infratemporal and middle cranial fossae.

3

Section 1: Central Nervous System

A

B

Superior orbital fissure

Pterygomaxillary fissure Vidian or pterygoid canal Foramen of Vesalius Foramen lacerum Carotid canal

Foramen ovale Foramen spinosum Carotid canal

Jugular canal

C

D Posterior clinoid process Vidian or pterygoid canal Foramen ovale

Anterior clinoid process

Foramen rotundum

Fig. 1.8 Cranial CT, bone algorithm. The skull base. Axial, (A) superior to (B).Coronal, (C) anterior to (D).

The foramen lacerum contains cartilage and is traversed only by small veins and nerves. It separates the petrous apex, the body of the sphenoid and the basiocciput and is crossed by the internal carotid artery. Smaller, inconstant foramina are sometimes encountered. The Vidian or pterygoid canal is found medial to the foramen rotundum. The foramen of Vesalius transmits an emissary vein and is medial to the foramen ovale. The temporal bone has four parts. The squamous part forms the lateral wall of the middle cranial fossa and is separated from the parietal bone by the squamosal suture. Its zygomatic process contributes to the zygomatic arch and the squamosal portion also bears the mandibular condylar fossa. The petromastoid portion forms part of the middle and posterior fossa floors. The styloid process passes inferiorly from the base of the petrous bone and the stylomastoid

4

Hypoglossal canal Foramen magnum

Fig. 1.9 Cranial CT, bone algorithm. The hypoglossal canal.

Chapter 1: The skull and brain

B

A Spheno-occipital synchondrosis

Fig. 1.10 (A) Cranial CT. Sagittal reformat bone algorithm. (B) Cranial MR. Midline sagittal section of the brain. Spheno-occipital synchondrosis.

foramen lies behind the styloid process transmitting the facial (VIIth) cranial nerve. The occipital bone forms most of the posterior cranial fossa walls. This is the largest of the three cranial fossae. It also gives rise to the occipital condyles which articulate with the atlas and the anterior condylar canals which transmit the hypoglossal (XIIth) cranial nerve (Fig. 1.9). Also inferiorly but more anteriorly, the occipital bone articulates with the sphenoid to form the clivus. The articulation is visible in children as the basisphenoid synchondrosis (Fig.1.10). In the adult the clivus is hyperintense on T1W MR images due to replacement of red marrow with fat. The transition from hypointensity occurs at around 7 years. Immature red marrow in children can enhance with intravenous gadolinium. The occipital bone is often devoid of a diploic space inferiorly. This accounts for the sparing of the occipital bone in thalassaemia major, where the response to chronic haemolysis causes reactive change (‘hair on end’ appearance) elsewhere in the skull vault.

Spheno-occipital synchondrosis

Venous impressions are larger than those due to arteries and vary in calibre. Arterial impressions have parallel walls and reduce in calibre only after branching. Normal vault lucencies and calcifications are listed in Table1.1 Table 1.1 Lucencies and calcifications seen on normal skull radiography Lucencies Sutures Vascular impressions Normal vault thinning, e.g. temporal bone Arachnoid granulations Pneumatization Calcifications (Fig. 1.12)

The skull radiograph (Fig. 1.11)

Pineal gland

Skull radiography is performed much less frequently now because of the versatility and reliability of cranial CT. The plain film images are complex with multiple overlapping lines and interfaces and of course give very limited and indirect evidence of cerebral pathology. When interpreting a skull radiograph perhaps the most important requirement is to distinguish a normal lucency from a fracture. Convolutional markings are absent at birth, most prominent at between 2 and 5 years and absent after about 12years. Vascular markings similarly do not develop until the postnatal period but then persist throughout life. They are less radiolucent than fractures, with indistinct margins and often branch. Diploic veins are responsible for the majority of impressions, although the dural venous sinuses (superior sagittal, lateral and sigmoid) cause depressions on the inner table, visible on plain radiographs. There is a vein running along the coronal suture large enough to be labelled the sphenobregmatic sinus, which gives rise to a prominent vascular impression.

Habenular commissure Choroid plexus Dural calcification including petroclinoid and interclinoid ‘ligaments’

The cerebral envelope See Fig.1.13 The meninges invest the brain and spinal cord. The three constituent parts are the outer, fibrous dura mater, the avascular, lattice-like arachnoid mater and the inner, vascular layer, the pia mater. Although the dura and arachnoid are applied closely, there is a potential space, known as the subdural space, between them into which haemorrhage may occur or pus form. Its existence in the normal individual is controversial. The subarachnoid space contains cerebral spinal fluid, which surrounds the cerebral arteries and veins. It is situated between the arachnoid and the pia, which is closely applied to the cerebral surface. The cranial dura has two layers, which separate to enclose the dural venous sinuses.

5

Section 1: Central Nervous System

A

B

Dural calcification

Pterion

Anterior clinoid process

Frontal sinus Saggital suture Lambdoid suture Lesser wing of sphenoid

Crista galli Cribiform plate Floor of the anterior cranial fossa

Orbital roof

Frontal sinus

Dorsum sellae

Frontal

Pineal gland calcification

Parietal

Floor of the anterior cranial fossa Cribiform plate

Calcified choroid plexus

Temporal

Greater wing of sphenoid

Innominate line

Superior orbital fissure

Anterior clinoid process

Lamina dura of pituitary fossa

Zygomatic bone

Zygomatic recesses of the maxillary antra

Sphenoid sinus

Maxilla

Occipital Normal temporal bone ‘thinning’ Clivus (basiocciput and basisphenoid)

Mandibular condyle

Fig. 1.11 Frontal (A) and lateral (B) skull radiographs.

The outer layer is the periosteum of the inner table of the skull (the endosteum). The inner layer covers the brain and gives rise to the falx and tentorium. Dura is hyperdense on CT images and relatively hypointense on MRI. It shows contrast enhancement on both modalities and since the falx may calcify or ossify, MRI may demonstrate focal regions of signal void due to calcification or of hyperintensity due to fat within marrow. The falx is a sickle-shaped fold of dura, comprising two layers, which forms an incomplete partition between the cerebral hemispheres. It extends from the crista galli to the internal occipital protuberance, where it joins the tentorium and is thinner anteriorly. The falx is demonstrated as a midline linear density on axial CT scan near to the vertex, but inferiorly and posteriorly assumes a triangular shape conforming to the superior sagittal sinus in cross-section. The tentorium cerebelli, another double dural fold, is attached from the

6

posterior clinoid processes along the petrous ridges to the internal occipital protuberance. Its upper, free, medial border surrounds the midbrain. This passes anteriorly through the opening, known as the tentorial hiatus or incisura. The uncus of the hippocampus and the posterior cerebral arteries lie above the free edge of the tentorium and both are at risk of compression against the tentorial edge when there is raised intracranial pressure in the supratentorial compartment (‘coning’). The free border anteriorly encloses the cavernous sinus on each side of the pituitary fossa before attaching to the anterior clinoid processes. For diagnostic purposes it is important to identify in which intracranial compartment a lesion is situated. On axial CT, structures medial to the line of the tentorial edge are in the infratentorial compartment; those lateral to that line are in the supratentorial compartment (Fig.1.14).

Chapter 1: The skull and brain

B

A

Calcified pineal body

C

Calcified pineal body

D

Calcified choroid plexus

Fig. 1.12 CT scans showing calcified pineal body in sagittal section (A), coronal section (B) and axial section (C), and choroid calcification in axial section (D).

Falx cerebri

Superior saggital sinus Inferior saggital sinus

Straight sinus

Great v. of Galen Transverse sinus Superior petrosal sinus

Cavernosus sinus

Superior petrosal sinus Optic n. Superior ophthalmic v. Facial v. Internal carotid a. Pterygoid venous plexus

Fig. 1.13 The cranial dura.

7

Section 1: Central Nervous System

Superficial temporal artery branches

Middle meningeal a. Middle deep temporal a. Anterior deep temporal a. Infraorbital a.

Internal maxillary a.

Fig. 1.15 External carotid angiogram, lateral projection.

The subarachnoid cisterns (Fig.1.16) Fig. 1.14 Axial CT with intravenous contrast showing the tentorial margins (arrow). Note that the dura continues anteriorly to form the lateral wall of the cavernous sinus.

The falx cerebelli is a small fold of dura attached superiorly to the posterior part of the tentorium in the midsagittal plane, which encloses the occipital sinus posteriorly. It terminates just above the foramen magnum and its free anterior border projects into the cerebellar notch. The diaphragma sellae is an incomplete roof over the pituitary gland and is pierced by the pituitary stalk. There is no subarachnoid space in the sella since the meningeal layers fuse. On both CT and MRI meningeal enhancement following intravenous contrast is a normal feature.

Meningeal blood supply and innervation The middle meningeal artery is the main arterial supply to the meninges (Fig. 1.15), but there are contributions from the cavernous carotid, the ophthalmic and vertebral arteries. There is also an accessory meningeal artery, which arises either from the maxillary or middle meningeal arteries and enters the skull through the foramen ovale. The middle meningeal artery is extradural and both it and the middle meningeal veins groove the inner table of the skull. Branches of the external carotid artery may often supply the lower cranial nerves. The middle meningeal arteries supply branches to both the trigeminal and the facial ganglia. The occipital artery gives branches which pass via the jugular foramen and condylar canal to supply the glossopharyngeal (IXth), vagal (Xth), accessory (XIth) and hypoglossal (XIIth) cranial nerves. Innervation of the dura is primarily from the trigeminal nerve, but also from the lower cranial nerves and the first three cervical segments. This may account for cervical pain in cranial subarachnoid haemorrhage.

8

Where the brain and skull are not closely applied, a number of subarachnoid cisterns are defined. They are situated at the base of the brain and around the brainstem, the free edge of the tentorium and the major arteries. The subarachnoid cisterns connect relatively freely with one another and their patency is essential for the normal circulation of cerebral spinal fluid. Although there are arachnoid membranes within the cisterns causing partial compartmentalization, the definition of a particular cistern is a result of the arbitrary division of what is effectively a single space. The cisterna magna lies between the medulla and the posteroinferior surface of the cerebellum and is triangular in sagittal section. It continues below the spinal subarachnoid space and receives cerebral spinal fluid from the fourth ventricle. It is sometimes punctured percutaneously in the midline to obtain cerebral spinal fluid for examination. The vertebral and posterior inferior cerebellar arteries travel through the lateral parts of the cisterna magna, which also contains the glossopharyngeal, vagus and spinal accessory nerves. In some, otherwise normal, individuals the system is very large and described as a mega-cisterna magna. The pontine cistern is anterior to both the pons and medulla and contains the basilar artery and cranial nerves V to XII. It is continuous around the brainstem, with the quadrigeminal plate cistern posteriorly and the interpeduncular cistern superiorly. The chiasmatic or suprasellar cistern extends from the infundibulum to the posterior surface of the frontal lobes and lies between the uncus on either side. It includes the proximal parts of the Sylvian fissures and contains the circle of Willis. Since the majority of berry aneurysms are borne on the circle of Willis, it can be appreciated that their rupture results in subarachnoid haemorrhage in the first instance.

Chapter 1: The skull and brain Interpeduncular cistern Cerebral aqueduct Quadrigeminal plate cistern Tectum Decussation of the superior cerebellar peduncles MIDBRAIN Chiasmatic cistern PONS ‘Belly’ MEDULLA Prepontine cistern Cisterna magna

Fig. 1.16 Cranial MR. Midline sagittal section of the brain. The subarachnoid cisterns.

The ambient cistern surrounds the midbrain and transmits the posterior cerebral and superior cerebellar arteries, the basal veins of Rosenthal and the trochlear nerves. The ‘wings’ of the ambient cistern are its lateral extensions posterior to the thalami. The quadrigeminal cistern (cistern of the great cerebral vein of Galen) lies adjacent to the superior service of the cerebellum and extends superiorly around the splenium of the corpus callosum. It contains the posterior cerebral, posterior choroidal and superior cerebellar arteries, and the trochlear (IVth cranial) nerves. It is also the location of the venous confluence where the vein of Galen joins the inferior sagittal and straight dural venous sinuses. The cistern of the lamina terminalis is superior to the chiasmatic cistern. It contains the anterior communicating artery and leads into the callosal cistern, through which the pericallosal artery travels.

The brainstem and cranial nerves The chiasmatic cistern leads posteriorly to the interpeduncular or intercrural cistern, which contains the terminal basilar artery and its branches and the oculomotor (IIIrd cranial) nerves. Blood within this cistern may be the only evidence of subarachnoid haemorrhage.

See Fig. 1.17 The brainstem consists of the midbrain, pons and medulla (Fig. 1.16). Even high field strength MRI shows little internal detail under normal scanning conditions. To demonstrate the exiting cranial nerves, high-resolution, heavily T2-weighted

Optic chiasm Cerebral peduncle Pineal gland Oculomotor (III) n.

Superior colliculus Inferior colliculus

Trochlear (IV) n. Trochlear (IV) n. Pons Trigeminal (V) n.

Superior cerebellar peduncle

Abducens (VI) n. Middle cerebellar peduncle

Facial (VII) n. Vestibulocochlear (VIII) n.

Inferior cerebellar peduncle

Glossopharangeal (IX) n. Vagus (X) n.

Lateral recess of IVth ventricle

Hypoglossal (X) n.

Cuneate nucleus

Accessory (XI) n.

Gracile nucleus

B. Posterior aspect

A. Anterior aspect

Fig. 1.17 The brainstem. A. Anterior aspect B. Posterior

9

Section 1: Central Nervous System

thin-section axial and coronal scans are required. These provide contour images of the nerves and brainstem against hyperintense (white) CSF. Even then the demonstration of the smaller nerves is inconstant.

The midbrain The midbrain has two prominent cerebral peduncles anteriorly and a dorsal tectum. Within the substance of the midbrain the red nuclei and the substantia nigra can be identified (Fig.1.18). The red nuclei are hypointense on T2W images due to their vascularity and the substantia nigra due to their iron content. As with the pons the appearance of the midbrain is very different from its axial appearance. In a midline section only the central tegmentum and dorsal tectum are seen, separated by the cerebral aqueduct. The tectum consists of four colliculi (‘hillocks’) or quadrigeminal bodies, which are involved in visual and auditory reflexes (Figs. 1.16, 1.28). Cranial nerves arising in the midbrain are the oculomotor (IIIrd) and the trochlear (IVth). Both have their nuclei in the periaqueductal grey matter.

The oculomotor (IIIrd) arises from the anterior midbrain, on the medial side of the cerebral peduncle (Fig. 1.19), and passes between the superior cerebellar and posterior communicating arteries. Aneurysms arising at the origins of either of these two arteries can cause a IIIrd nerve palsy, although posterior communicating artery aneurysms are much more common. The nerve then passes inferior to the posterior communicating artery, close to the free edge of the tentorium, into the cavernous sinus. Its cisternal portion is particularly well shown on axial FLAIR MR images. The trochlear (IVth) nerve is the smallest in calibre, has the longest intracranial course and is the only cranial nerve arising from the dorsal aspect of the brainstem (Fig.1.20).

The pons The pons has a bulbous anterior portion (the ‘belly’), seen prominently on sagittal images, and a dorsal tegmentum (Fig.1.16).

A

B Anterior communicating a. Chiasmatic cistern Middle cerebral a.

Chiasmatic cistern

Optic tract

Interpeduncular cistern

Mammillary body

Midbrain

Substantia nigra

Cerebral aqueduct

Red nucleus

Ambient cistern

Quadrigeminal plate cistern

Fig. 1.18 T2W axial MRI. The midbrain: (B) is cranial to (A).

A

B

Posterior cerebral a. Optic n. Optic chiasm

Oculomotor n.

Oculomotor n.

Superior cerebellar a.

Fig. 1.19 FLAIR axial MRI (A); T2W coronal MRI (B). The oculomotor (IIIrd cranial) nerve.

10

Chapter 1: The skull and brain

In axial section the lower pons is dominated by the posterolaterally directed middle cerebellar peduncles, giving it its descriptive bridge-like shape (Fig.1.21). Cranial nerves arising from the pons are, from above down, the trigeminal (Vth), the abducent (VIth), the facial (VIIth) and the vestibulocochlear (VIIIth). The trigeminal (Vth) is the largest of the true cranial nerves and arises at the junction of pons and middle cerebellar peduncle, the two combined motor and sensory roots passing directly forwards to Meckel’s cave (Fig.1.22). The abducent (VIth) has a relatively long intracranial course, passing in an anterolateral direction through Dorello’s canal into the cavernous sinus. It loops over the petrous apex and has its own bony sulcus (Fig.1.23).

The ‘Michelin-man’ appearance of the brainstem seen in Fig.1.24 denotes the pontomedullary junction (‘head’ basilar artery, ‘arms’ middle and ‘legs’ inferior cerebellar peduncles).

The medulla The lower part of the medulla encloses a central canal continuous with the spinal cord below, and the contours of the two are similar (Fig. 1.25). It becomes ‘open’ superiorly where it is related to the lower part of the fourth ventricle and takes on a more complex, square-like shape. The pyramidal and olivary eminences can be defined, separated by a sulcus. Pyramidal (motor) tracts are anteriorly situated through the brainstem.

VIIth (anterior) and VIIIth cranial n.

Middle cerebellar peduncle

Trochlear n.

Fourth ventricle Nodulus of the vermis

Fig. 1.21 T2W axial MRI. The pons.

Fig. 1.20 T2W axial MRI. The trochlear (IVth cranial) nerve.

A

B

Meckel’s cave

Trigeminal n. Trigeminal n.

Superior cerebellar peduncle

Fig. 1.22 T2W axial MRI (A); T2W coronal MRI (B). Thetrigeminal (Vth cranial) nerve.

11

Section 1: Central Nervous System

Meckel’s cave Middle cerebellar peduncle

Abducent n.

Inferior cerebellar peduncle

Middle cerebellar peduncle

Flocculus

Fig. 1.24 T2W axial MRI. The pontomedullary junction.

Fig. 1.23 T2W axial MRI. The abducent (VIth cranial) nerve.

A

B

Pyramidal eminence Central canal

Olivary eminence Inferior cerebellar peduncle

Fig. 1.25 T2W axial MRI. The closed medulla (A). The open medulla (B).

Cranial nerves arising from the medulla are, from above down, the glossopharyngeal (IXth), the vagus (Xth), the spinal accessory (XIth) and the hypoglossal (XIIth). The glossopharyngeal (IXth) and the vagus (Xth) cranial nerves arise from a sulcus posterolateral to the olive. Imaging usually fails to separate completely the IXth, Xth and XIth cranial nerves and they pass as a bundle to the jugular foramen. The hypoglossal (XIIth) nerve arises from the pre-olivary sulcus (Fig.1.26).

The cerebellum The cerebellum lies posterior to the brainstem, to which it is connected by the cerebellar peduncles. The cortical mantle overlies the white matter core as in the cerebral hemispheres but the cerebellar cortical ridges, known as the folia, and the intervening sulci are approximately parallel to one another (Fig.1.27).

12

The cerebellum consists of a narrow midline vermis and two hemispheres. The flocculus is largely separate from the rest of the cerebellum and extends laterally just inferior to the vestibulocochlear (VIIIth) cranial nerve (Fig.1.23). The normal flocculus appears to enhance more than the rest of the cerebellum on CT after intravenous contrast because of its proximity to the choroid plexus and anterior inferior cerebellar artery. It might therefore be mistaken for an acoustic Schwannoma, although the flocculus lies posterior to the porus acousticus. The nodule is the most ventral structure on the inferior vermian surface and is identified on axial scans indenting the fourth ventricle. On the inferior surface of the cerebellar hemispheres are the tonsils. Posterior and lateral to the tonsils are the biventral lobules.

Chapter 1: The skull and brain

Edge of the tentorium Hypoglossal n. (passing to the hypoglossal canal)

Glosspharyngeal vagus and accessory n. (passing to the jugular foramen)

Cerebellar white matter

Cerebellar grey matter as folia

Fig. 1.27 T2W coronal MRI. The cerebellum.

Inferior cerebellar peduncle

Fig. 1.26 T2W axial MRI. The glossopharyngeal (IXth cranial), vagus (Xth cranial), accessory (XIth cranial) nerve bundle. The hypoglossal nerve. Internal carotid a.

Vertebral a.

Common carotid a.

Quadrigeminal bodies Superior cerebellar peduncle Middle cerebellar peduncle Inferior cerebellar peduncle

Fig. 1.28 T2W coronal MRI. The cerebellar peduncles.

There are three cerebellar peduncles arising from the white matter core of the cerebellum on each side. The inferior cerebellar peduncle joins the medulla, the middle cerebellar peduncle (the largest) the pons and the superior, the midbrain. Their relationship is best seen on coronal MRI (Fig.1.28).

The intracranial circulation The brain is supplied by four arteries, the paired internal carotid and vertebral arteries (Fig.1.29).

