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is helpful to compare the two sides. The lateral view gives a relatively
clear view of the vault and pituitary fossa. One will also be in¬‚uenced (b)
Sphenoid sinus
by the external clinical ¬ndings as to where an abnormality might be Meckel™s cave
discovered. For practical purposes, the usual reason for requesting
Internal carotid artery
skull radiographs is to identify a fracture. There are normal vault Basilar artery
“lucencies,” which need to be considered, mainly due to blood vessel
Internal auditory canal
impressions, especially veins. A fracture will usually have a more
distinct margin and, unlike blood vessels, does not often branch. Middle cerebral peduncle
Inferior cerebellar
Normal calci¬cations may be encountered on skull radiographs peduncle
Fourth ventricle
arising in the pineal gland, choroid plexus, dura, and habenular com-
missure (Figs. 7.4, 7.17). (c)
Pyramid

The brainstem (Fig. 7.7)
Olive
The brainstem consists of medulla, pons, and midbrain. The medulla,
pons, and cerebellum together constitute the hindbrain.
The medulla commences at the foramen magnum as a continuation Fourth ventricle
of the spinal cord. Initially it is “closed,” possessing a central canal
like the spinal cord. More superiorly, it becomes “open” as the central Cerebellar
hemisphere
canal leads into the fourth ventricle. In the brainstem, the motor
tracts are generally anterior to the sensory, hence the clinical usage of
“anterior” columns meaning motor and “posterior” column, sensory.
A number of decussations occur within the brainstem where both
(d)
motor and sensory ¬bers cross the midline in accordance with the
general principle that functional control of one-half of the body is Trigeminal
nerve
largely exercised by the contralateral cerebral hemisphere. The
Pons
sensory decussation is craniad to the motor, but both occur in the
Semicircular
closed portion of the medulla. duct
The medulla leads superiorly into the pons, which has an anterior
Fourth
“belly” and a posterior tegmentum. ventricle
The midbrain has two cerebral peduncles transmitting the motor
tracts. Its posterior portion is pierced by the cerebral aqueduct
(of Sylvius), to connect the third and fourth cerebral ventricles. Fig. 7.7. T2 weighted axial MRI: (a) to (f), inferior to superior. The brainstem.


70
The skull and brain paul butler


(e)

Pituitary gland
Cavernous sinus


Pons
fornix
Tectum,
Superior cerebellar
consisting of superior
Superior cerebellar
peduncle
and inferior colliculi
peduncle
Middle cerebellar
peduncle

Inferior cerebellar
(f)
peduncle

Anterior communicating Middle cerebral artery
artery in Sylvian fissure

Optic tract
Fig. 7.8. T2 weighted coronal MRI. The cerebellar peduncles.
Substantia nigra

Red nucleus
Ambient cistern
Quadrigeminal
plate cistern
Tectum Optic nerve

CSF within sheath
Fig. 7.7. Continued
Meningeal sheath


The posterior portion is known as the tectum or tectal plate. It con-
sists of four colliculi or quadrigeminal bodies concerned with auditory
and visual re¬‚exes.


The cerebellum Fig. 7.9. T2 weighted axial MRI. The optic nerve.

The cerebellum consists of two hemispheres joined by a central
vermis. The cortical mantle of the cerebellum overlies the white
Fig. 7.10. FLAIR axial MRI.
matter core as in the cerebral hemispheres but the cerebellar cortical The oculomotor nerve.
ridges, known as the folia, and the intervening sulci are approxi- Oculomotor
nerve
mately parallel to one another and are linked to the brainstem by the
paired cerebellar peduncles (Fig. 7.8). They are named logically. The
inferior cerebellar peduncles join the medulla to cerebellum; the
middle cerebellar peduncles (the largest), pons to cerebellum; the
superior cerebellar peduncles, midbrain to cerebellum.