Internal carotid artery (ICA) The internal carotid artery arises at the carotid bifurcation at approximately the level of the third cervical vertebra.

Subclavian a. Brachiocephalic a.

Fig. 1.29 Time-of-flight (non-contrast) MR angiogram, frontal view. The greatvessels.

It is divided into seven segments, C1–C7 (Fig.1.30). No constant branches arise from its cervical (C1) segment and the artery enters the cranial cavity through the carotid canal in the petrous bone, running first vertically then horizontally (C2 segment) (Fig.1.31a). The short C3 segment runs vertically and medially between the petrous apex and the cavernous sinus above the foramen lacerum. The artery here is closely related to the trigeminal ganglion within Meckel’s cave (Fig.1.31b). C4 is the cavernous segment (Fig. 1.31c). The artery turns forwards in the cavernous sinus then upwards to form the clinoid (C5) segment. The artery then enters the subarachnoid space. The next segment is the ophthalmic (C6) segment, which extends just proximal to the posterior communicating artery. The distal segment (C7) extends to the terminal bifurcation. C4, C5 and C6 segments form the U-shaped carotid siphon, part of which is therefore intracavernous, part in the subarachnoid space.

13

Section 1: Central Nervous System

A

B C7 segment C7 segment C6 segment

C6 segment

C5 segment C5 segment C3 segment

C3 segment

C4 segment C4 segment

C2 segment

C2 segment

Fig. 1.30 (A) Internal carotid angiogram; (B) lateral projections. The arterial segments.

A

B

C2 segment horizontal

Meckel’s cave

C

C3 segment

C4 cavernous segment

Fig. 1.31 T2W axial MRI. The internal carotid artery at the skull base; (A)–(C), inferior to superior.

It should be noted that there are no angiographic indicators of the precise limits of the intracavernous portion of the internal carotid artery. Although there may be anatomical variation, lesions arising at, or distal to, the ophthalmic artery origin are taken to be within the subarachnoid space.

Branches of the ICA (Fig. 1.32) The ophthalmic artery is the first supraclinoid branch of the ICA recognizable on normal angiography. It arises in the subarachnoid space and runs forward through the optic canal within the optic nerve sheath (see Chapter 2). The posterior communicating artery is the second intracranial branch of the ICA and connects it to the posterior cerebralartery, just distal to the origin of the latter (see ‘Posterior

14

cerebral artery’ below). The oculomotor (IIIrd) cranial nerve passes between the posterior communicating artery above and the superior cerebellar artery below (Fig.1.19). The anterior choroidal artery arises from the posteromedial aspect of the ICA just distal to the posterior communicating artery and is well seen on the lateral angiogram. Its first or cisternal part lies between the uncus and optic tract. It then enters into the temporal horn of the lateral ventricle through the choroidal fissure into the choroid plexus. On the lateral angiogram there is an upwards ‘kink’, the plexal point, where the artery passes through the choroidal fissure (Fig.1.32a). The ICA divides, terminally, into the anterior and middle cerebral arteries. This T-shaped bifurcation is not normally in

Chapter 1: The skull and brain

B M4

A Anterior cerebral a.

M3

Frontopolar a. A2 M2 Angular a. Anterior choroidal a.

Orbitofrontal a. M1

Anterior choroidal a. ‘plexal point’

A1 Ophthalmic a.

Posterior communicating a. Ophthalmic a.

Fig. 1.32 Internal carotid angiogram, lateral (A) and frontal (B) projections. Major named arteries.

the true coronal plane since the MCA is directed posterolaterally. This requires an oblique angiographic projection to display the anterior and middle cerebral arteries en face.

The circle of Willis (Figs. 1.33, 1.34) Branches of the internal carotid and basilar arteries form an anastomotic ring on the ventral surface of the brain, known as the circle of Willis. This affords some protection against cerebralinfarction in the event of arterial occlusion. The participating arteries are the terminal ICAs, the first part of the anterior cerebral arteries (A1 segments), the anterior communicating artery, the posterior communicating arteries, the first parts of the posterior cerebral arteries (P1 segments) and the basilar artery. Small perforating arteries arise from the communicating arteries.

Anterior communicating a. Anterior cerebral a. Middle cerebral a. Internal carotid a. Posterior communicating a. Posterior cerebral a. Superior cerebellar a. Anterior inferior cerebellar a. Vertebral a. Posterior inferior cerebellar a.

Fig. 1.33 Time-of-flight (non-contrast) MR angiogram, axial view. The circle ofWillis.

In the axial plane the ‘circle’ has a polygonal configuration within the suprasellar cistern. Hypoplasia or aplasia of its component parts is common and the circle is complete in only a minority of individuals.

Extra- and intracranial arterial connections The circle of Willis is the ‘central’ anastomotic network linking the intracranial carotid circulations on each side and the vertebrobasilar circulation. There are also cortical connections between branches of the anterior, middle and posterior cerebral arteries. Numerous anastomotic paths exist between internal and external carotid arteries and between the external carotid and vertebral arteries. There may be persistent segmental connections between the internal carotid and basilar arteries, the commonest of which is the trigeminal artery arising from the lower part of the cavernous carotid.

The anterior cerebral artery (ACA) (Figs. 1.32, 1.33, 1.34) The anterior cerebral artery is the smaller terminal branch of the internal carotid artery and it runs anteromedially towards the midline, where it is seen above the optic nerve. This is the pre-communicating, horizontal or A1 segment. Both anterior cerebral arteries come to lie in close proximity at the base of the interhemispheric fissure, where they are usually linked by a short bridging vessel, the anterior communicating artery, within the cistern of the lamina terminalis. The A2 segment of the anterior cerebral artery extends from the anterior communicating artery to the origin of the frontopolar artery. Thereafter the A3 segment travels around the genu of the corpus callosum to the origin of the callosomarginal artery.

15

Section 1: Central Nervous System

A

Anterior cerebral a.

B

Middle cerebral a. branches (within triangle) Angular a. Anterior cerebral a.

Posterior cerebral a.

Posterior cerebral a. Posterior communicating a.

Middle cerebral a.

Basilar a. Superior cerebellar a. Vertebral a.

Ophthalmic a.

Basilar a.

Fig. 1.34 Internal carotid angiogram, lateral (A) and frontal (B) projections. The major named branches in a patient with a complete circle of Willis permitting opacification of the contralateral intracranial carotid circulation and the vertebrobasilar system.

ACA branches Heubner’s recurrent artery is the largest of the perforating medial lenticulostriate branches which course posterosuperiorly. The lenticulostriate arteries supply a number of important structures of the anterobasal brain. They are also ‘end’ arteries. Heubner’s artery can arise from the proximal A2 segment or A1 segments along with the majority of the medial lenticulostriate branches. The anterior communicating artery, although short, gives rise to several branches which course superiorly to supply the optic chiasm and other anterior midline structures. The orbitofrontal artery is usually the first cortical branch of the A2 segment, arising from the subcallosal segment to supply the inferior and inferomedial surfaces of the frontal lobe including the gyri recti. The frontopolar artery runs from the genu of the corpus callosum to the frontal pole and supplies the orbital gyri, olfactory bulb and tract and the anterior part of the superior frontal gyrus. The callosomarginal artery is present in approximately half of all cases. It runs through the cingulate sulcus above the cingulate gyrus and gives rise to anterior, middle and posterior internal frontal branches. These supply the superior frontal gyrus. The pericallosal artery is the continuation of the anterior cerebral artery beyond the origin of the callosomarginal artery. It arches posteriorly over the genu of the corpus callosum to lie on its superior surface as far as the splenium and below the cingulate gyrus. The anterior cerebral arteries are sometimes fused proximally to form a single trunk or azygos artery, which arises between the hemispheres before dividing near the genu of the corpus callosum.

16

The middle cerebral artery (MCA) (Figs. 1.32, 1.33, 1.34) This is the larger terminal branch of the internal carotid artery. Its proximal portion, the M1 segment, runs laterally to the horizontal limb of the Sylvian fissure between the frontal and temporal lobes. At the anteroinferior aspects of the insula the middle cerebral artery turns upwards, forming its genu (the distal limit of the M1 segment) and its branches (the M2 segment), then runs over the surface of the insula in the depths of the Sylvian fissure. At the superior limit of the insula they turn inferiorly and then laterally under the frontoparietal operculum (M3 segment) to emerge from the lateral aspect of the Sylvian fissure and spread out over cortical surfaces of the frontal, parietal, occipital and temporal lobes (M4 segments).

MCA branches A variable number of lateral lenticulostriate branches arise from the M1 segment, which supply the basal ganglia, internal capsule and caudate nucleus. The anterior temporal arteries usually arise from the M1 segment and course over the anterior pole of the temporal lobe. The terminal arterial division of M1 is termed the MCA trifurcation but more properly comprises two sequential bifurcations. A number of variable cortical branches extend over the surface of the hemispheres, the largest and most posterior of which is the angular artery.

Posterior cerebral artery (see p.22) Figure 1.35 illustrates the territories supplied by the various arteries. There is some individual variation.

Chapter 1: The skull and brain

MCA

LSA

PCA

H

ACA

AChA

BA

SCA

AICA

PICA

Fig. 1.35 The vascular territories. Brain arterial distributions. ACA = anterior cerebral artery, H = recurrent artery of Heubner, MCA = middle cerebral artery, LSA = lenticulostriate artery, AChA = anterior choroidal artery, PCA = posterior cerebral artery, BA = basilar artery, SCA = superior cerebellar artery, AICA = anterior inferior cerebellar artery, PICA = posterior inferior cerebellar artery.

The dural venous sinuses (Fig. 1.36) The dural sinuses are valveless trabeculated venous channels and may conveniently be divided into a superior group related to the vault and the basal group found at the skull base. The sagittal, transverse and straight sinuses are the main components of the superior group. The basal group comprises the cavernous, petrosal and sphenoparietal sinuses.

The superior sagittal sinus, which is triangular in crosssection, increases in size from back to front and usually begins near the crista galli, although it may not develop anterior to the coronal suture. In the majority of individuals, most of its flow is directed to the right transverse sinus with the straight sinus draining to the left transverse sinus. Cortical veins enter perpendicular to the superior sagittal sinus anteriorly but the angle becomes shallower more posteriorly with the veins entering against the direction of flow. As with venous systems elsewhere, normal anatomical variants are common. The superior sagittal sinus may bifurcate well above its normal termination at the internal occipital protuberance (‘torcular’). This early separation may lead to an erroneous diagnosis of sagittal sinus thrombosis on CT, if the intervening space is mistaken for non-enhancing thrombus (a false positive empty triangle or empty delta sign). The inferior sagittal sinus is the marker for its inferior margin of the falx. It is not uncommon to identify it at catheter angiography and on gadolinium-enhanced T1-weighted MRI in the midsagittal plane (Fig.1.37). The transverse sinuses commence at the torcular and lie within the outer margins of the tentorium (Figs. 1.36, 1.38). The right is usually dominant and larger than that on the left and receives almost the entire output of the superior sagittal sinus. The sinus on one side can be poorly developed or even absent. In order to distinguish such a variant from sinus occlusion, it is often helpful to examine with CT the bony depressions in the vault in which the sinus runs to the jugular foramen, both of which will be correspondingly underdeveloped in the congenital variant. The transverse sinuses become the sigmoid sinuses at the posterior petrous edge continuing towards the jugular bulb. The transverse and sigmoid sinuses are together known as the lateral sinus. Occasionally one encounters intraluminal filling defects in the transverse sinus due to prominent arachnoid granulations. Where the sigmoid sinus is adjacent to the petrous bone, there can be pseudo-erosive changes in the bone margin. Normal petromastoid aeration is a useful guide to this variant. The straight sinus lies at the junction of the falx and the tentorium and the torcular is where the straight, transverse and superior sagittal sinuses meet (venous confluence 1). The vein of Galen (the great cerebral vein) joins the inferior sagittal and straight sinuses at the venous confluence within the quadrigeminal plate system (venous confluence 2) (Fig.1.36a). Although functionally a single unit, the paired cavernous sinuses are situated on either side of the pituitary fossa and receive the superior and inferior ophthalmic veins and the sphenoparietal sinuses (Figs. 1.36, 1.39). They connect with each other through the intercavernous sinuses and posteriorly they communicate with the transverse sinuses, by the superior petrosal sinus on each side. Each is a trabeculated, extradural venous channel lying on the body of the sphenoid bone. The internal carotid artery pursues an S-shaped course through the sinus before piercing its dural roof, medial to the anterior

17

Section 1: Central Nervous System

Septal v.

A

Thalamostriate v.

B

Superior sagittal sinus Internal cerebral v. V. of Galen Venous confluence #2

Venous confluence #2 Straight sinus

Transverse sinus

V. of Labbe Venous confluence #1

Anterior tip of the superior sagittal sinus

Superior petrosal sinus Transverse sinus

Sigmoid sinus

Sigmoid sinus Sphenoparietal sinus

Cavernous sinus

C

Superior sagittal sinus Inferior sagittal sinus Septal v. Internal cerebral v. V. of Galen

Fig. 1.36 Internal carotid angiogram, venous (late) phase, lateral (A) and frontal (B)projections. Panel (C) is from a different patient illustrating individual variation. Note that there are two ‘venous confluences’. The first is in the quadrigeminal cistern involving the great vein of Galen, the straight sinus and the inferior sagittal sinus. Thesecond is at the torcular and involves the superior sagittal sinus, the straight sinus and the transverse sinuses.

Fig. 1.37 Contrast-enhanced T1W cranial MR. Midline sagittal section of the brain. The cerebral venous system.

B

A

Sigmoid sinus

Transverse sinus

18

Fig. 1.38 Contrast-enhanced T1W MRI: (A) coronal, (B) axial scans. Transverse and sigmoid sinuses.

Chapter 1: The skull and brain

A

B

Intracranial optic n.

Intracavernous carotid a.

Oculomotor n. Abducent n. V2 V3

C

Optic chiasm

Pituitary gland

Oculomotor n.

Abducent n.

Mandibular division of the trigeminal n. in the foramen ovale

Fig. 1.39 Contrast-enhanced T1W cranial MR: axial scan (A): coronal scans;(B)isanterior to (C). The cavernous sinuses.

clinoid process. The abducent nerve lies free within the sinus applied to the lateral wall of the artery. From above down, the oculomotor, trochlear, ophthalmic and maxillary nerves run in a common dural tunnel in the lateral wall of the sinus to reach the superior orbital fissure (Fig.1.39). The cavernous sinuses enhance with intravenous contrast on both CT and MRI. Fat deposits can occur normally within the sinus and are demonstrated by CT as hypodense foci. The normal sinus has a concave lateral wall and the two sinuses should be symmetrical. Inferiorly the ophthalmic division of the trigeminal nerve, also embedded in the lateral wall, courses towards the trigeminal ganglion. The trigeminal ganglion contains the cell bodies of the sensory root of the trigeminal nerve. It is crescentic in shape and occupies a dural recess in the medial wall of the middle fossa at the petrous apex posterior to the cavernous sinus. This recess, known as Meckel’s cave, is in continuity with the prepontine system and is of cerebral spinal fluid density and signal

intensity on CT and MRI, respectively. It will also be opacified during CT cisternography. The petrosal and sphenoparietal sinuses drain the cavernous sinuses on each side. The superior petrosal sinuses are situated at the junction of the tentorium and the petrous bone. They drain to the transverse sinuses. The inferior petrosal sinuses lie between the clivus and petrous apex and run medial to the superior sinus to the jugular bulb. The sphenoparietal sinus is a medial extension of the sylvian vein and courses around the greater sphenoid wing.

The supratentorial venous system See Fig. 1.36 Blood within the superficial cerebral veins flows in a centrifugal direction, radially towards the dural venous sinuses or adjacent lacunae. The veins are valveless. Almost all of the superficial veins are unnamed and inconstant, with three exceptions. The superficial middle cerebral (sylvian) vein forms along the surface of the Sylvian fissure and is convex anteriorly on a lateral projection. It is continuous with the sphenoparietal sinus. The anastomostic veins of Trolard, superiorly, and Labbe, inferiorly, connect the superficial middle cerebral vein with superior sagittal and transverse sinuses, respectively. It is uncommon for both anastomotic veins to be well developed inan individual. Blood in the deep cerebral veins flows centripetally (i.e. centrally). Medullary veins drain to subependymal veins along the walls of the lateral ventricles. The thalamostriate vein is a member of the subependymal group and runs across the floor of the lateral ventricle over the thalamus to enter the internal cerebral vein behind the foramen of Monro. The septal vein, another subependymal vein, passes around the head of the caudate nucleus and travels posteriorly in the septum pellucidum. It too enters the internal cerebral vein behind the foramen of Monro. The venous angle, at the confluence of the thalamostriate and septal veins, denotes the posterior margin of the foramen on the lateral angiogram.

19

Section 1: Central Nervous System

A

B

Internal cerebral v.

Septal v. Thalamostriate v.

Basal v. of Rosenthal

Internal cerebral v.

V. of Galen

Fig. 1.40 Contrast-enhanced T1W cranial MR, axial scans. The deep cerebral veins: (A) is superior to (B).

The basal vein of Rosenthal forms in the Sylvian fissure and travels in the ambient cistern around the midbrain to enter thevein of Galen, along with the internal cerebral vein. Boththe basal vein and internal cerebral vein are paired structures, the latter running along the roof of the third ventricle, from the foramen of Monro in the cistern of the velum interpositum. The vein of Galen is a short (1–2 cm) single midline vessel and originates under the splenium of the corpus callosum, curving posteriorly and superiorly towards the straight sinus. Elements of the deep cerebral venous system can be identified on intravenous contrast-enhanced CT and MRI (Fig.1.40).

The vertebrobasilar arterial system There are four vertebral artery segments. The first, extraosseous segment (V1) extends from the subclavian artery origin to the C6 foramen transversarium. Then the osseous V2 segment passes through foramina transversaria of the cervical column to C1. The arterial course here is vertical to C2. Then it turns laterally and once again vertically to the C1 foramen transversarium. V3 is the extraspinal segment directed superomedially to the foramen magnum. V4 is the intracranial segment, within the subarachnoid space (Figs. 1.41, 1.42). The normal anatomy of the intracranial vertebrobasilar arterial is subject to some individual variation in the origins, course and distribution of the component arteries (Fig. 1.43). There is also a well-developed network of anastomoses between these arteries.

V3 segment

Left vertebral a.

V2 segment

V1 segment

Subclavian a.

Fig. 1.41 CT angiogram, frontal (coronal) reconstruction. The vertebral arteries. The vertebral artery segments.

20

Chapter 1: The skull and brain

V4 (intradural segment) Posterior inferior cerebellar a. V3 (extraspinal) segment C1 vertebral level

C2 vertebral level

hypoplastic then the other will be well-developed. In a small proportion of cases, one, usually hypoplastic, vertebral artery terminates as the PICA. The PICA first winds around the olive of the medulla and comes near to the biventral lobule of the cerebellum. This is the anterior medullary segment (Fig.1.43b(1)). The vessel then courses around the brainstem as the lateral medullary segment, which corresponds to the caudal loop seen on the lateral projection at angiography(2). This curves around the inferior margin of the cerebellar tonsil. The posterior medullary segment ascends to the superior part of the tonsil and, at the apex of the cranial loop, gives off branches which supply the choroid plexus of the fourth ventricle(3). The PICA then proceeds to supply the undersurface of the cerebellar hemisphere. Meningeal branches may also arise from it.

Basilar artery V2 (intraforaminal -C6-C1) segment

Fig. 1.42 Left vertebral catheter angiogram, frontal projection. The vertebral artery segments.

The basilar artery forms from the confluence of the vertebral arteries at the pontomedullary junction. It ascends approximately in the midline in the pontine system and grooves the surface of the anterior pons. Superiorly it courses a little posteriorly before dividing into the posterior cerebral arteries. Throughout the length of the basilar artery, small penetrating branches pass posteriorly into the brainstem, which are at risk during vascular interventional procedures.

The anterior inferior cerebellar artery (AICA) The posterior inferior cerebellar artery (PICA) PICA arises from the vertebral artery as its largest and most distal branch. It usually rises well above the foramen magnum but may arise below it. There is a reciprocal arrangement with the anterior inferior cerebellar artery such that if one is

A

AICA passes laterally from the basilar artery, closely related to the abducent nerve. It traverses the cerebellar pontine angle system, usually anterior and medial to the neural bundle. A lateral branch then courses around the flocculus and a medial branch supplies the biventral lobule and cerebellar hemisphere. A labyrinthine artery supplies the inner ear.

B

Parietooccipital a.

Posterior cerebral a. (P3) Superior cerebellar a. Basilar a. Anterior inferior cerebellar a.

Fig. 1.43 Left vertebral catheter angiogram: (A) frontal, (B) lateral projections. The vertebrobasilar arterial system. Posterior inferior cerebellar artery. (1) anterior medullary segment, (2) lateral medullary segment, (3) supratonsillar segment.

21

Section 1: Central Nervous System

The superior cerebellar artery The superior cerebellar artery arises from the basilar artery near to its terminal division. It runs laterally around the brainstem and comes to lie inferior to the oculomotor nerve, which separates it from the posterior cerebral artery. At the lateral border of the pons it turns posteriorly over the middle cerebellar peduncle as the ambient segment and the tentorium maycome into contact with the artery. The ambient segment parallels the course of the trochlear nerves, and it is notable that the basal vein, the posterior cerebral artery and the free edge of the tentorium are also in this plane. In the quadrigeminal cistern both superior cerebellar arteries approach the midline. They supply the cerebellar hemispheres and superior vermis.

The posterior cerebral artery Each posterior cerebral artery can be divided into a number of segments (Fig. 1.44). The P1 or pre-communicating segment extends from the basilar bifurcation to the origin of the posterior communicating artery. It lies within the interpeduncular fossa and thalamoperforating arteries rise from both this P1 segment and from the posterior communicating artery. These branches have an extensive distribution to the thalamus, hypothalamus, the oculomotor and trochlear nerves and to the internal capsule. The P2 or ambient segment runs around the brainstem in the ambient cistern, parallel to the basal vein. It courses around the cerebral peduncles to lie above the tentorium. The P2 segment may be compressed against the tentorial edge when there is uncal pressure on the midbrain in the presence of raised intracranial pressure. Infarction of the occipital lobe is thus a recognized consequence. The P2 segment usually gives rise to the inferior temporal artery and asingle medial and multiple lateral posterior choroidal arteries.

A

The P3 segment extends from the quadrigeminal plate cistern to the calcarine fissure. The two major terminal branches of the posterior cerebral artery are the parietooccipital and calcarine arteries. The smaller calcarine artery is seen angiographically to pursue a straight course, running between the parieto-occipital branch posteriorly and the posterior temporal branch inferiorly on the lateral projection. The posterior pericallosal arteries arch over the splenium and arise from either the posterior cerebral or parieto-occipital arteries. There is some variation between individuals as to the origin of the posterior cerebral artery branches. It is not uncommon to encounter the so-called fetal origin of the posterior cerebral artery. In this case the precommunicating (P1) segment is undeveloped, and the posterior cerebral artery fills exclusively from the internal carotid artery, and not from the basilar artery.

Diencephalon The diencephalon comprises a large aggregate of grey matter, which lies between the cerebral hemispheres and brainstem and which borders the third ventricle. The thalamus is the largest structure in the diencephalon and is made up of a number of functionally important nuclei. The most dorsal nucleus is called the pulvinar (Fig. 1.45). The two thalami are apposed (not in continuity) in the midline at the interthalamic adhesion or massa intermedia. The hypothalamus forms the roof of the interpeduncular fossa and the floor of the third ventricle. The pineal gland (or body) hangs by a stalk joined to the posterior aspect of the diencephalon and third ventricle. It lies in the midline above the superior colliculi (Fig.1.46). In adults, it is almost invariably calcified, when seen on CT (Fig.1.12). It is not protected by the blood–brain barrier and consequently enhances avidly with contrast. B

Posterior pericallosal a.

Calcarine a. Parietooccipital a.

Posterior cerebral a. (P3)

Posterior choroidal As. Parietooccipital a. Calcarine a. Thalamoperforate As.

Posterior inferior cerebellar a.

Fig. 1.44 Left vertebral catheter angiogram: (A) frontal, (B) lateral projections. The posterior cerebral arteries.

22

Chapter 1: The skull and brain

The pineal stalk consists of two laminae, forming the habenular commissure superiorly and the posterior commissure inferiorly. The habenular commissure can calcify and forms a C shape, the open part of the C directed posteriorly.

The pituitary gland The pituitary gland occupies the pituitary fossa in the body of the sphenoid bone, situated in the midline above the sphenoidsinus in between the cavernous sinuses (Fig.1.47). It is suspended from the pituitary stalk, or infundibulum, which A

arises from a hollow eminence of grey matter called the tuber cinereum, the inferior part of the hypothalamus. The tuber cinereum lies posterior to the optic chiasm and anterior to the mamillary bodies. Like the pineal gland, the pituitary gland, the infundibulum and the tuber cinereum enhance normally with contrast due to the absence of a blood–brain barrier. The anterior lobe (adenohypophysis) can be distinguished from the posterior lobe (neurohypophysis) on sagittal MRI scans. The neurohypophysis often has a conspicuous appearance on T1W images due to the presence of vasopressin/ oxytocin – the so-called pituitary ‘bright spot’ (Fig.1.47a). B

Gyrus rectus Temporal lobe Prepontine cistern

Sylvian fissure Amygdala

Pons

Suprasellar cistern

Superior cerebellar peduncle

Cerebral peduncle Inferior colliculus

Fourth ventricle

Occipital lobe Occipital lobe

Cerebellar hemisphere

C

Interhemispheric fissure

D

Medial orbital gyrus Gyrus rectus Uncus

Mammillary body Ambient cistern

Inferior recess of third ventricle

Cerebral aqueduct with periaqueductal grey matter

Quadrigeminal cistern

Calcarine sulcus

Fig. 1.45 Axial T1-weighted images through the brain: (A)–(K), inferior to superior.

23

Section 1: Central Nervous System

E

External capsule

F Genu of corpus callosum

Insula

Head of caudate nucleus Claustrum

Anterior limb of internal capsule

Anterior commissure

Globus pallidus Putamen

Third ventricle

Thalamus

Habenula

Splenium of corpus callosum

Pulvinar of thalamus Hippocampal tail

Supramarginal gyrus

Pineal body Angular gyrus

Cistern of the velum interpositum Occipital horn of the lateral ventricle

G

H

Body of caudate nucleus Frontal operculum Corona radiata

Parietooccipital sulcus

Fig. 1.45 (cont.)

The presence of the bright spot is variable both between individuals and in the same individual scanned at different times. Normal sizes (measured from superior to inferior) of the gland are: • 6 mm or less in children • 8 mm in males • 10 mm in females • 12 mm in pregnant/lactating females. The superior margin of the gland is normally concave but can be convex in the neonate and in females of reproductiveage.

24

The basal ganglia The basal ganglia comprise several deep grey matter nuclei within the forebrain, midbrain and diencephalon (Figs.1.18b, 1.48, 1.49, 1.50): • caudate nucleus • putamen • globus pallidus (also referred to as the pallidum) • subthalamic nucleus • substantia nigra. Knowledge of the rather complex three-dimensional anatomy of these nuclei is invaluable when interpreting CT or MR images. The head of the caudate nucleus indents the frontal

Chapter 1: The skull and brain

I

J

Centrum semiovale

Motor ‘hand knob’

Central sulcus

K Superior frontal sulcus Precentral sulcus Precentral gyrus

Body of corpus callosum

Central sulcus

Splenium of corpus callosum

Postcentral gyrus

V. of Galen Quadrigeminal cistern Pineal body Pars bracket Tectum of midbrain Genu of corpus callosum

Fig. 1.46 T2 sagittal MRI. Pineal gland.

Fig. 1.45 (cont.)

A

Fornix

B

Optic chiasm

Mammillary body

Infundibular recess of third ventricle

Tuber cinereum

Infundibulum or pituitary stalk

Pituitary gland Posterior pituitary ‘bright spot’

Fig. 1.47 T1 sagittal MRI. Pituitary gland: (A) pre, (B) post contrast.

25

Section 1: Central Nervous System

Head of caudate n. Insula

External capsule Claustrum Putamen Globus pallidus Third ventricle Habenula Pineal body

Fig. 1.48 Skull-stripped axial T1-weighted image. The basal ganglia.

horn of the lateral ventricle. Its body curves upwards and posteriorly from the head, following the contour of the body of the lateral ventricle before continuing in an arch to its thinnest part, the tail, which comes to lie immediately superior to the temporal horn of the lateral ventricle The subthalamic nucleus is an ovoid aggregation of grey matter that lies medial to the internal capsule, lateral to the hypothalamus and superolateral to the red nucleus (Fig.1.51). It establishes connections with both internal and external segments of the globus pallidus and with the thalamus. Damage to this nucleus results in contralateral hemiballismus – uncontrolled jerks of the limbs.

A

The limbic system The limbic system is a complex arrangement of interrelated cortical and subcortical structures that play a major role in memory, olfaction and emotion. The following is a list of its core components: • hippocampal formation • parahippocampal gyrus • amygdala • hypothalamus. The limbic lobe refers to the cortical parts of the limbic system. It forms a border (limbus) around the diencephalon and midbrain, which is composed of three C-shaped arches one inside the other, viewed from a lateral perspective (Fig.1.52). Outer arch: • parahippocampal gyrus • cingulate gyrus • subcallosal gyrus Middle arch: • hippocampus proper (cornu ammonis) • dentate gyrus • indusium griseum (supracallosal gyrus) • paraterminal gyrus Inner arch: • fornix and fimbria. The hippocampus incorporates several structures, which together may be called the hippocampal formation. During development, this area of cortex becomes rolled up into an S-shape, which forms at the medial (also called mesial) aspect of the temporal lobe (Figs. 1.53, 1.54). It comprises the hippocampus proper (also called the cornu ammonis), the dentate gyrus and the subiculum. The subiculum lies inferior to the hippocampus proper and blends into the

B Caudate nucleus

Body of caudate n. Thalamus

Claustrum Claustrum Temporal stem

Putamen

Third ventricle

Basilar a.

Fig. 1.49 Coronal inversion recovery MR image (3T): (A) is anterior to (B).

26

Chapter 1: The skull and brain

adjacent parahippocampal gyrus. The hippocampi are closely scrutinized by the neuroradiologist for mesial temporal sclerosis in the context of temporal lobe epilepsy. The hippocampus can be recognized in the coronal plane as a protrusion into the medial temporal horn of the lateral ventricle. The border between the parahippocampal gyrus (medially) and the occipitotemporal gyrus (also known as the fusiform gyrus) is demarcated by the collateral sulcus (also identifiable in the coronal plane). The uncus is the most medial portion of the temporal lobe and is continuous with the parahippocampal gyrus posteriorly (Fig. 1.45). The amygdala is just lateral to the uncus and situated anterior to the temporal horn of the lateral ventricle. The amygdala is thus anterior and superior to the hippocampus.

A

The fimbria of the hippocampal formation continues as the crus of the fornix, a fibre bundle that sweeps backwards, upwards and medially around the posterior aspect of the thalamus (Figs. 1.47a, 1.50). The two crura then pass forwards and converge in the midline, forming the body, where they are attached to the septum pellucidum. The body continues forwards before separating, just above the foramen of Monro, into the columns of the fornices. The fibres terminate in septal nuclei and the mammillary bodies of the hypothalamus. Only the hippocampus proper and the subicular region project fibres into the fornix. The hippocampal tail tapers into a thin neuronal lamina, the indusium griseum, which arches around the corpus callosum along the inner border of the cingulate gyrus,

B Genu corpus callosum

Head of caudate nucleus Claustrum

Anterior commissure

Putamen Third ventricle Pituitary stalk

Median eminence of hypothalamus Amygdala

C

D Parietal operculum

Frontal horn of lateral ventricle Sylvian fissure

Mammillary body Temporal horn of lateral ventricle

Insula

Temporal operculum Hippocampal head Parahippocampal gyrus

Collateral sulcus

Fig. 1.50 Coronal T1-weighted images through the brain: (A)–(G), anterior to posterior.

27

Section 1: Central Nervous System

E

F

Corona radiata

Cingulate gyrus

Body of corpus callosum

Splenium, corpus callosum

Fornix

Cingulate gyrus Occipitotemporal gyrus

Parahippocampal gyrus

G

Subthalamic nucleus

Parietooccipital fissure Occipital horn, lateral ventricle

Internal capsule

Calcarine sulcus

Substantia nigra Red nucleus

28

Fig. 1.50 (cont.)

Fig. 1.51 T2 coronal MRI. Subthalamic nucleus.

becoming part of the paraterminal gyrus in behind the subcallosal area. The olfactory nerve, like the optic nerve, is really part of the CNS and not strictly a cranial nerve. The olfactory bulb receives tiny fibres from the nasal mucosa through the cribriform plate and projects an axonal bundle, the olfactory tract, that runs posteriorly along the inferior surface of the frontal lobe. It divides into medial and lateral tracts at the olfactory trigone, a point in the basal forebrain just in front of the anterior perforated substance (so-called because it is the point of entry for multiple small striate arteries) (Fig.1.55). The mamillary bodies (or nuclei) are part of the hypothalamus and are situated at the ends of the columns of the fornices.

They relay impulses from the hippocampal formation and amygdala nuclear complexes to the thalamus (along the mammillothalamic tract (Fig. 1.56)). They are of particular relevance in patients with Wernicke-Korsakoff syndrome, in whom they may be atrophied as a result of chronic thiamine deficiency.

The cerebral hemispheres The cerebral cortex is organized into folds called gyri between which there are CSF-filled grooves called sulci. The deeper and more anatomically constant sulci are known as fissures. The lateral (Sylvian) fissure marks the superior margin of the temporal lobe, while the parieto-occipital fissure divides the parietal and occipital lobes. The central sulcus marks the

Chapter 1: The skull and brain

Cingulate gyrus and cingulum

A

Indusium griseum

Choroid plexus

Septum pellucidum

Alveus

Column of fornix Fimbria of fornix Dorsal fornix Isthmus Olfactory bulb Uncus

Fimbria of fornix

Parahippocampal gyrus

Dentate gyrus

Subiculum

Amygdala nuclear complex

Tail of caudate nucleus Anterior commissure

B

Temporal horn of ventricle

Hippocampus

Ammon’s horn

Fornix

Fimbria

Fig. 1.52 (A) Medial aspect of cerebral hemisphere showing the limbic system. (B) The hippocampus viewed from above.

division between the frontal and parietal lobes. The insula is an area of invaginated cortex lying deep within the Sylvian fissure, covered by the frontal, temporal and parietal opercula (Latin for ‘little lids’) (Figs. 1.45, 1.48, 1.49, 1.50). Cortical anatomy is subject to individual variation but the more constant gyri and sulci are illustrated in Fig.1.57. The key facts concerning the functional anatomy of specific cortical areas are given here.

Frontal lobe • • •

Anterior to central sulcus Precentral gyrus contains primary motor cortex Lateral surface of precentral gyrus supplies head and face • Medial surface supplies lower limb • Upper limb takes up the largest area of cortical representation (situated between lower limb and head/face) • Premotor cortex lies anterior to precentral gyrus • Three further frontal lobe gyri: superior, middle, inferior, separated by the superior and inferior frontal sulci. The dominant hemisphere of the frontal lobe also contains Broca’s area (involved with motor aspects of speech). It is situated in the pars opercularis, which lies in the posterior aspect of the inferior frontal gyrus. A V-shaped area of cortex immediately anterior to it is a useful and constant cortical landmark, called the pars triangularis (Fig.1.58d).

Dentate gyrus

Fig. 1.53 The limbic lobe.

• • • •

The parietal lobe also contains two further important gyri: the supramarginal and angular gyri, which are involved in visuospatial processing (especially in the non-dominant hemisphere). The angular gyrus is found on the lateral surface of the cerebrum at the posterior termination of the Sylvian fissure (Fig.1.45f ). The supramarginal gyrus lies in front of the angular gyrus. The medial surface of the parietal lobe is called the precuneus (Figs. 1.57, 1.58).

Temporal lobe • •

Inferior to lateral (Sylvian) fissure Transverse gyrus (of Heschl) contains primary auditory cortex (at the superior surface of the temporal gyrus within the Sylvian fissure (Fig.1.58d)) • Middle and inferior temporal gyri contain large areas of association cortex • Medial temporal lobe contains limbic structures (parahippocampal gyrus, uncus). The temporal lobe also contains Wernicke’s area (in the dominant hemisphere), which is involved with the receptive aspects of speech and is situated in the posterior part of the superior temporal gyrus, inferior to the angular gyrus.

Occipital lobe

Parietal lobe

• •

• •

Posterior to central fissure Lies above and in front of occipital lobe (divided by parietooccipital fissure (Fig.1.58d))

Postcentral gyrus contains primary somatosensory cortex Inferolateral surface supplies face, lips and tongue Superolateral surface supplies upper limb Medial aspect supplies lower limb.

Posterior to parieto-occipital fissure Primary visual cortex is situated on medial occipital lobe (calcarine cortex) Anterior margin marked by the temporo-occipital incisure.

29

Section 1: Central Nervous System

A

B Amygdala

Tail of caudate nucleus

Temporal horn of lateral ventricle

Head of hippocampus with interdigitation

Hippocampal head

Superior temporal gyrus

Middle temporal gyrus

Inferior temporal gyrus

Fusiform

Ambient cistern

C

Choroidal fissure

Fimbria

Alveus

Temporal horn of lateral ventricle

D

MEDIAL

LATERAL

Exploded view

ParaSubiculum hippocampal gyrus

Collateral sulcus

Fusiform gyrus

Stratum radiata

Fig. 1.54 The hippocampus, (A) T2 parasagittal MRI. T2 coronal MRI (B); thehead (C); the body (D); the tail (E). E

The medial surface of the occipital lobe is made up of the cuneus (above) and the lingula (below) (Fig.1.58).

The central sulcus Fornix

Tail of hippocampus

30

The central sulcus is a useful landmark for neuroradiologists and neurosurgeons alike but identifying it with certainty can present some difficulty on CT and MR images. There are, however, a number of useful tips, which can help determine the position of the central sulcus. • On axial scans, follow the superior frontal sulcus from anterior to posterior until it meets and forms an angle with the precentral sulcus – the central sulcus is the next one behind (Fig.1.45k). • On lateral sagittal images note the Y-shaped sulcus of the pars triangularis at the anterior end of the Sylvian fissure. The next major fissure posterior to the Y is the precentral sulcus (Fig.1.58d). • On medial sagittal images follow the cingulate sulcus as it ascends superiorly and posteriorly towards the vertex

Chapter 1: The skull and brain

A

B

Olfactory bulb

Olfactory bulb

Olfactory trigone Lateral olfactory stria Anterior perforated substance Medial olfactory stria Uncus

Fig. 1.55 (A) Line diagram showing the olfactory pathways. (B) T2 coronal MRI. Olfactory bulbs.

A

B

Mammillothalamic tract

Mammillothalamic tract

Fig. 1.56 T2 axial MRI. Mammillothalamic tracts: (B) is superior to (A).

• •

as the pars marginalis (Fig.1.58a), which on axial images looks like a bracket (Fig.1.45k). The central sulcus indents the medial part of the paracentral lobule at the vertex on the medial surface of the cerebrum just in front of the pars marginalis. The precentral gyrus is usually larger than the postcentral gyrus (and the cortex is slightly thicker). The precentral gyrus contains an area at its superiorlateral part, which resembles an upside-down omega

(Ω) – an area of cortex that represents the motor-hand area (Fig.1.45j).

White matter tracts The internal capsule The internal capsule is a thick band of projection fibres carrying axons going to and from the cerebral cortex at the level of the basal ganglia and thalamus.

31

Section 1: Central Nervous System

Central sulcus

B

Pr ec un

r pe

io

ta l ron rf

Cuneus

M

f dle id

u gyr

ta ron

ior fer In

rine sulcus Calca

Parahippoc a

mpal gyr us

Medial oc cipitotem poral gyr Lateral us occipit otemp oral gy rus

s

r us l gy

nta fro

l gy

rus

rs Pa ularis ng tria

Superio

al mpor

or Inferi

pe rio rp ul e

lob

Parietooccipital sulcus

Inferior p ar lobu ietal le

gyr oral r temp

te Middle

Uncus

Su

al ie t ar

Su

gy rus

yrus

nta l

Postcentral g

Med ial f ro

eu s

al lobule centr Para

Precentral gyrus

A

us

s gyru

yrus o r al g temp

Temporooccipital incisure

Fig. 1.57 The cortical gyri: (A) medial and (B) lateral aspects.

Central sulcus

A

Pars marginalis

B

Body of caudate

Cingulate sulcus Precuneus Parietooccipital fissure Cuneus Calcarine sulcus

Head of caudate 4th ventricle

C

D

Precentral sulcus Central sulcus Pars opercularis Pars triangularis

Body of hippocampus

Transverse gyrus (of Heschl) Pars frontalis

Fig. 1.58 Sagittal T1-weighted images through the brain: (A)–(D), medial to lateral.

32

Chapter 1: The skull and brain

Projection fibres, by definition, pass to or from the more caudal parts of the neuraxis (the term for the brain and spinal cord). Superiorly the fibres fan out as the corona radiata as they pass the body of the lateral ventricle to continue as the supraventricular white matter tracts, which radiologists refer to as the centrum semiovale (Fig.1.45i). When viewed axially on CT and MR, the internal capsule resembles a V-shaped tract composed of anterior and posterior limbs, joined by the genu (Fig.1.45). Voluntary movement is initiated in the premotor and precentral gyri of the frontal lobe, which send projection fibres to brainstem motor nuclei and spinal cord via the corticospinal and corticobulbar tracts, respectively. The corticobulbar tracts are at the genu. In some individuals the corticospinal tracts, in the posterior limb of the internal capsule, show a relatively high T2 signal because of low myelin density. This should not be mistaken for a lesion (Fig.1.59). The high density of axons in the internal capsule means that even a small lacunar stroke occurring within it can precipitate profound and extensive neurological deficits. The blood vessels supplying the internal capsule are given below: • anterior limb – recurrent artery of Heubner (branch of the anterior cerebral artery) • genu – lenticulostriate arteries (branches of middle cerebral artery) • posterior limb – anterior choroidal artery (arises from the internal carotid artery).

The commissural tracts These link equivalent sites across the cerebral hemispheres.

Corpus callosum The corpus callosum is the largest commissure. It forms a C-shaped structure, concave inferiorly. The rostrum is the portion that projects infero-posteriorly from the anteriormost genu. The body curves upwards and posteriorly towards the splenium (Fig. 1.46). The genu fibres curve forward into the frontal lobes, forming forceps minor. Similarly, the fibres of the splenium curve backward into the occipital lobes, as forceps major (Fig.1.60).

Corticospinal tract

Fig. 1.59 T2 axial MRI. The corticospinal tracts.

The ventricular system The cerebral ventricles are cavities situated deep within the brain. They are lined by ependymal cells and contain the choroid plexus, which produces CSF. There are four ventricles in total: the two paired lateral ventricles and the midline third and fourth ventricles (Fig. 1.62). Each lateral ventricle drains into the third ventricle via the foramen of Monro. The third ventricle communicates with the fourth via the cerebral aqueduct (of Sylvius) (Figs. 1.45c, 1.61). Each lateral ventricle has a body, atrium and three hornsnamed after the lobe in which they lie: frontal, occipital, temporal. The relations of the frontal horn are the corpus callosum superiorly, the head of the caudate nucleus laterally and

Anterior commissure The anterior commissure is a phylogenetically older structure, which comprises a transversely oriented fibre bundle, linking the olfactory tracts and structures within the anterior temporal lobes including the amygdala nuclear complexes. It lies in front of the columns of the fornices, embedded in the anterior wall of the third ventricle (the lamina terminalis) (Figs. 1.45e, 1.50b, 1.61).

Forceps major

Forceps minor

Posterior commissure The posterior commissure (also known as the epithalamic commissure) runs within the posterior pineal lamina to connect the right and left midbrain. It carries fibres responsible for bilateral pupillary light reflex.

Fig. 1.60 Diffusion anisotropy (MR) image showing the major white matter tracts. Note that the colour coding denotes vectors. Red: transverse fibres. Blue:vertical fibres. Green: anteroposterior fibres.

33

Section 1: Central Nervous System

Foramen of Monro

Lateral

Anterior commissure Lamina terminalis

Third Optic recess of third ventricle

Cerebral aqueduct Fourth

Infundibular recess of third ventricle

Cerebral aqueduct

Fig. 1.61 T2W sagittal. The third ventricle. Note that the patient has hydrocephalus.

inferiorly and the septum pellucidum medially. The body is formed by the corpus callosum (roof), dorsal part of the thalamus (floor), fornix (medially) and the body and tail of the caudate laterally. The temporal horn (sometimes called the inferior horn) has the tail of the caudate as its roof, the hippocampus medially and inferiorly and the optic radiation and associated white matter tracts medially. The occipital horn is surrounded by the forceps major, a white matter tract of the corpus callosum. The atrium represents the confluence of the three horns and contains the choroid plexus, which is highly vascular and usually calcified on CT (Fig.1.12d). The cavum septi pellucidi is a fluid-filled extrapial cavity between the two laminae of the septum pellucidum. It is seen in virtually all neonates and regresses by adulthood. It persists into adulthood in about 10% of people. The cavum vergae is the posterior continuation of the cavum septi pellucidi beneath the splenium and above the fornix. The cavum closes in posterior to anterior. Therefore in an adult a cavum septi pellucidi can exist in isolation or with a cavum vergae but a cavum vergae cannot persist as the solitary cavum. The velum interpositum is a cisternal space created by infolding of the tela choroidea beneath the fornix. Inferiorly, it opens into the quadrigeminal cistern. It contains the internal cerebral veins and does not extend anterior to the foramen of Monro, allowing it to be distinguished from the cavum vergae (Fig.1.45e).

34

Fig. 1.62 The cerebral ventricles.

The third ventricle The third ventricle is a narrow slit-like vertical cavity between the right and left diencephalon (Figs. 1.45, 1.48, 1.50). Its anterior wall is called the lamina terminalis, which contains the anterior commissure at its superior border. It possesses two recesses anteriorly; the more superior lies above the optic chiasm (the optic or chiasmatic recess) and the more inferior lies behind the infundibulum of the pituitary gland (the infundibular recess) (Fig.1.61). Two further recesses are present posteriorly, one above the pineal gland (the suprapineal recess), the other directed into the pineal gland (pineal recess).

The fourth ventricle The fourth ventricle is within the dorsal pons and upper medulla. It appears as a diamond-shaped cavity on coronal scans, an inverted U on axial scans and as a triangle on sagittal images. There are paired lateral apertures (the foramina of Luschka) and a single median aperture (of Magendie), which transmit CSF into the cisterna magna. These provide routes of spread for disease out of the ventricular system and into the subarachnoid space. The fourth ventricle is bounded by a tent-shaped roof, made up of the superior medullary velum and inferior medullary velum. The dorsal surface of the pons and medulla form the anterior wall and it is enclosed laterally by the middle cerebellar peduncles (Figs. 1.21, 1.28, 1.46).

Section 1 Chapter

2

Central Nervous System

The orbit and visual pathway Indran Davagnanam and Jonathan L. Hart

Plain film Plain film radiography is no longer used routinely for the evaluation of orbital pathology, but familiarity with normal anatomy remains important when reviewing emergency department trauma radiographs (Fig.2.1).

Cross-sectional anatomy The primary imaging modalities used to examine the orbit and visual pathways in clinical practice are CT and MRI. The divergent, conical anatomy of the orbits and their contents means that a combination of axial, coronal and parasagittal scan planes may be required to delineate anatomical structures optimally. CT demonstrates orbital anatomy well due to the substantial differences in attenuation of bone, air in adjacent paranasal sinuses, orbital fat and soft tissues. In particular, helical CT with multiplanar reconstructions provides excellent bony anatomical detail. Coronal reformatted images are important for the bony anatomy at the orbital apex, the orbital floor and roof.

Frontal sinus

MRI is valuable for evaluation of intra-orbital soft-tissue anatomy and is unhindered by artefacts from surrounding bone. Imaging protocols usually include axial and coronal sequences, including thin-section coronal T2-weighted scans with fat suppression. Intravenous gadolinium-enhanced T1-weighted imaging is also combined with fat suppression so that enhancing structures are not obscured by the intrinsic high-T1 signal of normal orbital fat. Acquisition times should be short to minimize the effects of eye movement. MRI is the preferred technique for demonstration of the intracranial optic nerves, optic chiasm and tracts.

The orbit The orbits are pyramidal bony cavities with the apex lying posteriorly and the base anteriorly (Fig.2.2). The long axes of the orbits are divergent by approximately 45° and the medial walls are roughly parallel. The fragile medial (lamina papyracea) and inferior walls are vulnerable to blowout fractures in blunt trauma (Fig. 2.3). Paranasal sinus pathology may involve the orbits by direct extension.

Crista galli

Superior orbital margin

Lesser wing of sphenoid

Planum sphenoidale Nasal bone Lamina papyracea

Anterior clinoid process

Frontozygomatic structure

Superior orbital fissure

Ethmoid sinuses

Greater wing of sphenoid

Innominate line (temporal fossa) Zygomatic arch Inferior margin of orbit Malar process of the zygoma

Nasal septum

Floor of pituitary fossa

Maxillary antrum

Fig. 2.1 Occipitofrontal radiograph of the bony orbits.

Applied Radiological Anatomy, 2nd Edition ed. Paul Butler, Adam W.M. Mitchell and Jeremiah C. Healy. Published by Cambridge University Press. © Cambridge University Press 2011.

35

Section 1: Central Nervous System

Anatomical relationships

Osseous anatomy of the orbital walls (Fig.2.4)

• • • •

Superior: anterior cranial fossa and frontal sinus Medial: nasal cavity, ethmoid and sphenoid sinus Inferior: maxillary sinus Posterolateral: temporal fossa and middle cranial fossa

Nasolacrimal duct

A

Roof: frontal bone (predominantly), lesser wing of sphenoid posteriorly Medial: (anterior to posterior) frontal process of maxilla, lacrimal bone, ethmoid bone, small sphenoid contribution at apex

Nasolacrimal duct

B

Lower pole of eye

Lower pole of eye

Inferior oblique muscle

Inferior oblique muscle

Orbital fat

Orbital fat

Temporal fossa

Inferior rectus muscle Middle cranial fossa (temporal lobe) Bony portion of nasolacrimal duct

Bony portion of nasolacrimal duct

Lateral wall of orbit

Floor of orbit

Temporal fossa

Suture between greater wing of sphenoid and zygoma

Inferior orbital fissure

Pterygopalatine fossa

Carotid canal

Zygomatic arch Sphenoid sinus Medial canthus

Lids

Lateral rectus muscle Lens

C

Outer coats of eye Vitreous Medial rectus muscle Lateral rectus muscle Orbital fat Annulus of Zinn Pituitary gland Cavernous sinus Middle cranial fossa (temporal lobe) Lamina papyracea

D

Extraconal orbital fat Vitreous Inferior pole of lacrimal gland Medial rectus muscle Optic disc Optic n.-sheath complex Intraconal orbital fat Optic n. (intracranial) Middle cranial fossa (temporal lobe) Infundibulum Lamina papyracea

Zygoma

Zygoma

Temporal fossa

Ethmoidal air cells

Ethmoidal air cells Greater wing of sphenoid

Temporal fossa Greater wing of sphenoid Optic canal

Sphenoid sinus

Superior orbital fissure Anterior clinoid process

Dorsum sellae

Pituitary fossa

Superior orbital fissure

Fig. 2.2 Paired (soft tissue and bone window settings) axial CT sections through the orbit in a caudal to cranial sequence at the levels of: (A) the pterygopalatine fossa and bony nasolacrimal ducts, (B) the inferior orbital fissure, (C) the superior orbital fissure, (D) the optic canal, (E) the superior rectus muscle, (F) the tendons of superior oblique muscle.

36

Chapter 2: The orbit and visual pathway

Upper lid (tarsal plate)

Upper lid (skin)

Superior oblique tendon Upper lid (tarsal plate)

E

Position of trochlea

Upper lid (skin) Upper lid (tarsal plate)

F

Lacrimal gland

Superior portion of the eye Lacrimal gland

Top of eye Levator palpebrae superioris m.

Superior oblique m. Superior ophthalmic v. Superior rectus-levator palpebrae superioris complex

Orbital fat

Orbital foramina (Fig.2.2) Crista galli Lacrimal fossa Superior orbital fissure

Greater wing of sphenoid

Fig. 2.2 (cont.)

Floor: (medial to lateral) orbital plate of maxilla and zygomatic bone, orbital process of the palatine bone posteriorly Lateral: zygomatic bone and frontal bone

The optic canal is formed by two roots of the lesser wing of the sphenoid bone and is intimately related to the sphenoid sinus and posterior ethmoid air cells (rarely, the entire circumference may be pneumatized). Its diameter is 3–4 mm. It communicates with the middle cranial fossa and transmits the optic nerve and ophthalmic artery. Onaxial CT images it courses below and medial to the anterior clinoid process (Fig.2.2d). The superior orbital fissure (SOF) lies between greater and lesser wings of the sphenoid and is separated from the optic canal by the optic strut. It communicates with the middle cranial fossa and the cavernous sinus lies posteriorly. The SOF transmits: • oculomotor (IIIrd cranial) nerve (supplying the superior, medial and inferior recti; inferior oblique and levator palpebrae superioris muscles) Lacrimal gland

A

Superior oblique tendon

Vitreous

Medial rectus m.

Nasociliary n.

Levator palpebrae superioris m.

B Superior rectus

Trochlear fossa

Superior oblique tendon

Vitreous

External coats of the eye

Lens

Inferior oblique m.

Air under the lower eyelid

Nasolacrimal duct

Orbital septum and soft tissue of the lower lid

Orbital fat Orbital plate of the frontal bone

Superior orbital margin

Crista galli Frontal sinus

Cribriform plate

Nasal bone

Lamina papyracea (ethmoid bone)

Floor of the orbit Maxillary antrum Frontozygomatic suture

Malar process of the frontal bone

Fig. 2.3 Coronal reconstructed CT sections of the orbits, from anterior to posterior, imaged on soft-tissue (top panel) and bone (bottom panel) windows at the levels of: (A) the reflected portion of the superior oblique tendon, (B) the mid-globe, (C) the posterior pole of the globe, (D) the extra-ocular muscles, (E) just anterior to the apex of the orbit.

37

Section 1: Central Nervous System

Lateral rectus m.

Inferior rectus m.

Superior rectus-levator palpebrae superioris complex

Superior ophthalmic v. Superior oblique m.

C

Superior rectus-levator palpebrae superioris complex Superior oblique m.

D

Nasociliary n.

Superior ophthalmic v.

Optic n.-sheath complex

Optic n.-sheath complex

Intraconal fat

Medial rectus m. Lateral rectus m.

Posterior pole of eye

Medial rectus m. Infraorbital n.

Inferior rectus m.

Orbital plate of the frontal bone

Ethmoid air cells

Cribriform plate

Lateral wall of orbit

Lamina papyracea

Temporal fossa

Floor of the orbit

Lamina papyracea

Osteomeatal complex

Infraorbital groove

Zygoma Infraorbital groove

Osteomeatal complex

Maxillary antrum

Middle turbinate

Inferior turbinate Optic n.-sheath complex

Frontal lobes

E Rectus muscles

Sphenoid bone; lesser wing Frontal bone

Optic canal

Supraorbital notch Ethmoidal foramina; anterior and posterior

Superior orbital fissure Sphenoid bone; greater wing

Planum sphenoidale

Ethmoid bone; orbital plate Lacrimal bone Palatine bone; orbital process

Inferior orbital fissure

Infraorbital groove

Zygomatic bone

Infraorbital foramen Maxilla

Fig. 2.3 (cont.)

• • • • •

trochlear (IVth cranial) nerve – superior oblique muscle abducent (VIth cranial) nerve – lateral rectus muscle branches of the ophthalmic (Vi) division of the trigeminal (Vth cranial) nerve carotid sympathetic plexus branches superior and inferior ophthalmic veins.

A small fat pad in the SOF is an important normal anatomical finding on CT or MRI; effacement may be the only sign of subtle pathology involving the superior orbital fissure.

38

Fig. 2.4 Bony constituents of the orbit and principal foramina: schematic diagram (frontal view of the right orbit).

Small nerves in the orbit, particularly the branches of the ophthalmic division of the trigeminal nerve, the inferior division of the oculomotor nerve and the infraorbital nerve, may be seen on MRI (particularly on coronal sections). The inferior orbital fissure (IOF) is located in the orbital floor between the greater wing of the sphenoid and orbital plate of the maxilla. It communicates with the pterygopalatine fossa and the masticator space. The IOF transmits: • branches of the internal maxillary artery

Chapter 2: The orbit and visual pathway

Aqueous

Cornea

A

B Ciliary body

Capsule and nucleus of the lens

Choroid/sclera

Vitreous Vitreous Lacrimal gland Optic disc Lateral rectus m.

Orbital fat

Medial rectus m.

Lateral rectus m.

Ophthalmic a. Medial rectus m.

Optic n. sheath complex

Internal carotid a.

C

Superior ophthalmic v.

Superior rectus-levator palpebrae superioris complex

Optic n. sheath

Medial rectus m.

Superior oblique m.

Lateral rectus m. Ophthalmic a. Inferior rectus m.

Fig. 2.5 Spin-echo MRI of the right orbit demonstrating the variation in the normal appearances of the eye depending on the sequence. (A) Axial T2-weighted, (B) axial and (C)coronal T1-weighted post-gadolinium contrast, fat suppressed. Note the normal intense enhancement of the extra-ocular muscles and ciliary body. Contrast enhancement of the meningeal sheath of the opticnerve outlines the nerve itself on the coronalimage.

communications between the inferior ophthalmic vein and the pterygoid venous plexus • branches of the zygomatic and infraorbital nerves The supraorbital foramen transmits the supraorbital nerve(Vi). The infraorbital canal / infraorbital foramen transmits the infraorbital nerve (Vii).

The anterior and posterior ethmoidal foramina transmit the anterior and posterior ethmoidal vessels and nerves (Vi), respectively.

The orbital compartments The soft tissue structures of the orbit are surrounded by the orbital fat which fills the cavity. The orbital septum is a fascial

39

Section 1: Central Nervous System

Superior and inferior canaliculi

Common canaliculus

Corpus callosum

Valve of Rosenmüller

Optic chiasm

Optic recess

A

Inter-thalamic adhesion/ massa intermedia

Contrast medium in conjunctival sac

Third ventricle

Lacrimal sac

Superior and inferior colliculi

Nasal septum Valve of Krause Mamillary bodies

Osseous and meatal segments of the nasolacrimal duct

Interpeduncular fossa Midbrain Cerebellar vermis

Valve of Hasner (plica lacrimalis)

Fourth ventricle

Middle and inferior conchae

Pons Pituitary infundibulum Pituitary gland Sphenoid sinus Infundibular recess

Fig. 2.6 Dacryocystogram, anteroposterior supine projection. B

layer attached to the orbital margin which separates the extraorbital pre-septal space from the orbital post-septal space (Fig.2.2c). Division of the orbital space into intra- and extraconal compartments is of importance in the differential diagnosis of retro-orbital masses and inflammatory and infiltrative lesions. The intraconal space is a cone formed by four rectus musclesand fasciae and, importantly, includes the muscles themselves (Fig.2.5).

Ethmoid sinuses

Optic n.

Optic chiasm Optic tract Hypothalamus Third ventricle

The globe The lens (due to its low water content) and ciliary bodies are demonstrated as dense structures distinct from the fluid of the anterior chamber and vitreous on CT (Fig. 2.2). The normal aqueous and vitreous humours are of similar attenuation to CSF, although streak artefact from the bone may produce areas of apparent high density. MRI with surface coils provides superior anatomical detail, but involuntary eye movements inevitably cause some motion artefact (Fig.2.5). Even with maximal spatial resolution, MRI does not permit differentiation of the three primary layers of the globe (sclera, uvea and retina). However, some ocular diseases are accompanied by detachment and/or effusion, which may permit the different potential spaces between the layers to be visualized. After intravenous gadolinium contrast, the choroid, ciliary body and iris enhance strongly (Fig.2.5b). The globe is divided into anterior and posterior segments. The anterior segment, containing aqueous humour, is anterior to the lens and its supporting circumferential ciliary body, which is attached to the lens by zonule fibres, the contraction of which allows accommodation. The anterior segment is further divided by the iris into: • the anterior chamber – the major chamber between cornea and iris • the posterior chamber – a potential space between iris and lens ligament complex.

40

Position of lateral geniculate nucleus

C

Optic tract

Optic chiasm Chiasmatic cistern

Optic n.

Fig. 2.7 Sagittal (A), axial (B) and oblique (C) reformatted volume T1-weighted MR images demonstrating the intracanalicular and intracranial segments of the optic nerves and the optic chiasm. There is normal interindividual variation in the position of the chiasm relative to the pituitary fossa.

Chapter 2: The orbit and visual pathway

The posterior segment (vitreoretinal portion of the eye) contains the vitreous humour. Its three layers comprise (from inside out) retina, uveal tract (choroid, ciliary body and iris) and sclera (the fibrous layer to which the extra-ocular muscles are attached). The optic disc lies slightly medially on the posterior concavity of the globe. The globe may be evaluated for proptosis on cross-sectional imaging. On an axial section containing the optic lens and optic nerve head, a line is drawn between the bony lateral orbital margins. Normally, one-third of the globe is located behind this interzygomatic reference line.

The extra-ocular muscles (Figs. 2.2, 2.3, 2.5) Four fusiform rectus muscles move the eyeball, but the divergent geometry of the orbits is such that the actions of the superior and inferior recti do not occur strictly in the orthogonal planes. The oblique muscles are necessary to assist in direct upward and downward globe movements. The extraconal levator palpebrae superioris elevates the upper eyelid.

The extra-ocular muscles are of normal soft tissue attenuation on CT and signal intensity on MRI. Intravenous gadolinium improves their conspicuity on T1-weighted MRI (when combined with fat saturation sequences) as they enhance strongly due to the lack of blood–tissue barrier. The levator palpebrae superioris is not always identified separately from the superior rectus on standard imaging, and the muscles are sometimes referred to together as the superior muscle complex. Normal measurements of extra-ocular muscles are described (with maximal diameters of the order of 5 mm), but morphology is at least as important a marker of pathology as muscle size. Measurements also vary with age, sex and interzygomatic distance. The eye position (appreciated from the lens or optic nerve) should be accounted for when assessing relative sizes of the muscles. Divergence of the eyes may be normal in the sleeping patient. • The rectus muscles arise from a common annular tendon at the orbital apex (Zinn’s ligamentous ring). The medial rectus is larger than the opposing lateral rectus muscle.

Lateral ventricle Head of caudate nucleus

A

B

Anterior cerebral a.

Internal capsule Lentiform nucleus

Optic chiasm

Pre-chiasmatic optic n.

Middle cerebral a.

Uncus of temporal lobe

Chiasmatic cistern

Pituitary infundibulum

Pituitary gland Sphenoid sinus

Basisphenoid Internal carotid a.

Fig. 2.8 Coronal MRI. (A) T1-weighted image through the infundibulum of the pituitary gland, demonstrating division of the optic chiasm into the optic tracts; (B)slightly more anterior T2-weighted coronal image through the optic chiasm itself. Lateral ventricle Caudate nucleus Third ventricle Foramen of Monro

A

B

Thalamus

Optic tracts Hippocampus

Lateral geniculate body Hypothalamus Chiasmatic cistern Pituitary infundibulum

Ambient cistern

Interpeduncular fossa

Sphenoid sinus

Fig. 2.9 Retrochiasmal optic pathways: (A) optic tracts, (B) lateral geniculate bodies on coronal reformatted volume T1-weighted inversion recoveryMRI.

41

Section 1: Central Nervous System

Hippocampus Uncus of temporal lobe

A

Lateral ventricle

B

Interpeduncular fossa

Thalamus (pulvinar)

Cerebral peduncle

Superior colliculus

Ambient cistern Choroidal fissure

Aqueduct of Sylvius

Inferior colliculus

Superior colliculus Trigone of the lateral ventricle

Cerebellar hemisphere

Quadrigeminal cistern

Fourth ventricle

Fig. 2.10 Superior colliculi. (A) Axial T1-weighted MRI through the midbrain and (B) coronal T2-weighted image through the quadrigeminal plate.

Third ventricle

Third ventricle

Thalamus

A

Thalamus

B

Lateral ventricle

Occipital lobe

Lateral geniculate nucleus Optic radiation

Primary visual cortex

Fig. 2.11 Retrochiasmal optic pathways: axial diffusion tensor fractional anisotropy maps demonstrating the optic pathways from lateral geniculate nucleus to the primary visual cortex. Images provided by Dr. L Mancini, Department of Neuroimaging Physics, National Hospital for Neurology and Neurosurgery, UK.

The superior oblique muscle is the longest and thinnest muscle and arises from the body of the sphenoid. Its tendon loops around the trochlea (L. pulley) on the superomedial orbital wall to insert into the posterosuperior sclera. It may not be distinguishable from medial rectus on axial imaging. Calcification of the trochlea may be a normal finding on CT. The inferior oblique muscle is relatively short and thick. It arises from the medial orbital floor and inserts into the inferolateral sclera. Levator palpebrae superioris arises from the lesser wing of the sphenoid and penetrates the orbital septum to insert into the superior tarsal plate.

The lacrimal apparatus (Fig.2.6) The lacrimal gland and nasolacrimal duct are well demonstrated on cross-sectional imaging, but dacrocystography is required to optimally delineate the lacrimal canaliculi, lacrimal

42

sac and nasolacrimal duct pathway. A number of valves are described along the lacrimal pathway, but these are of little functional importance. • The lacrimal gland lies in the lacrimal fossa on the lateral orbital roof and is divided into orbital and palpebral lobes by the orbital septum. It drains via multiple ducts into the superior fornix. • The lacrimal sac lies in the lacrimal groove between the maxilla and lacrimal bone. It is filled via the lacrimal canaliculi. • The nasolacrimal duct opens into the anterior part of the inferior meatus of the nasal cavity.

The optic nerve The optic nerve is an evagination of cerebral white matter and is therefore surrounded by all of the normal meningeal layers. The ‘optic nerve-sheath complex’ is formed by the optic nerve and the dural and leptomeningeal coverings. The dura blends

Chapter 2: The orbit and visual pathway

A

B

Parietal lobe

Corpus callosum (splenium) Parietooccipital sulcus Midbrain (tectum): superior and inferior colliculi

Occipital lobe

Midbrain (tegmentum) Calcarine fissure (anterior limb) Cerebellar vermis

Calcarine fissure (posterior limb)

Pons

Medulla oblongata

Fig. 2.12 Visual cortex. (A) Midline sagittal T1-weighted MR image. (B) Similar section 5 mm from the midline demonstrating the calcarine sulcus more clearly.

Parieto-occipital fissure

A

B

Interhemispheric fissure

Parietal lobe Falx cerebri Occipital lobe

Calcar avis

Occipital horn of lateral ventricle

Superior cerebellar vermis Calcarine fissure (anterior limb)

Cerebellar hemisphere Posterior temporal lobe

Fig. 2.13 Calcarine fissure (anterior limb) in the coronal plane. (A) T1-weighted MR image and (B) T2-weighted images through the occipital lobes. The deep fissure and the grey matter lining indents the occipital horns producing the calcar avis.

with the sclera anteriorly and is tightly adherent to the bone of the optic canal posteriorly. Intracranial pressure changes are transmitted to the optic nerve-sheath complex, resulting in papilloedema. The individual components of the complex are not separated on CT (Fig.2.2c), but on MRI the optic nerve, the dura and the CSF-containing subarachnoid space can be identified

separately, particularly with high-resolution T2-weighted and gadolinium-enhanced T1-weighted images (Fig. 2.5). Unenhanced T1-weighted images do not resolve the components of the normal optic nerve-sheath complex. The segments of the optic nerve are: • Intra-ocular: < 1 mm; not normally visible unless pathological (i.e. papilloedema)

43

Section 1: Central Nervous System

Parietal lobe

A

B

Parietooccipital fissure

Dural venous sinus confluence (Torcular Herophili)

Calcarine sulcus (location of primary visual cortex)

Head of caudate nucleus

Interhemispheric fissure

Cerebellar hemisphere

C

Thalamus

Fig. 2.14 Coronal (A) T1-weighted MR image and (B) T2-weighted images through the occipital lobes (posterior to those in Fig. 2.13) showing the position of the calcarine fissure (posterior limb) and visual cortex. (C) Axial T1-weighted image in the plane of the calcarine fissure. Note the complex infolding of the cortex lining the calcarine fissure, as compared with the adjacent areas.

Corpus callosum (splenium) Temporal lobe Sylvian/ lateral fissure Occipital horn of the lateral ventricle Occipital lobe Visual cortex

• •

Intra-orbital: a laxity and a sinusoidal course are normal in the neutral position; CSF space is often widest immediately posterior to the optic disc Canalicular: CSF around the nerve is normally effaced in the intracanalicular segment Intracranial: closely related to the terminal ICA and the A1 segment of the anterior cerebral artery (located superior to the nerve).

Intracranial visual pathways

wall of the third ventricle, with the optic recess above and the infundibular recess below (Fig.2.7a).

The optic tracts (Figs. 2.9, 2.10) The optic tracts run posterolaterally between the crus cerebri and uncus (inferior to the anterior perforated substance). They merge with brain substance as they course posteriorly to the lateral geniculate nucleus (LGN), an elevated region of grey matter on the posterior aspect of the thalamus, lateral to the pulvinar. Fibres from the LGN and visual cortex project to the superior colliculi, which are involved in the control of eye movements (Fig.2.7a).

The optic radiation (Fig.2.11) Two groups of fibres run to the primary visual cortex. • The inferior visual field fibres pass directly to the occipital cortex, lateral to the occipital horn of the lateral ventricle. These parallel, compact, myelinated fibres can be identified on axial T2-weighted MRI. • The superior visual field fibres sweep inferiorly around the temporal horn, forming Meyer’s loop. These fibres are not readily apparent on MRI.

The optic chiasm (Figs. 2.7, 2.8) The optic chiasm lies in the chiasmatic (or ‘suprasellar’) cistern, above the pituitary fossa. The pituitary stalk lies posterior to the chiasm. The position of the chiasm varies with respect to the sella from a more anterior (pre-fixed) location to a posterior (postfixed) position. The posterior part of the chiasm contributes to the anterior

44

The visual cortex (primary) (Figs. 2.12, 2.13, 2.14) The visual cortex is located along the superior and inferior margins of the calcarine fissure on the medial aspect of the occipital lobe. The inferior contralateral visual field lies on the superior aspect of the fissure, the superior contralateral visual field on its inferior aspect.

Chapter 2: The orbit and visual pathway

BB

AA 1

Optic n.

2

1

Left inferior visual field

2

Macula

Optic chiasm 3

Left superior visual field

3

Pituitary stalk

Optic tract 4 4 Lateral geniculate nucleus 5

5

6

1 Optic nerve: monocular visual loss 2 Optic chiasm: bitemporal hemianopia 3 Optic tract: homonymous hemianopia 4 Optic radiation, temporal lobe (Meyer’s loop): homonymous superior quadrantopia 5 Optic radiation, parietal lobe: homonymous inferior quadrantopia 6 Optic radiation, posterior fibres: homonymous hemianopia. 7 Calcarine cortex, occipital lobe: homonymous hemianopia with macular sparing; e.g. posterior cerebral artery occlusion (the macula is represented in the posterior visual cortex, with supply from middle cerebral artery branches)

6

7 7

Occipital cortex

7

Fig. 2.15 Lesions of the visual pathway (A) Diagram showing the effect on the visual field of lesions at various points along the visual pathway (B) Primary visual cortex along the right calcarine sulcus. A lesion superior to the calcarine sulcus in the right primary visual cortex results in a defect in the left inferior visual field. Note the over-representation of the macula at the posterior aspect of the sulcus (cortical magnification).

Supraorbital a. Intracanalicular segment of the ophthalmic a. Internal carotid a. (cavernous segment) 'Choroidal crescent' of eye Ciliary a. Terminal branches including frontal, dorsal nasal, palpebral Inferior muscular a.

Fig. 2.16 Selective injection of the internal carotid artery on lateral projection digital subtraction angiography in the arterial phase, centred on the ophthalmic artery.

The cortical representation of the central visual field and fovea is located around and lateral to the occipital pole. The two occupy a disproportionate extent of cortex compared to the peripheral field (‘cortical magnification’). Fig.2.15 shows the visual field defects due to interruption of the visual pathway at various points.

Vascular anatomy The orbit Arterial supply The ophthalmic artery (Figs. 2.5, 2.16) is the first angiographically visible branch of the intradural internal carotid artery. It runs through the optic canal in the dural sheath, inferolateral to the nerve at the orbital apex and then crosses (usually superiorly) to the medial aspect of the nerve. Its major branch, the central retinal artery, pierces the nerve inferomedially, 10 mm posterior to the globe, and runs centrally inside the nerve to the globe.

45

Section 1: Central Nervous System

Superficial temporal a.

Middle meningeal a.

Ethmoidal arterial branch Deep temporal a. Infraorbital a. Sphenopalatine a. Descending palatine a. Transverse facial a.

Internal maxillary a.

Venous drainage The superior ophthalmic vein (SOV) (Figs. 2.2e, 2.5c) is intraconal, coursing inferior to the superior rectus muscle. It provides venous drainage from the face via the angular and supraorbital veins. The SOV is routinely visualized on CT and MRI. Its diameter is variable (approximately 2 mm is usual) and minor asymmetry is not uncommon. The inferior ophthalmic vein (IOV) drains into the SOV or directly to the cavernous sinus. It communicates with the pterygoid venous plexus via the IOF and is not consistently demonstrated on cross-sectional imaging. The central retinal vein drains to the SOV, another orbital vein or directly to the cavernous sinus. There is no functionally significant collateralization within the bulb, hence glaucoma and haemorrhage may occur as a result of its occlusion.

The visual pathways Arterial supply

Fig. 2.17 Selective injection of the external carotid artery on lateral projection digital subtraction angiography in the arterial phase: artefact from eyelid motion shows the position of the anterior margin of the orbit.

Other branches include the long and short posterior ciliary, lacrimal, posterior and anterior ethmoidal, supraorbital and palpebral arteries. There are extensive anastomoses with the external carotid artery (ECA), notably the middle meningeal and internal maxillary branches, which can put the ophthalmic artery at risk during particulate embolization of lesions supplied by the ECA.

46

• • • • •

Optic chiasm: internal carotid A, anterior cerebral branches Optic tract: posterior communicating A and anterior choroidal A Lateral geniculate nucleus: anterior choroidal and posterior cerebral A Optic radiations: anterior choroidal, middle cerebral and posterior cerebral A Visual cortex: posterior cerebral A (with a variable contribution from the middle cerebral A)

Section 1 Chapter

3

Central Nervous System

The petrous temporal bone Tim Beale and Simon Morley

Imaging methods

Middle cranial fossa

High-resolution computerized tomography (HRCT) and magnetic resonance imaging (MRI) are used in a complementary fashion when assessing the anatomy and pathology of the petrous temporal bone.

Mandibular fossa EAC

External auditory canal (EAC) The S-shaped EAC extends from the external auditory meatus (EAM) to the tympanic membrane. The lateral one-third is cartilaginous and the medial two-thirds bony. The bony EAC is narrowed focally at the isthmus (Fig. 3.1). The meatus is oval in sagittal cross section and lined closely by skin that attaches directly to the periosteum. The anatomical relations of the EAC are (Fig. 3.2): • anteriorly: the mandibular fossa containing the mandibular condyle and temporomandibular joint • posteriorly: the mastoid process • inferiorly: the parotid gland and infratemporal fossa • superiorly: the middle cranial fossa and the temporal lobe. The nodal drainage from the EAC is to the intraparotid group.

Vidian canal

Foramen ovale Foramen spinosum Mandibular condyle

Mandibular condyle Mastoid air cells

Fig. 3.2 Sagittal HRCT. Relations of the EAC.

The tympanic membrane (TM) The conical tympanic membrane is set at an angle to the floor of the canal and separates the middle ear (mesotympanum) from the external ear (Figs. 3.1, 3.3). The handle (manubrium) and the lateral (short) process of the malleus are embedded in the TM. From the malleal prominence the anterior and posterior malleal folds divide the TM into a smaller, thinner pars flaccida above and a larger pars tensa below. The TM is usually visible in the coronal plane as a thin line on HRCT and is attached superiorly to the scutum (shield) (Figs. 3.3b and 3.6) and peripherally to a bony annulus.

The middle ear and mastoid The middle ear (ME) cavity is divided in the coronal plane into hypotympanum, mesotympanum or epitympanum (attic) along the superior and inferior margins of the EAC. The anatomy is complex but well shown by HRCT (Figs. 3.4–3.6).

Bony isthmus The inferior TM

Fig. 3.1 Axial HRCT. The external auditory canal.

Medial wall There are several important anatomical landmarks along the medial wall of the ME cavity. The cochlear promontory overlies the basal turn of the cochlea, with the oval window superiorand the round window posteroinferior to it. The cochleariform process is a depression in the anterior aspect of the medial wall marking the point where the tensor

Applied Radiological Anatomy, 2nd Edition ed. Paul Butler, Adam W.M. Mitchell and Jeremiah C. Healy. Published by Cambridge University Press. © Cambridge University Press 2011.

47

Section 1: Central Nervous System

A

Head of the malleus

B

Lateral malleal ligament The neck of the malleus

Proximal tympanic and labyrinthine segments of facial n. Note the “snakes eyes” appearance Head The tensor tympani muscle

Lateral process of malleus

Scutum

Handle of malleus

Lateral process Handle Calcified TM.

The boundaries of Prussak’s space

Fig. 3.3 Coronal HRCT. The malleus.

Lateral wall The lateral wall comprises mainly the scutum and superior to it a thin wall covering the tegmental air cells superior to the medial EAC.

Posterior wall The pyramidal eminence separates the sinus tympani medially from the facial recess laterally (Figs. 3.5b and 3.7). The stapedius muscle extends from the pyramidal eminence to attach to the neck of the stapes (Fig. 3.7). The aditus or passageway extends between the posterior wall of the attic and the antrum (Fig. 3.5e).

Roof and floor Fig. 3.4 Coronal HRCT. The compartments of the middle ear (ME) cleft; red, epitympanum (or attic); yellow, mesotympanum; and blue, hypotympanum.

tympani muscle turns laterally to attach to the neck of the malleus (Fig. 3.5d). More posteriorly the lateral semicircular canal (LSCC) protrudes into the epitympanum, with the tympanic segment of the facial nerve passing just inferior to it and lateral to the oval window (Fig. 3.6b). A

Horizontal intrapetrous ICA

Tensor tympanic m.

The tegmen is the thin bony plate covering the roof of the tympanic cavity separating it from the middle cranial fossa (Fig. 3.6). The floor consists of bone of variable thickness, which overlies the internal carotid canal anteriorly and the jugular bulb posteriorly.

Aberrant internal carotid artery (ICA) This is a collateral circulation secondary to failure of development of the first embryonic segment of the ICA. The collateral Neck of malleus

B Incudostapedial joint (ISJ) Pyramidal eminence Facial recess

Handle of malleus Basal turn of cochlea Long process of incus Round window Round window niche Sinus tympani

Fig. 3.5 Axial HRCT middle ear. a–e, inferior to superior.

48

Sinus tympani Posterior semicircular canal (PSCC) Descending (mastoid) segment facial n. Vestibular aqueduct

Chapter 3: The petrous temporal bone

C

Tensor tympanic muscle

D

Anterior crus of stapes

Anterior malleal ligament Processus cochleariformis

Posterior crus of stapes

Oval window and footplate

Mastoid antrum

E

Posterior semicircular canal (PSCC)

Neck of malleus

Vestibular aqueduct

Vestibular aqueduct

Anterior epitympanic recess Tympanic facial n.

Head of malleus Body of incus Middle turn of cochlea Short process of incus Vestibule Aditus ad antrum

Fig. 3.5 (cont.)

Incus The incus consists of a body and short, long and lenticular processes. The short process extends posteriorly within the fossa incudis just inferior to the aditus ad antrum (Fig. 3.5e). The long and lenticular processes meet at an angle of approximately 90°. This ‘hockey stick’ appearance is best demonstrated in the coronal plane (Figs. 3.6a, 3.8). The body and short process ‘ice cream cone’ is best seen on axial images (Fig. 3.5e) The cup-shaped lenticular process of the incus articulates with the head (capitulum) of the stapes at the incudostapedial joint (ISJ), which is again a synovial diarthrodial articulation (Figs.3.5b, 3.11).

Stapes route is via the inferior tympanic artery, which is markedly enlarged, anastomoses with the caroticotympanic branch of the ICA and connects with the horizontal intrapetrous ICA. It does this by coursing over the cochlear promontory and is seen as a retrotympanic vascular mass. This can be recognized on imaging (Fig. 3.8).

The auditory ossicles Malleus The malleus is made up of the head, neck, lateral (short) process, anterior process and handle (manubrium). The lateral process and handle of the malleus are embedded in the TM (Fig. 3.3). The malleus articulates with the larger body of the incus at the synovial diarthrodial malleoincudal joint within the attic and is best visualized on axial (Fig. 3.5e) or sagittal oblique images. On axial images the malleoincudal joint resembles an ice cream (head of malleus) sitting on a cone (body of incus) and on sagittal reformatted images a molar tooth (Fig. 3.9).

The stapes consists of the head (capitulum) anterior and posterior crura and the tympanic portion of the footplate (Figure3.5c,d). These components together are described as the stapes superstructure. The space between the stapes crura is called the obturator foramen.

Ossicular ligaments The superior, lateral and anterior malleal ligaments, supporting the malleus, can be seen on HRCT (Fig. 3.5d). The others cannot.

Prussak’s space Prussak’s space is the most common site of origin for acquired cholesteatoma and is located between the lateral mallear ligament superiorly, the lateral (short) process of the malleus inferiorly, the pars flaccida of the TM laterally and the neck of the malleus medially (Fig. 3.3a).

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Section 1: Central Nervous System

A

Arcuate eminence over superior SCC Tegmen tympani Vestibule Lenticular process of incus Basal turn of cochlea / cochlea and overlying cochlear promontory Long process of incus Scutum

Superior SCC

B Tegmen tympani Lateral SCC Round window Tympanic facial n. canal Scutum Tympanic membrane

Fig. 3.6 Coronal HRCT middle ear (A is anterior to B).Tegmen (red), lateral SCC (green), superior SCC (blue) and Scutum (yellow) coloured.

Neck of malleus

Long process of incus

The inferior extension of the antrum is called the central mastoid tract and the more peripheral air cells the peripheral mastoid area. Asymmetrical pneumatization of the petrous apices is a normal anatomical variation (Fig. 3.12). On MRI the T1 high signal from fatty marrow on the non-pneumatized side can be mistaken for pathology.

Koerner’s septum Facial recess

Sinus tympani

This is a bony septum of variable thickness and is part of the petrosquamosal suture. It is a surgical landmark and passes through the antrum, where it can be confused with the medial wall of the antrum at surgery (Fig. 3.13).

The inner ear Posterior SCC

Fig. 3.7 Axial HRCT middle ear: tensor tympani (red) and stapedius (turquoise) muscles and pyramidal eminence (yellow) coloured.

This consists of the bony labyrinth demonstrated by HRCT surrounding the membranous (endolymphatic) labyrinth, shown on MRI. The perilymphatic fluid is interposed between the two.

The cochlea The mastoid antrum and petromastoid aircells There is great variability in the degree of temporal bone pneumatization, which can be very extensive, but which is usually symmetrical. The mastoid antrum communicates with the epitympanum via the aditus. A

Horizontal intrapetrous ICA

The cochlea is located anteriorly and converts mechanical energy from movement of the stapes footplate into electrical energy. It consists of 2½ to 2¾ turns with its base at the lateral end of the internal auditory canal (IAC). The modiolus is the central axis from which a thin bony plate (the spiral lamina) projects and attaches to the outer cochlea wall. In addition there is a thicker plate of bone (the interscalar septum) that separates the individual turns of the B Crista falciformis Long process of incus

Basal turn of cochlea

Lenticular process of incus

Round window niche

Aberrant component of ICA extending over cochlear promontory

Fig. 3.8 HRCT middle ear. (A) Axial and (B) coronal.

50

Aberrant component of ICA extending over cochlear promontory

Chapter 3: The petrous temporal bone

Body of incus

Handle malleus

Long process incus Vestibule

Glaserian fissure Handle malleus

Singular canal

Inferior (mastoid segment) facial n. canal Course of chorda tymapni nerve

Fig. 3.9 Sagittal HRCT ‘molar tooth’ sign. Fig. 3.10 Axial HRCT magnified. The ossicles.

Pneumatized

Lenticular process of incus ISJ

Stapes

Non-pneumatized

Fig. 3.12 Axial HRCT. Asymmetrical petrous apex pneumatization. Fig. 3.11 Axial HRCT magnified. The ossicles: note the alignment of the stapes and lenticular process of incus in a normal ISJ.

sequences (Fig. 3.15).The cochlear duct, however, because of its small size cannot yet be clearly differentiated as a separate entity. Geniculate ganglion

The cochlear aqueduct

Labyrinthine segment facial n.

This is a narrow channel containing perilymph that connects the scala tympani of the basal turn with the subarachnoid space. It is parallel and inferior to the IAC and best identified from its lateral aspect in the round window region.

Koerner’s septum

The vestibule

Middle turn cochlea

Fig. 3.13 Axial HRCT. Koerner’s septum.

cochlea and also projects out from the modiolus. The modiolus, spiral lamina and interscalar septum are all clearly visible on both HRCT and MRI (Fig. 3.14). The cochlear nerve passes through the central core of the modiolus. The scala tympani and vestibuli are the perilymph channels that parallel and surround the endolymph (cochlear duct). As the perilymph of the scala tympani and vestibuli closely resembles CSF, both scala are clearly visualized on T2-weighted MR

The vestibule (Figs. 3.5e, 3.6, 3.16) contains the saccule (anterior) and the utricle (posterior) which cannot yet be clearly differentiated on routine imaging. There are five openings in the posterior vestibule (utricle) for the three semicircular canals (the posterior and superior semicircular canals have a common crus).

The vestibular aqueduct The vestibular aqueduct arises from the posterosuperior vestibule and is directed posteriorly and inferiorly. It is seen on axial images but multiplanar reformatted images in particular in the parasagittal plane are sometimes required to assess the whole course (Fig. 3.5c).

51

Section 1: Central Nervous System

Anterior malleal ligament

A

B

Interscalar septum

Tensor tympani Spiral lamina Central channel of modiolus

Scala vestibuli middle turn

Facial recess Pyramidal eminence

Central channel of modiolus

Singular canal Scala tympani middle turn

Sinus tympani

Fig. 3.14 Axial HRCT (A is inferior to B). Scala vestibuli middle turn Interscalar septum m

Scala tympani middle turn Spiral lamina Cochlear n. Inferior vestibular n.

Fig. 3.15 Axial T2 MRI. Cochlea. m = modiolus.

petrous bone that can be seen also on skull radiographs (Fig. 3.6a). Passing inferior to the SSCC is the petromastoid canal, which contains the subarcuate artery. It is of variable size, extends from the posterior fossa to the mastoid antrum and can be mistaken for a fracture (Fig. 3.16a). The bony covering of the lateral semicircular canal (LSCC) is a prominent landmark on the medial wall of the middle ear cleft (Fig. 3.6b). Commonly the nerve of the PSCC passes initially in aseparate canal, the singular canal, which is clearly visible onHRCT prior to joining the inferior vestibular nerve in the IAC (Fig. 3.14a).

The internal auditory canal

The semicircular canals There are three semicircular canals, the lateral, superior and posterior. Each semicircular canal describes two-thirds of a circle and is orthogonal with the others. The superior semicircular canal (SSCC) is perpendicular to and the posterior semicircular canal (PSCC) parallel to the long axis of the petrous bone (wedge) (Fig. 3.16). The bony covering of the superior semicircular canal forms the arcuate eminence, a ridge on the superior surface of the A

There is individual variation in size and shape of the normal internal auditory canal (IAC) but the right and left IACs are symmetrical. The IAC: • is cylindrical in shape and lies in a horizontal plane; it can therefore be clearly seen in both the axial and coronal planes,(Figs. 3.8b, 3.16c) • is lined with dura and contains cerebrospinal fluid • transmits the facial (VIIth cranial) and vestibulocochlear nerves (Fig. 3.17). B

Petromastoid canal

Superior SCC

Common crus Posterior SCC

Fig. 3.16 Axial HRCT. Semicircular canals (SCCs) and vestibule (A–D, superior to inferior): superior SCC (turquoise), posterior SCC (red), lateral SCC (green), common crus (yellow) and vestibule (purple) coloured.

52

Chapter 3: The petrous temporal bone

C

D

Geniculate ganglion Vestibule IAC Vestibule Lateral SCC Common crus

Posterior SCC

Posterior SCC

Fig. 3.16 (cont.) Scala vestibuli middle turn

A

B

Scala vestibuli middle turn

Scala tympani middle turn Scala tympani middle turn

Cochlear n.

Inferior vestibular n. Facial n. Vestibule Superior vestibular n. Posterior SCC Crus communis

Facial n.

C

D Superior vestibular n. Facial n.

C

Vestibulocochlear n.

C

Inferior vestibular n. Cochlear n. Basal turn of cochlea

Fig. 3.17 Axial (A,B) and sagittal oblique (C,D) MRI through IAC. lmage C through porus of IAC with facial n. (red) and vestibulocochlear n. (turquoise). Image D through fundus of IAC with superior vestibular n. (purple), inferior vestibular n. (pink) and cochlear n. (yellow).

At the porus acousticus, the medial opening of the IAC, there are usually only two distinct nerves visible (Fig. 3.17c), the larger and more posterior vestibulocochlear (VIIIth cranial) nerve and the smaller, more anterior facial nerve.

At the fundus, the lateral portion of the IAC, the vestibulocochlear (VIIIth cranial) nerve has divided into a separate cochlear nerve anterorinferiorly seen entering the modiolus and the superior and inferior vestibular nerves in the

53

Section 1: Central Nervous System

posterosuperior and posteroinferior quadrants, respectively (Fig. 3.17d). The fundus is divided into halves by the horizontal crista falciformis, a bony bar of variable size that is clearly seen on HRCT (Fig. 3.8b). There is also a vertical crest of bone at the fundus called Bill’s bar, which further divides the fundus into quadrants; however, this cannot yet be clearly identified on imaging.

Proximal basal turn of cochlea

Cerebellar arterial loop

Facial n.

Vestibulocochlear n.

The facial (VIIth cranial) nerve The facial (VIIth cranial) nerve consists of a larger motor and smaller sensory root.

Inferior crus of posterior SCC

Intracranial course

Flocculus of cerebellum

High-resolution axial T2-weighted MRI demonstrates the nerve as it exits the brainstem at the pontomedullary junction, courses through the cisternal space anterior to the larger VIIIth nerve (Fig. 3.18) and enters the anterosuperior quadrant of the IAC (Fig. 3.17d).

Intratemporal course MRI and HRCT are complementary when assessing the intratemporal course of the facial nerve, which is divided into labyrinthine, tympanic and mastoid segments. A

Greater superficial petrosal n.

Fig. 3.18 Axial T2 MRI.

The first (labyrinthine) segment of the facial nerve is the narrowest and shortest segment and extends anterolaterally from the IAC superior to the cochlea, terminating at the geniculate ganglion (Fig. 3.19a). The most proximal branch of the facial nerve is the greater superficial petrosal nerve, which extends anteriorly from the geniculate ganglion in a small groove to supply secretomotor fibres to the lacrimal gland and Head of malleus

B

Body of incus Short process of incus

Geniculate ganglion

Tympanic facial n.

Labyrinthine facial n. Intracanalicular facial n. Aditus ad antrum Lateral SCC

C

Proximal tympanic facial n.

D

Labyrinthine facial n. Tensor tympani m.

Lateral SCC Tympanic facial n. Vestibule Oval window Scutum Basal turn of cochlea

Head of malleus Neck of malleus Lateral (short) process of malleus

Fig. 3.19 HRCT. The facial nerve. Axial (A is superior to B); coronal (C is anterior to D), (E) sagittal. Facial nerve: intracanalicular (yellow), labyrinthine (purple), geniculate ganglion (turquoise), tympanic segment (green). In panel E: C, mandibular condyle; E, external auditory canal.

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Chapter 3: The petrous temporal bone

E

Incus

Chorda tympani

C

Horizontal intrapetrous ICA

E Descending (mastoid) segment facial n.

Pars nervosa jugular foramen (turquoise) Jugular spine

Fig. 3.19 (cont.) C, mandibular condyle, E, external auditory canal.

receive taste fibres from the hard palate. The facial nerve then turns posteriorly at an acute angle at the geniculate ganglion or anterior genu to form the tympanic segment (Fig. 3.19b). The second or tympanic segment is further subdivided. The proximal tympanic segment is just superior to the tensor tympani tendon (Fig. 3.19c), the mid-portion immediately inferior to the LSCC (Fig. 3.19d) and the posterior segment inferior to the short process of the incus. The third or mastoid segment originates at a second, posterior genu where the facial nerve turns inferiorly at an obtuse angle to descend and exit the skull base at the stylomastoid foramen (Fig. 3.19e). The two main branches of the mastoid segment are the nerve to stapedius and the chorda tympani. The chorda tympani originates just above the stylomastoid foramen and extends anteriorly and superiorly in its own canal to enter the middle ear cleft lateral to the long process of the incus. It carries afferent taste fibres from the anterior two-thirds of the tongue and efferent secretomotor fibres to sublingual, submandibular and minor salivary glands (Figs. 3.9, 3.19e). The classic ‘snakes eyes’ appearance of the facial nerve is seen on coronal scans (Fig. 3.3b) when it passes through both the distal labyrinthine and proximal tympanic segments on the same image just posterior to the geniculate ganglion. Axial scans are best for assessing the labyrinthine segment on both HRCT and MRI. The tympanic segment can be seen on both axial and coronal sequences, although coronal CT best demonstrates the relationship of the nerve to the oval window and stapes and the presence of any dehiscence of the bony canal wall. Although the posterior genu can be seen in both the axial and coronal planes, sagittal oblique reformatted images are usually best for assessing this region. The normal facial nerve will enhance with gadolinium toaminor degree at both the geniculate ganglion and proximal tympanic segment due to a perineural capillary-venous plexus.

Pars vascularis jugular foramen (blue)

Fig. 3.20 Axial CT. Jugular foramen.

The intracanalicular, labyrinthine and mastoid segments never normally enhance. T1-weighted axial scans at the stylomastoid foramen clearly demonstrate the dark dot of the facial nerve surrounded by fat and it is a useful review area when assessing for possible pathology at this site such as possible involvement of the facial nerve by a deep lobe of parotid lesion.

The jugular foramen The jugular foramen (Fig. 3.20) connects the sigmoid sinus to the internal jugular vein and is divided into two parts, asmaller anterior pars nervosa, which despite its name contains only the glossopharyngeal (IXth cranial) nerve, and Jacobsen’s nerve (the tympanic branch of the IXth nerve), and drains the inferior petrosal sinus and a larger posterior pars vascularis that contains the vagus (Xth cranial), spinal accessory (XIth cranial) and the auricular branch of vagus (Xth cranial) nerve (Arnold’s nerve) as well as draining the jugular bulb and sigmoid sinus.

Jugular bulb The jugular bulb is the dilatation of the jugular vein within the superior aspect of the jugular foramen. In the coronal plane it is inferior to the vestibule in the posterior hypotympanumand on MRI is in the same plane as the odontoid peg. Thejugular foramen is commonly larger on the right side but when normal its cortical margins and the jugular spine are preserved. A high-riding jugular bulb is defined when the superior aspect of the bulb extends above the floor of the internal auditory canal (IAC).

55

Section 1 Chapter

4

Central Nervous System

The extracranial head and neck Tim Beale

The suprahyoid neck The suprahyoid neck extends from the skull base to the hyoid bone. It is divided into a number of spaces by the three layers of deep cervical fascia, which act as barriers to the spread of disease (Fig.4.1). This method also simplifies the differential diagnosis and has largely replaced the divisions based on various muscular triangles.

The three layers of deep cervical fascia are the superficial (investing) layer, the middle (visceral) layer and the deep (prevertebral) layer. The superficial layer invests all the deep structures apart from the platysma and superficial nodes and sends slips that envelop the sternocleidomastoid and trapezius muscles and the parotid and masticator spaces.

Pharyngeal mucosal space

Masticator space

Superficial (investing) layer (continuous line)

Parotid space

Middle (visceral) layer (interrupted line)

Carotid space Deep (prevertebral) layer (dotted line) Perivertebral space (both prevertebral and paraspinal)

Fig. 4.1 Axial T2 MRI. The suprahyoid spaces (coloured lines left half of figure) and deep cervical fascia (white lines right half of figure). Note the central location of the parapharyngeal space (white colour and star). The carotid space is made up of all three layers of deep cervical fascia. There are two layers of prevertebral fascia and one layer of visceral fascia anterior to the prevertebral muscle. These form the boundaries of three potential spaces, the prevertebral space, the ‘danger’ space and the retropharyngeal space (from posterior to anterior). These spaces are not normally visible and all extend inferiorly to approximately T3. The danger space extends further inferiorly and is a conduit for infection into the mediastinum. (Reference Hamsberger. H. Handbook of Head and Neck imaging (Masby)).

Applied Radiological Anatomy, 2nd Edition ed. Paul Butler, Adam W.M. Mitchell and Jeremiah C. Healy. Published by Cambridge University Press. © Cambridge University Press 2011.

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Chapter 4: The extracranial head and neck

The middle layer is more complex. It lies deep to the anterior strap muscles, surrounds the constrictor muscles, oesophagus and thyroid and contributes to the carotid sheath and the anterior aspect of the retropharyngeal space. The deep layer encircles the vertebrae, paraspinal and prevertebral muscles.

The parapharyngeal space (PPS) The PPS is located centrally within the suprahyoid neck and is an inverted pyramid extending from the skull base to its apex at the hyoid bone (Fig.4.2). Since it contains mainly fat it is easily identified on both CT and MR. Lesions arising in the parapharyngeal fat space are rare but the direction of its displacement by adjacent pathology yields important clues.

The PPS is sometimes divided further into prestyloid and retrostyloid components. In this chapter we refer to the retrostyloid PPS as the carotid space and the prestyloid PPS as the parapharyngeal space proper (Fig.4.3).

The salivary glands The paired parotid, submandibular and sublingual salivary glands are known as the major salivary glands. Minor salivary glands are also present throughout the whole upper aerodigestive tract and heterotopic salivary tissue has been recorded in the external auditory canal, middle ear, neck and even the mandible. Ultrasound is usually the first-line imaging modality and for salivary masses can be combined with fine-needle aspiration cytology (FNAC). MRI is often used in a complementary

Lateral pterygoid m. Parotid space

The parapharyngeal space Medial pterygoid m.

The masticator space

Masseter m.

Fig. 4.2 Coronal T2 MRI. Parapharyngeal space. Theparapharyngeal space (PPS) extends from the skull base to the hyoid and opens into the posterior submandibular space (not shown). There is no fascial barrier between the PPS and the posterior submandibular space. The masticator space (MS) and parotid space (PS) are also shown.

Prestyloid

Parapharyngeal space Carotid space

Retrostyloid

Fig. 4.3 Axial T2 MRI. The suprahyoid neck. The carotid space is equivalent to the retrostyloid parapharyngeal space and the parapharyngeal proper is equivalent to the prestyloid parapharyngeal space.

57

Section 1: Central Nervous System

fashion to assess the local extent of any lesion, in particular as to whether there is ‘deep lobe’ involvement or perineural extension. CT may be helpful in the assessment of multiple salivary calculi or acute salivary infection when there is clinical suspicion of an abscess. MR sialography is increasingly used instead of conventional sialography, with the latter reserved for when radiological intervention such as ductal dilatation (sialoplasty or stone extraction) is being considered.

The parotid gland The parotid is the largest of the major salivary glands. Anteriorly it is palpable superficial to the ramus of the mandible and the posterior aspect of the masseter muscle. The posterior relations are the mastoid process and sternocleidomastoid muscle. On its deep aspect it is separated from the carotid sheath by the posterior belly of digastric muscle styloid process and styloid muscles (Fig. 4.4). The parotid extends from the level of the external auditory canal inferiorly to the angle of the mandible. The gland is encapsulated by the superficial layer of cervical fascia (forming the parotid space) and is described as having superficial and deep ‘lobes’ separated by the facial nerve. More correctly the parotid is superficial when it is external to the mandible and retromandibular when it is deep to the mandible. The larger superficial component lies on the masseter, with the deeper component extending between the styloid process and posterior ramus of the mandible. Indeed a radiological sign of tumours arising from the deep parotid is widening of the stylomandibular ‘notch’. The facial nerve can be identified in the stylomastoid foramen as it exits the skull base on both CT and MRI because it is surrounded by a ‘cuff ’ of fatty tissue (Fig.4.5). Its extracranial course follows the lateral aspect of the posterior belly of digastric muscle before entering the posterior parotid, where it divides into five main branches (temporal, zygomatic, buccal, mandibular and cervical) that pass lateral to the retromandibular vein and exit the anterior aspect to supply the muscles of facial expression. The facial nerve can sometimes be visualized within the posterior parotid on MRI before it has divided.

Mandible (ramus) Inferior alveolar n. Medial pterygoid m. Styloid process and muscles

Masseter m.

ECA (External carotid a.) RMV (Retromandibular v.)

Posterior post belly digastric Mastoid process

Fig. 4.4 Axial T1 MRI. The relations of the parotid gland (outlined) to the surrounding structures. ECA = external carotid artery; RMV = retromandibular vein.

58

Internal carotid a.

Internal jugular v.

Fig. 4.5 Axial T2 MRI. At level of stylomastoid foramen. The facial nerve (arrow) shown surrounded by a cuff of fat is easily visible at the stylomastoid foramen. This is a review area when assessing patients with parotid malignancy and/or facial nerve palsy.

One method of mapping the course of the facial nerve is to mark a line between the lateral surface of the posterior belly of digastric muscle and the lateral surface of the ramus of the mandible (Fig.4.6). The parotid duct is around 7 cm long and emerges from the anterior parotid superficial to the masseter muscle, then turns 90° to pierce the buccinator muscle to open into the oral cavity at the level of the second upper molar (Figs. 4.6, 4.7). Accessory parotid tissue anterior and separate from the parotid is present in 20% of people and is found along the course of the duct up to the cheek and can be mistaken for a mass.

The submandibular gland The submandibular gland has superficial and deep parts that connect around the posterior border of the mylohyoid muscle (Fig. 4.8a,b). The palpable and larger superficial part located between the mandible and external surface of the mylohyoid muscle is the main component of the submandibular space. The smaller deeper part is superior to the mylohyoid muscle and palpable intraorally. The mandibular branch of the facial nerve is a superficial relation to the gland and therefore the surgical approach is usually made more inferiorly to avoid any damage. The tortuous facial artery passes from the posterior to the superior aspect of the gland, which it indents, then extends inferiorly onto the deep and inferior aspects of the mandible before passing superiorly on the lateral surface, where it is palpable, and associated with a facial node, to supply the face. The main duct emerges anteriorly from the gland where it angles sharply around the posterior border of the mylohyoid muscle and it is at this site where approximately a third of calculi are found. It then passes anteriorly between the mylohyoid and hyoglossus muscles along with the lingual vein and nerve and the hypoglossal nerve before opening at the sublingual papilla in the anterior floor of mouth (Figs.4.8, 4.9). The submandibular gland is separated from the anterior parotid only by the stylomandibular ligament. Lesions arising from the tail of parotid lesions can therefore mistakenly be thought to arise from the submandibular gland.

Chapter 4: The extracranial head and neck

Zygomaticus major m.

Facial a. Parotid duct

Buccinator m.

Masseter m.

Medial pterygoid m.

Mandible (ramus)

Posterior belly digastric m.

Fig. 4.6 Axial T1 MRI. Left parotid gland and cheek. The white line between the lateral surface of the posterior belly of digastric muscle and lateral surface of the ramus of the mandible is a method to identify the course of the intraparotid facial nerve. The nerve divides the superficial and deep lobes of the parotid gland.

Bucccinator m.

Facial a.

Parotid duct

Masseter m. Medial pterygoid m.

Fig. 4.7 Axial T2 MRI. The relations of the parotid duct (white line). Note the duct pierces the buccinator muscle to open into the cheek at the level of the upper second molar.

59

Section 1: Central Nervous System

A

Mylohyoid m. Duct Geniohyoid m.

Hyoglossus m. Submandibular salivary gland

B

Mylohyoid m. Posterior belly digastric m. Facial a. Platysma Hyoid bone

Fig. 4.8 Axial (A) and coronal (B) T2 MRI. The course and relations of the submandibular duct and submandibular salivary gland (SMG).

60

The sublingual gland

The pharynx

The almond-shaped sublingual gland is the smallest of the major salivary glands and lies deep to the anterior floor of mouth mucosa, superior to the mylohoid muscle. It lies on the sublingual groove of the mandible, with the distal submandibular duct separating it from the more medial genioglossus muscle (Fig.4.10a,b).

The pharynx is a midline fibromuscular tube extending from the skull base to the lower border of the cricoid cartilage, where it is continuous with the cervical oesophagus. It is divided into the nasopharynx, oropharynx and hypopharynx (Fig.4.11). The muscular layer is made up of the three constrictors and the palato- and stylopharyngeus muscles.

Chapter 4: The extracranial head and neck

Nasopharynx

Fig. 4.9 Right submandibular sialogram. Note mild intraglandular sialectasis.

The nasopharynx acts as a conduit for air and secretions from the nasal cavity. The anterior limit of the nasopharynx is the nasal choanae; the sloping roof is formed by the central skull base (body of sphenoid and clivus), which slopes down to become the posterior wall overlying the anterior arch of the atlas and the atlantoaxial joint and lateral to this the prevertebral muscles. The division between the nasopharynx and oropharynx is the hard and soft palate at approximately the C1/2 level. The lateral wall is composed of the cartilaginous Eustachian tube (torus tubarius), with the eustachian tube orifice seen just anterior and the lateral pharyngeal recess or fossa of Rosenmuller just posterior to it. Two muscles that affect the function of the Eustachian tube are the tensor and veli palatini muscles (Fig.4.12).

A Temporalis m.

Hard palate Mucosa and submucosa lining inferior surface of hard palate

Masseter m.

Maxillary alveolus

Hyoglossus m. Mandible (Body)

Genioglossus m.

Mylohyoid m. Geniohyoid m.

Platysma

Anterior belly digastric m.

B

Sublingual salivary gland Submandibular duct

Genioglossus m.

Mylohyoid m. Geniohyoid m. Anterior belly diagastric m. Platysma

Fig. 4.10 Coronal images through floor of mouth demonstrating the relation of the sublingual salivary gland to the submandibular duct and adjacent muscles. (A) is posterior to (B).

61

Section 1: Central Nervous System

Nasopharynx

Oropharynx

Hypopharynx Supraglottis (larynx)

Glottis (larynx) Subglottis (larynx)

Fig. 4.11 Sagittal CT. The divisions of the pharynx and larynx.

Tensor veli palatini m.

Eustachian tube orifice Torus tubarius (cartilage)

Levator veli palatini m.

Styloid process

Lateral pharyngeal recess (fossa of Rosenmuller)

Internal jugular v. (IJV)

Internal corotid a. (ICA)

Prevertebral m. (longus colli)

Facial nerve in stylomastoid foramen

Fig. 4.12 Axial T2 MRI. The nasopharynx.

The fossa of Rosenmuller is difficult to examine clinically but is the commonest site of origin of nasopharyngeal carcinomas. Nasopharyngeal carcinoma may extend intracranially by eroding the fibrocartilage covering the foramen lacerum and gain access to the intrapetrous internal carotid and cavernous sinus but more commonly tumour extends laterally through the pharyngobasilar fascia into the parapharyngeal and then masticator spaces and from there via the mandibular division

62

of the trigeminal (fifth cranial) nerve (V3) gains access to the middle cranial fossa via the foramen ovale (Fig.4.13).

Oropharynx The oropharynx extends from the inferior surface of the soft palate superiorly to the valleculae inferiorly and includes the tongue base (largely comprising the lingual tonsils), anterior

Chapter 4: The extracranial head and neck

Mandibular division trigeminal nerve (v3) exiting foramen ovale (white line) Tensor veli palatini m. Torus tubarius (cartilage)

Lateral pterygoid m.

Levator veli palatini m.

Medial pterygoid m.

Palatopharyngeus m. and constrictor

Fig. 4.13 Coronal T2 MRI. The nasopharynx.

and posterior tonsillar pillars, faucial tonsils, soft palate and part of the superior and middle constrictors deep to the oropharyngeal mucosa (Figs. 4.11, 4.14). The boundaries of the valleculae are medially the midline glosso-epiglottic fold, anteriorly the tongue base, laterally the pharyngo-epiglottic folds and posteriorly the pharyngeal surface of the suprahyoid epiglottis. The anterior and posterior tonsillar pillars are both mucosal folds overlying muscles (palatoglossus and palatopharyngeus, respectively) that merge superiorly into the soft palate. The anterior tonsil and anterior tonsillar pillar is the commonest site of oropharyngeal cancer, which may then spread to the glossotonsillar sulcus and tongue base anteriorly and the soft palate superiorly (Fig.4.15a,b).

Hypopharynx The hypopharynx extends from the oropharynx to the cervical oesophagus posterior to the larynx and includes mucosa, minor salivary glands and the inferior constrictor muscle. The latter consists of two parts, the more superior obliquely oriented thyropharyngeus and the horizontal more inferior cricopharyngeus; between the two is a potential weak area through which a pharyngeal pouch may form The hypopharynx consists of three subsites: laterally the paired pyriform sinuses, in the shape of upturned cones, with

the (pyriform) apex inferiorly at the level of the true cords, anteriorly the postcricoid region and posteriorly the posterior hypopharyngeal wall. The boundaries of the pyriform sinus are laterally the posterior thyroid lamina, posteriorly the lateral aspect of the posterior pharyngeal wall and anteromedially the aryepiglottic fold, which separates it from the larynx (Fig.4.16). On imaging, the pyriform sinuses are often collapsed but distend when the patient performs a valsalva manoeuvre.

Larynx The larynx is divided radiologically into the supraglottis, glottis and subglottis (Fig.4.11). The supraglottis extends from the tip of the epiglottis to just superior to the true cords. The subglottis extends from the inferior surface of the true cords to the inferior aspect of the cricoid cartilage. The larynx has a cartilaginous skeleton consisting of the cricoid, thyroid and arytenoid cartilages and the covering epiglottis. The cricoid ring supports the larynx and consists of a cricoid arch and a larger cricoid lamina that faces posteriorly. The thyroid cartilage protects the larynx and comprises two laminae that meet anteriorly and posterior extensions called

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Section 1: Central Nervous System

Faucial tonsil

Palatoglossus m. (anterior tonsillar pillar)

Medial pterygoid m.

Palatopharyngeus m. (posterior tonsillar pillar)

Constrictors

Prevertebral m. (longus colli and capitis)

Posterior belly digastric m.

Fig. 4.14 Axial T2 MRI. The oropharynx.

A

Levator veli palatini m. Lateral pterygoid m.

Tensor veli palatini m. Palatopharyngeus m. (proximal)

Medial pterygoid m.

Constrictors Palatoglossus m.

Masseter m.

B

Lateral pterygoid m.

Levator veli palatini m.

Medial pterygoid m. Masseter m.

Palatopharyngeus m.

Fig. 4.15 Coronal T2 MRI. The oropharynx. (A) At the level of the anterior tonsillar pillar (palatoglossus muscle with overlying mucosa). (B) At the level of the posterior tonsillar pillar (palatopharyngeus muscle with overlying mucosa).

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Chapter 4: The extracranial head and neck

Strap m. Thyroid lamina (light brown) Preepiglottic fat Paraglottic fat Aryepiglottic fold (yellow) Pyriform sinus (white) Constrictors (brown)

Fig. 4.16 Axial T2 MRI at the level of the supraglottic larynx and pyriform sinus.

inferior and superior cornua. The inferior cornua articulates with the lateral cricoid at the cricothyroid joint. The arytenoid cartilages are pyramidal in shape, sit on top of the cricoid lamina and consist of anterior vocal processes to which the posterior vocal cord is attached, a superior process and a larger posterolateral muscular process (Figs. 4.16, 4.17a,b). The epiglottis protects the larynx and is made of flexible fibrocartilage. On both sides aryepiglottic folds extend from the lateral surface of the epiglottis and attach posteriorly to the superior process of the arytenoid. The inferior margin of the aryepiglottic fold is free and the mucosal surface of the aryepiglottic folds forms the false cords. The laryngeal ventricle is between the inferior free margin of the false cords and the true cords. The laryngeal vestibule is the air space within the supraglottic larynx. The pre-epiglottic and paraglottic fat spaces are two readily identifiable, connected regions within the supraglottic larynx between the mucosa and cartlilage (Fig.4.16).

The retropharyngeal space The retropharyngeal space is a potential space between the middle and deep layers of cervical fascia, extending from the skull base to T4. It provides a potential route of spread of infection to the mediastinum. Retropharyngeal nodes are seen only in the suprahyoid neck and must be assessed in patients with naso- and oropharyngeal pathology.

The mandible and masticator space Each half of the mandible consists of a vertical ascending ramus and a horizontal body, the two joining posteriorly at the mandibular angle, with each body fusing anteriorly at the mental symphysis. The posterior ramus extends superiorly to become the condylar neck and head and the anterior ramus the coronoid process, the two separated by the sigmoid notch. The tooth-bearing area or mandibular alveolus opposesthe maxillary alveolus above. The mandibular foramen, whichcontains the inferior alveolar nerve (a branch of the mandibular

division of the trigeminal nerve) and the inferior alveolarvessels, opens onto the lingual surface of the ramus and exits anteriorly as the mental branch on the buccal surface at the mentalforamen. The muscles of mastication insert onto the mandible: • the lateral pterygoid onto the medial mandibular condyle • the medial pterygoid on the inner surface of the angle • the temporalis onto the coronoid process and ramus • the masseter onto the outer surface of the coronoid process ramus and angle (Figs. 4.7, 4.18). The mandible and surrounding muscles of mastication are invested by the superficial layer of deep cervical fascia to form the masticator space, which contains the mandibular division of the trigeminal nerve (Fig.4.1a,b). The commonest pathology arising from this space is dental infection. Squamous cell carcinoma arising from the mucosa overlying the mandible may extend proximally along the inferior alveolar nerve and from there via the more proximal mandibular division intracranially (Fig.4.13).

Infratemporal fossa The infratemporal fossa is the area between the pterygopalatine fossa medially and the zygomatic arch laterally and is part of the masticator space and is therefore also called the nasopharyngeal masticator space. It communicates superiorly with the temporal fossa between the zygomatic arch and lateral skull vault, which is thus also known as the suprazygomatic masticator space. Other muscles that are attached to the mandible are the mylohyoid muscle on the oblique mylohyoid line on the lingual surface of the body, the geniohyoid and genioglossus muscles on the genial tubercles on the inner surface of the mentum and the anterior belly of digastric muscles from fossae either side of the lingual surface of the mentum (Fig.4.10).

Teeth There are two sets of dentition, deciduous and permanent. The 20 deciduous teeth with the central incisors appear first at approximately 6 months, the remainder appearing up to 3 years of age. The deciduous teeth are made up of two incisors, one canine and two molars in each quadrant. The first deciduous teeth to be replaced are the incisors, followed by the molars and lastly the canines. The 32 permanent teeth are made up of two incisors, one canine, two premolars and three molars in each quadrant. Thefirst molar tooth is the first permanent tooth to erupt at about 6 years with the remainder appearing up to 21 years of age (Fig.4.19).

Temporomandibular joint (TMJ) The condyle of the mandible lies within the mandibular (or glenoid) fossa of the temporal bone. Both are covered by a layer of fibrous tissue and separated by a biconcave fibrous disc. The posterior wall of the glenoid fossa is also the anterior wall of the bony external auditory canal. The bony prominence anterior to the glenoid fossa is the articular eminence (or tubercle).

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Section 1: Central Nervous System

A Strap muscles (thyrohyoid (deep) and sternohyoid)

Vocal cords (vocalis and thyroarytenoid muscles)

Lamina of thyroid cartilage

Platysma Lamina of cricoid cartilage

Common carotid a. (CCA)

Post cricoid region of hypopharynx (cricopharyngeus m.)

Internal jugular v. (IJV) External jugular v. (EJV)

Vertebral a. Arytenoid cartilage

B

Vallecula

Arytenoid cartilage Lamina of thyroid cartilage

Lamina of cricoid cartilage

Fig. 4.17 The larynx. (A) Axial T1 MRI at the level of the vocal cords. (B) Coronal T2 MRI.

Temporalis m.

Ramus of mandible

Lateral pterygoid m. Medial pterygoid m.

Masseter m.

Fig. 4.18 Coronal T2 MRI. The masticator space and ramus of mandible.

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Chapter 4: The extracranial head and neck

Articular eminence Posterior band Bilaminar zone External auditory canal Molars

Mandible (condyle) (green star)

Premolars Central incisor Lateral incisor

Anterior band

Canine

Lateral pterygoid m.

Fig. 4.19 Orthopantomogram (OPG).

The thicker margins of the disc are called the anterior and posterior bands, with the posterior band located superior to the condyle and the thinner central portion between the condyle and articular eminence (Fig. 4.20). The disc divides the joint into two separate compartments and is attached medially and laterally to both the joint capsule and condyle. The posterior disc attaches to the condyle and temporal bone by retrodiscal soft tissue called the bilaminar zone that allows forward translational movement. Both rotation and translational movement occur in the TMJ. Rotational movement occurs between the condyle and inferior aspect of the disc whereas translational movement takes place between the glenoid fossa and superior surface of the disc. MR in the sagittal oblique plane performed in the mouth in closed and open positions and supplemented by a coronal sequence clearly shows the dynamic relationship of the disc to the condyle. CT is useful in trauma or when a biomodel joint replacement is being considered.

Fig. 4.20 Sagittal oblique ‘proton density’ MRI. The temporomandibular joint.

A

Superficial temporal a. Middle meningeal a. Loop of Maxillary a. In pterygopalatine fossa Maxillary a. Occipital a.

Main trunk of ECA Facial a. Superior thyroid a. Lingual a.

B

The cervical arteries and venous drainageofthe neck Arterial supply In the majority of individuals, the right common carotid artery (CCA) arises from the brachiocephalic artery just posterior to the right sternoclavicular joint, whereas the left CCA arises from the aortic arch. Both carotid arteries are encased within a dense sheath that is composed of all three layers of deep cervical fascia which extends to the skull base (Fig.4.1). This sheath of fascia forms the carotid space, which also contains the internal jugular vein (IJV), lateral to the CCA but posterolateral to the internal carotid artery (ICA) (Fig.4.5). The carotid space also contains the following cranial nerves, from the skull base to approximately the C1–2 level: the glossopharyngeal, vagus, accessory and hypoglossal nerves (IX–XII). Only the vagus (Xth cranial) nerve continues in the infrahyoid carotid space between the posterior aspect of both CCA and IJV and can be clearly seen on US (Fig.4.21). Asympathetic plexus and cervical nodes are also found within the carotid space.

Superficial temporal a.

Loop of STA over zygoma Occipital a.

Maxillary a.

Fig. 4.21 The external carotid artery and its major branches. (A) Lateral and (B)frontal projections.

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Section 1: Central Nervous System

Middle meatus Hiatus semilunaris Infundibulum (ethmoid) Ostium of antrum Osteomeatal unit

Direction of mucociliary flow

Fig. 4.22 Coronal CT. The ostiomeatal unit.

The bifurcation of the CCA into internal and external branches (ICA and ECA) occurs at the level of C4 (hyoid bone) and both can be used as a marker of the junction of upper and mid deep cervical nodal levels. The smaller ECA courses anteromedial to the ICA. The ECA supplies the face, scalp, dura and upper cervical organs (Fig.4.21a,b). There are many and variable anastomoses between the individual branches of the ECA, and the branches of the ECA and ICA and ECA and vertebral artery. The vertebral artery is the first branch of the subclavian and passes through the foramina transversarium of the upper six cervical vertebrae. Between the foramina of the atlas and axis it has a posterior convexity (which allows for rotational movement) before entering the skull via the foramen magnum.

Venous drainage The internal jugular vein (IJV) emerges from the jugular foramen posterior to the ICA and receives the inferior petrosal sinus just inferior to the skull base. It joins the subclavian vein to form the brachiocephalic at the level of the sternoclavicular joints. It receives a variable number of tributaries in its course. The internal jugular veins are commonly asymmetrical in size, usually right larger than left (as are therefore the jugular foramen). This asymmetry should not be mistaken as pathological. The anterior face drains via the facial veins which communicate via the ophthalmic veins with the cavernous sinus. Orbital infection can therefore lead to ophthalmic vein and then cavernous sinus thrombosis. The cavernous sinus drains externally via the pterygoid venous plexus into the retromandibular vein within the parotid, which is joined by the posterior auricular vein to form the external jugular vein, which in turn drains into the subclavian vein just above the clavicle. There are variable anterior jugular veins (usually one either side of the midline) that drain just above the sternum into subclavian or external jugular veins.

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The paranasal sinuses and nasal cavity The anatomy of this region is best assessed by CT acquired axially with multiplanar reformatted images (MFR) in the coronal and sagittal planes. CT is performed prior to functional endoscopic sinus surgery (FESS) whose aim is to restore the normal drainage pathways. The radiologist must therefore understand the anatomy of the mucociliary drainage pathways, notably the ostiomeatal unit and the sphenoethmoidal recess, and the clinically relevant anatomical variations commonly found in this region.

The ostiomeatal unit The ostiomeatal unit (OMU) drains the frontal, anterior ethmoidal and maxillary sinuses. The OMU is sited in the area of the superomedial maxillary sinus and middle meatus and includes the maxillary sinus ostium, ethmoid infundibulum, hiatus semilunaris, and frontal recess and is best demonstrated on coronal CT. The coordinated action of the cilia clears mucus towards the ostia (Fig.4.22).

The frontal sinus and frontal recess The frontal sinuses are asymmetrical extensions from the anterior ethmoidal air cells between the tables of the frontal bones. They are the last paranasal sinus to aerate and are not fully developed until just after puberty. Aplasia or lack of any extension into the frontal bone is present in between 5 and 8% of people and hypoplasia in 4%. The frontal sinus drainage pathway is via the frontal recess, which is the shape of an inverted funnel, measures approximately 13 mm long and is formed by the walls of the adjacent air cells, hence the term ‘recess’ rather than ‘duct’. The frontal recess has an oblique course 50° to the orbitomeatal plane and is therefore best demonstrated on the sagittal reconstructed images (Fig.4.23). The usual boundaries of the frontal recess are posteriorly the ethmoidal bulla, anteriorly and inferiorly the agger nasi air cell, medially the olfactory fossa and middle turbinate, laterally the lamina papyracea and superiorly the roof of the anterior ethmoidal air cells (fovea ethmoidalis) (Fig.4.24).

Chapter 4: The extracranial head and neck

Frontal sinus Ethmoidal bulla Frontal sinus ostium Frontal recess Agger nasi air cell

Middle turbinate Inferior turbinate

Fig. 4.23 Sagittal CT. The frontal recess.

Fovea ethmoidalis (interrupted white line)

The frontal recess anatomy, however, is complex due to the variable accessory air cells that may form part of its boundaries, and drains either into the ethmoid infundibulum or middle meatus depending on the superior attachment of the uncinate process. These frontal region accessory air cells are as follows. • The agger nasi air cell (ANC) is the most anterior of the ethmoidal air cells and may vary in size. If large it can displace the frontal recess posteriorly and narrow the ostium. • The frontoethmoidal air cells are variably classified depending on their number and extent. They are located superior to the ANC and extend into the frontal sinus (Fig.4.25). • The frontal bulla cell is an extension of the ethmoidal bulla into the frontal region. The suprabulla air cell is an air cell just superior and anterior to the ethmoidal bulla and the supraorbital air cell usually arises from the anterior ethmoidal air cell and extends into the orbital plate of the frontal bone. When assessing the frontal sinus and recess region the priority must be first to identify the frontal drainage pathway and to then clearly describe the site of origin, size and relationship of the adjacent air cells forming the frontal recess.

Vertical lamella Cribriform plate (white line)

The maxillary sinus

Frontal recess (dotted white line)

The maxillary sinus or antrum is the first aerated sinus to form and may be hypoplastic in up to 10% of people. The roof forms the orbital floor in which runs the infraorbital canal and the floor is formed by the maxillary alveolus. The medial wall also forms the lateral wall of the nasal cavity. The main ostium arises in the superior aspect of the medial wall and opens into the ethmoid (maxillary) infundibulum, which is a narrow channel between the uncinate process inferiorly and the lamina papyracea and ethmoidal bulla superiorly. The infundibulum opens into the hiatus semilunaris (Fig.4.22). The anatomical variants of the maxillary sinus are sinus septations, accessory sinus ostia and sinus hypoplasia. The maxillary sinus septum may be fibrous or bony and often extends from the infraorbital canal to the lateral wall. The accessory ostium is seen posterior to the natural ostium and is present in approximately 10% of the population (Fig.4.26). There may

Ethmoidal bulla Middle turbinate

Lamina papyracea

Fig. 4.24 Coronal CT through frontal recess and anterior cranial fossa (ACF).

Frontal sinus Accessory sinus ostia Frontal recess (opacified) Frontoethmoidal air cell Agger nasi air cell

Middle turbinate Inferior turbinate

Fig. 4.25 Sagittal CT. The frontal ethmoidal air cells.

Fig. 4.26 Coronal CT. Bilateral accessory sinus ostia.

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Section 1: Central Nervous System

Medial wall and floor of orbit Partially pneumatised uncinate process Infundibulum (ethmoid)

Secondary hypoplastic maxillary antra

Infraorbital air cell

Fig. 4.27 Coronal CT. Hypoplastic maxillary antra. Note the bilateral atelectatic uncinate processes (continuous white line) with secondary hypoplastic maxillary antra. Dotted white line = medial wall and floor of orbits.

Fig. 4.28 Coronal CT through outflow of left antrum.

Anterior clinoid process Posterior ethmoidal air cell (PE)

Sphenoid sinus Posterior ethmoidal air cell

Nasolacrinal duct

Sphenoethmoidal recess

Fig. 4.30 Sagittal CT. The sphenoethmoidal region.

Sphenoid sinus astium

Sphenoid sinus

Fig. 4.29 Axial CT. The relations of the sphenoethmoidal recess (coloured red).

be circular flow of mucus from the natural ostium inferiorly into the accessory ostium, leading to recurrent sinusitis. Maxillary sinus hypoplasia may be seen in association with an atelectatic uncinate process (Fig.4.27) or following surgery, in particular the Caldwell-Luc approach. Infraorbital (Haller) air cells are centred inferior to the ethmoidal bulla, extend along the orbital floor and may compromise the outflow (Fig.4.28).

The sphenoethmoidal region The sphenoid sinus develops in the body of the sphenoid and drains via a sinus ostium in the medial aspect of the anterior wall into the sphenoethmoidal recess (Fig.4.29). The adjacent posterior ethmoidal air cells drain via individual ostia into the superior meatus. The degree of pneumatization of the sphenoid is highly variable and may extend into

70

the greater and lesser wings of the sphenoid and pterygoid processes. This variable pneumatization needs to be carefully assessed prior to endoscopic transsphenoidal surgery. The posterior ethmoidal air cells may extend above the sphenoid sinus (sphenoethmoidal air cells), displacing the sinus inferiorly (Fig.4.30). The surgeon needs to be informed of this variable anatomy prior to endoscopic surgery. There are also a number of important structures, closely related to the sphenoid sinus, which may project into the sinus and which may have a dehiscent bony covering. These are the optic nerve, the maxillary nerve, the vidian canal and the intracavernous segment of the internal carotid artery (Fig.4.31).

The nasal cavity The nasal cavity extends from the palate to the skull base, is divided by the nasal septum and opens posteriorly via the choanae into the nasopharynx and anteriorly via the piriform aperture into the nares or nostrils. The nasal septum comprises the septal cartilage anteriorly and the perpendicular plate of the ethmoid and the vomer posteriorly (Fig.4.32). Nasal septal spurs and septal deviation are common.

Pterygopalatine fossa The pterygoplatatine fossa (PPF) is an elongated space, wider superiorly and located between the vertical plate of the palatine

Chapter 4: The extracranial head and neck

Optic n.

Inferior aspect of perpendicular plate of ethmoid

Maxillary division (V2) of trigeminal n.

Cartilaginous septum Vomer

Vidian canal

Fig. 4.32 Sagittal CT through midline of nasal cavity demonstrating the components of the nasal septum.

A The pterygopalatine fossa (PPF)

Fig. 4.31 Coronal CT. The sphenoid sinus. Note the important structures that are intimately associated with the sinus and at risk during endoscopic surgery.

bone (which is fused with the posterior wall of the antrum) anteriorly and the pterygoid process of the sphenoid posteriorly (Fig.4.33a,b). It is the ‘junction box’ of the deep neck as it communicates with multiple anatomical sites: • medially, the nasal cavity via the sphenopalatine foramen • posteriorly, the middle cranial fossa via the foramen rotundum and the foramen lacerum and facial nerve via thevidian canal • laterally, the infratemporal fossa (or nasopharyngeal masticator space) via the pterygo-maxillary fissure • superiorly, the orbit via the inferior orbital fissure • inferiorly, the palate via the greater and lesser palatine canals. The PPF is easily visualized on MR and CT as a largely fatfilled space. It also contains the sphenopalatine ganglion and transmits the maxillary nerve and internal maxillary (Fig.4.33b).

The oral cavity The oral cavity and the oropharynx are separated by a line of structures comprising, superiorly, the junction of soft palate and hard palate, laterally, the anterior tonsillar pillars and inferiorly the vallate papillae on the surface of the tongue. The tongue base is posterior to the vallate papillae and lies within the oropharynx. It consists largely of the lingual tonsils, which form the posterior third of the tongue. The contents of the oral cavity are the hard palate, maxillary and mandibular alveolar ridges, retromolar trigone, buccal mucosa, floor of mouth and anterior two-thirds of the tongue. The vestibule is the space between the cheek and lips externally and the teeth and gums internally and contains the superior and inferior gingival sulci. The hard palate forms the roof of the oral cavity, separating it from the nasal cavity, with the maxillary alveolus and teeth forming the anterior and lateral boundaries of the hard palate. The lateral wall of the oral cavity is the buccal mucosa (cheek)

Maxillary antrum Infratemporal fossa Nasal cavity Medial and lateral pterygoid plates

B

Inferior orbital fissure Pterygopalatine fossa (PPF)

Fig. 4.33 The pterygopalatine fossa. (A) Axial CT. The pterygopalatine fossa (PPF) outlined red. Note medial relations of nasal cavity and laterally the infratemporal fossa. (B) Sagittal T2 MR. PPF (white star). Note the maxillary antrum anteriorly and the orbital apex superiorly connected by the inferior orbital fissure.

and inferiorly lie the floor of the mouth (mylohyoid muscle), mandibular alveolus and teeth.

The tongue The tongue is a muscular organ made up of intrinsic transverse, vertical, inferior and superior fibres visible on ultrasound (Fig.4.34) and MR. It is supported by three paired extrinsic muscles. The largest of these is the genioglossus, which arises anteriorly from the genial tubercle on the lingual surface of the anterior mandible and fans out posteriorly into the tongue (Fig.4.35). The hyoglossus arises from the hyoid and extends superiorly and laterally to blend with the styloglossus muscle, which arises from the styloid process and stylohyoid ligament.

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Section 1: Central Nervous System

Dorsal surface of tongue

Intrinsic muscles (horizontal)

Sublingual salivary gland Neurovascular pedicle

Genioglossus m.

Lingual septum

Fig. 4.34 Coronal ultrasound through the tongue. Note the horizontal intrinsic muscle, the midline lingual septum and the paramedian genioglossus muscles.

Genioglossus m.

Vallecula

Geniohyoid m.

Mylohyoid m.

Platysma Preepiglottic fat

Epiglottis

Fig. 4.35 Sagittal T2 MRI. The tongue.

The hyoglossus muscle is a useful landmark that can be readily visualized on CT and MRI with the lingual artery and glossopharyngeal nerve on its medial aspect and the lingual vein, submandibular duct, lingual and hypoglossal nerves on its lateral aspect. A midline fibrous septum divides the tongue and is an important radiological landmark when staging oral cancer (Fig.4.36).

Sublingual space The sublingual space is lateral to the genioglossus muscles and separated from the submandibular space by the mylohyoid muscle. The sublingual space contains the following; anterior hyoglossus muscles, lingual, glossopharyngeal and hypoglossal nerves, lingual artery and vein, sublingual and deep portion of the submandibular salivary glands. The submandibular duct is within the oral tongue but inferior to the intrinsic tongue muscles.

Thyroid gland The thyroid gland consists of two lobes on either side of the trachea separated by an isthmus. It is invested by the mid-layer

72

of deep cervical fascia and due to its superficial location is ideally visualized with ultrasound (Fig. 4.37). A pyramidal lobe may extend superiorly, usually arising from the left side of the isthmus. The strap muscles lie superficial to the gland. During development the thyroid gland descends from the foramen caecum in the midline of the tongue base on the end of the thyroglossal duct. Rarely descent is incomplete, resulting in a lingual thyroid. More frequently a thyroglossal sinus or cyst may persist, the latter presenting as an anterior cervical swelling.

Parathyroid glands There are usually four but occasionally up to six parathyroid glands, which measure approximately 6 × 2 × 2 mm and are most frequently found in the tracheo-oesophageal groove posterior to the mid to inferior lobes of the thyroid. Their position may vary and they can be found within the thyroid gland itself or the mediastinum. The normal parathyroid gland is occasionally visible on ultrasound, which, in conjunction with sestamibi radionuclide imaging, is commonly used to identify overactive glands. The normal parathyroid glands are not seen routinely on MRI or CT.

Chapter 4: The extracranial head and neck

Temporalis m.

Bony hard palate

Masseter m.

Submucosa of hard palate

Maxillary alveolus

Hyoglossus m. Body of mandible

Genioglossus m.

Mylohyoid m. Geniohyoid m. Platysma Anterior belly digastric m.

Fig. 4.36 Coronal T2 MRI. The floor of the mouth.

Sternohyoid m. (SH) Sternothyroid m. Sternocleidomastoid m.

The thyroid gland Trachea

Common carotid a.

Oesophagus (cervical)

Internal jugular v.

Fig. 4.37 Panoramic axial ultrasound. The thyroid gland (continuous white line) and the trachea (interrupted white line).

The cervical lymphatic system There are approximately 300 lymph nodes within the neck(of the 800 in total throughout the body). A palpable lymphnode is a frequent presentation of head and neck malignancy. The location of the node(s) can sometimes point to the likely site of malignancy and their involvement influences prognosisadversely. The cervical nodes are commonly classified clinically into seven levels, superior to inferior (Fig.4.38). Levels 1a and b are submental and submandibular, respectively, and drain the lips, anterior floor of mouth and anterior tongue. Levels 2 to 4 are synonymous with the upper, mid and lower deep cervical chains and follow the internal jugular vein deep to sternocleidomastoid muscle. The most important node of this chain is the jugulodigastric node in the upper deep cervical region as it is the most frequently involved node in

squamous cell carcinomas of the tonsil, lateral tongue base and supraglottic larynx. Level 5 nodes are those within the posterior triangle, located between the anterior border of the trapezius and posterior border of the sternocleidomastoid muscles. These nodes are also known as the spinal accessory and transverse cervical chains. Level 5 is divided into 5a (above) and 5b (below) relative to the course of the accessory nerve. The most common origin of a malignant 5a node is the naso-pharynx. Level 6 comprises the anterior cervical nodes, which include the pre- and paratracheal and the less frequently involved prelaryngeal nodes. When a prelaryngeal node is involved in a patient with laryngeal squamous cell carcinoma, subglottic involvement should be sought. Level 7, the superior mediastinal nodes, should be assessed in cervical oesophageal, thyroid and subglottic laryngeal cancers.

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Section 1: Central Nervous System

IIa

IIb

Ib Ia III Va VI Vb

IV

Fig. 4.38 Illustration of right lateral neck with right submandibular salivary gland and sternocleidomastoid muscle (SCM) removed and internal jugular vein (IJV) tied off. Level Ia: submental. Between anterior bellies of digastric muscles. Level Ib: submandibular. Lateral to anterior belly of digastric muscle around submandibular salivary glands and superior to hyoid bone. Levels II to IV are the deep cervical chain, deep to the SCM muscle surrounding the carotid sheath. Only level II is split into levels IIa and IIb. Level IIa: anterior upper deep cervical including jugulo-digastric node. Anterior, medial and if touching posterior to right IJV above level of hyoid (another land mark is approximately level of carotid bifurcation). Level IIb: posterior upper deep cervical. Posterior to IJV with fat plane between node and IJV and deep to SCM muscle. Level III: mid deep cervical. From hyoid to inferior margin of cricoid cartilage. Level IV: lower deep cervical. From inferior cricoid cartilage to clavicle. Level V: posterior triangle. Between anterior border of trapezius and posterior border of SCM muscles. Separated into upper (Va) from skull base to lower border of cricoid and lower (Vb) lower border cricoid to clavicle. Level VI: anterior neck. From hyoid to manubrium including pre- and paratracheal and prelaryngeal (Delphian) nodes. Level VII: superior mediastinum. Superior aspect of manubrium to inominate vein between carotid arteries (not highlighted on this figure). Note that there are other head and neck nodal groups not includedin this classification, including intra- and periparotid, facialand retropharyngeal nodes.

The retropharyngeal and parotid nodes are not included in the above classification. The retropharyngeal node should be assessed in any patient with nasopharyngeal or oropharyngeal malignancy. The parotid nodes can be around or actually within the gland and drain the adjacent scalp, external auditory canal and pinna and nasopharynx.

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The lymph drains ultimately into the thoracic duct on the left, which is frequently visible on CT, and on the right, either directly or via a lymphatic duct, into the junction of subclavian and internal jugular veins.

Section 1 Chapter

5

Central Nervous System

The vertebral column and spinal cord Asif Saifuddin

Radiographic anatomy Introduction Radiography remains an important investigation for the assessment of spinal anatomy, with all areas adequately assessed by a combination of anteroposterior (AP) and lateral views. These can be supplemented by: • AP open mouth view of the odontoid peg and atlanto-axial articulation (Fig.5.1) • AP view of the lumbosacral junction with ~25° cranial angulation (Ferguson view) demonstrating the L5/S1 disc space tangentially and the L5 pars en face (Fig.5.2). A major advantage of radiography is that it can be obtained in the erect position, allowing accurate assessment of spinal alignment and overall spinal balance in the coronal and sagittal planes. A major limitation is the inability to assess the soft tissues of the spinal column, which include the intervertebral discs, spinal ligaments, spinal cord and paraspinal musculature: • these require the additional techniques of CT and MRI.

Odontoid process (dens) Right lateral mass of C1 Left atlantoaxial joint Right lateral mass of C2

Fig. 5.1 AP open mouth radiograph of the atlanto-axial joint.

Left L5 pars interarticularis L5/S1 intervertebral disc

Left S1 ventral root foramen Left sacroiliac joint

Fig. 5.2 Ferguson view of the lumbosacral junction.

Therefore, the radiographic anatomy of the spinal column is essentially limited to assessment of the vertebrae, the joints and spinal alignment.

The vertebral column The vertebral column commences at the craniocervical junction(C0–C1 articulation) and terminates at the tip of the coccyx. It comprises seven cervical, 12 thoracic, five lumbar, five sacral and three to five coccygeal vertebrae (Fig.5.3): • variation in the numbering of the last lumbar vertebra occurs with lumbosacral transitional junctions, occurring in ~16% of the population and also being termed ‘lumbarization’ or ‘sacralization’ • the lumbosacral transitional vertebra (LSTV) has unilateral or bilateral enlarged transverse processes, which attach to the superior aspect of the sacrum by either a pseudarthrosis or a complete bony ankylosis (Fig.5.4). Spinal alignment is assessed in the coronal and sagittal planes: • in the coronal plane, the C7 spinous process should lie vertically above the mid-sacral line • in the sagittal plane, the C2 body should lie vertically above L4 and the hips. Four curvatures are seen in the sagittal plane in adults (Fig.5.3): • cervical lordosis; from C1 to T2, ranging from 30 to 40°

Applied Radiological Anatomy, 2nd Edition ed. Paul Butler, Adam W.M. Mitchell and Jeremiah C. Healy. Published by Cambridge University Press. © Cambridge University Press 2011.

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Section 1: Central Nervous System

Enlarged right transverse process (TP)

Transitional lumbar vertebra (LSTV)

Cervical spine

Ankylosis between TP and sacrum Left sacral ala

Thoracic spine

Fig. 5.4 AP radiograph showing lumbosacral transition.

The atlanto-axial articulation (C1–C2) comprises four synovial joints between the atlas (C1) and the axis (C2): • the atlas is a bony ring arising from three primary ossification centres, the anterior arch and two neural arches ·

the neural arches fuse by age 3 years to form the posterior arch and fuse with the anterior arch by age 7years – failure of fusion of the anterior and/or posterior arches may result in congenital defects which can mimic fractures

Lumbar spine

·

the atlas thus comprises an anterior arch, which fuses at the anterior tubercle, the posterior arch and two lateral masses (Figs.5.1, 5.5)

the axis is formed from four primary ossification centres, one for each neural arch, one for the body and one for the odontoid process (dens)

Sacrum

Fig. 5.3 Sagittal T1W SE MRI showing the spinal curvatures.

• • •

Anterior atlanto-dens interval

thoracic kyphosis; from T2 to T12, ranging from 20 to 40° lumbar lordosis; from L1 to L5, ranging from 20 to 40° sacrococcygeal kyphosis (pelvic curvature); from the lumbosacral junction to the tip of the coccyx.

Anterior tubercle of C1 Odontoid process (dens) Neural arch of C1

The cranio-cervical junction The cranio-cervical junction is composed of the occiput (C0), the atlas (C1) and the axis (C2), forming a bony canal that protects the cervicomedullary junction of the spinal cord. The occipito-atlantal (C0–C1) articulation comprises two synovial joints formed between the occipital condyles and the lateral masses of the atlas: • it requires either CT or MRI for optimal evaluation.

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Body of C2

Spinous process of C2 Pedicle of C2

Fig. 5.5 Coned lateral radiograph of the cranio-cervical junction.

Chapter 5: The vertebral column and spinal cord

A

B C3/C4 intervertebral disc C4 vertebral body

C3 vertebral body

C4 spinolaminar line C5 lamina

Left C5/C6 neurocentral joint (of Luschka) Right C6 uncinate process

C4/C5 intervertebral disc C6 spinous process C6 pedicle

C6 spinous process

Left C7 pedicle

Fig. 5.6 (A) Coned AP radiograph of the lower cervical spine. (B) Coned lateral radiograph of the lower cervical spine.

·

·

·

a secondary ossification centre forms at the tip of the dens, fusing by age 12 years, with failure of fusion resulting in an os odontoideum the odontoid fuses to the C2 body by age 3–6 years and the neural arches fuse posteriorly by 2–3 years andwith the body by 3–6 years the axis thus comprises a body, a superiorly protruding dens, two lateral masses and two pedicles which fuse to form a large, commonly bifid spinous process (Figs.5.1,5.5)

the 4 joints of the C1–C2 articulation are: · ·

· ·

two lateral joints between the lateral masses of C1 and C2 (Fig.5.1) two median joints, one between the dens and anterior arch of the atlas (Fig.5.5) and one between the dens and the transverse atlantal ligament, which is not appreciated radiographically the normal anterior atlanto-dens interval is

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