The cranial nerves
There are 12 paired cranial nerves, most of which are analogous to seg-
mental nerves arising from the spinal cord. They variously provide The oculomotor (third) cranial nerve supplies the extraocular
sensory and motor nerves to structures in the extracranial head and muscles with the exceptions of the lateral rectus and superior oblique.
neck and their distribution is complex. It arises in the midbrain from a nucleus at the level of the superior
The olfactory (¬rst) cranial nerve consists of about 20 bundles of colliculi and emerges medial to the cerebral peduncle and is often
sensory nerves, which pass through the cribriform plate from the seen on FLAIR sequence MR images (Fig. 7.10). It passes anteriorly
nose to the olfactory bulb inferior to the frontal lobe. The ¬bers pass between the posterior cerebral and superior cerebellar arteries to
posteriorly from the bulb along the olfactory tract and thence to the enter the superior part of the cavernous sinus and thence to the orbit
olfactory cortex. through the superior orbital ¬ssure, accompanied by the trochlear
The optic (second) cranial nerve is not a true nerve but rather an (fourth) and abducent (sixth) cranial nerves.
evagination (outpouching) of the brain. The nerve carries with it a The oculomotor nerve is accompanied by parasympathetic ¬bers,
meningeal sheath and is surrounded by CSF (Fig. 7.9). It passes, along which constrict the pupil. An intracranial aneurysm arising at the
with the ophthalmic artery, into the orbit through the optic canal. origins of either the posterior communicating or superior cerebellar
arteries may result in an oculomotor palsy. This will be accompanied
The two optic nerves converge to form the optic chiasm, which lies in
by dilatation of the pupil because the parasympathetic constrictor
the suprasellar CSF cistern above the pituitary gland. From the chiasm,
¬bers travel peripherally in the nerve making them vulnerable
two optic tracts diverge toward the lateral geniculate bodies on each
to extrinsic pressure. The nerve also supplies levator palpebrae
side of the midbrain. From there, the optic pathway continues through
superioris “ so that a third nerve palsy is associated with ptosis.
the temporal lobes towards the visual cortex in the occipital lobes.

71
The skull and brain paul butler


The trochlear (fourth) cranial nerve is the only one arising from the
posterior surface of the brainstem, looping around the midbrain and
passing with the oculomotor nerve between the superior cerebellar
and posterior cerebral arteries. It is not seen on routine MRI scans.
The trigeminal (¬fth) cranial nerve is the largest and most complex. Bundle containing
The nerve arises from the pons and passes anteriorly to the trigeminal glossopharyngeal,
vagus, and spinal
ganglion located in Meckel™s cave at the posterior end of the cav- accessory nerves

ernous sinus (Fig. 7.11).
The motor root, supplying the muscles of mastication, travels
beneath the ganglion and exits the skull through the foramen ovale.
The ophthalmic (VI) division exits through the superior orbital ¬ssure
and the maxillary (VII) division through the inferior ¬ssure. The
mandibular (VIII) division does not enter the cavernous sinus but exits Fig. 7.13. T2 weighted axial MRI. The glossopharyngeal, vagus and spinal
inferiorly through the foramen ovale. The motor ¬bers to the muscles accessory nerves.
of mastication are con¬ned to the mandibular divisions of the nerve.
The abducent (sixth) cranial nerve supplies the lateral rectus muscle
and has a long intracranial course from pons to cavernous sinus, (a)
which makes it vulnerable in trauma to the skull base. The nerve may
be seen on thin section MR images (Fig. 7.12).
Hypoglossal
The facial (seventh) cranial nerve, which innervates the muscles nerve
of facial expression, passes with the vestibulocochlear (eighth)
cranial nerve from the pons to the internal auditory canal across the
cerebellopontine angle cistern and these are routinely visualized on
MRI (Fig. 7.12).
There is a sensory root, the intermediate nerve, which transmits
secretomotor ¬bers to the lacrimal, submandibular, and sublingual
glands and ¬bers conveying taste from the anterior two-thirds of the (b)
tongu (the chorda tympani).




Hypoglossal
Cavernous sinus nerve (anterior
condylar) canal

Meckel™s cave


Fig. 7.14. (a) T2 weighted axial MRI, (b) Axial CT on bone algorithm. The
Trigeminal nerve
hypoglossal nerve and canal.




The glossopharyngeal (ninth), vagus (tenth) and spinal accessory
(eleventh) cranial nerves are not seen on routine cranial MRI but
Fig. 7.11. T2 weighted axial MRI. The trigeminal nerve.
can be resolved on special sequences. They arise from the medulla
and form a bundle which leaves the cranium through the jugular
foramen (Fig. 7.13).
The hypoglossal (twelfth) cranial nerve can be identi¬ed exiting
through the hypoglossal, or anterior condylar, canal after emerging
from the medulla between the olive and pyramid (Fig. 7.14). Again the
nerve is not often seen on routine MRI.
The diencephalon, between the brainstem and cerebral hemi-
Abducent nerve
spheres includes the thalamus, hypothalamus, and pineal gland,
which all border the third ventricle. The thalami are paired, olive-
Facial nerve

shaped nuclear masses extending anteriorly as far as the foramen of
Vestibulo-
cochlear nerve Monro and forming most of the lateral walls of the third ventricle
(Fig. 7.15). Medially, the thalami are apposed (not joined) at the massa
intermedia or interthalamic adhesion. Laterally, the posterior limb of
the internal capsule separates thalamus and lentiform nucleus. The
posterior part of the thalamus is the pulvinar, which overlies the
Fig. 7.12. T2 weighted axial MRI. The abducent, facial and vestibulocochlear
midbrain.
nerves.


72
The skull and brain paul butler


Head of caudate
Fornix
nucleus Genu of corpus
callosum Internal cerebral
vein
Fornices Massa intermedia
Splenium of corpus
Hypothalamus callosum

Foramen of
Lentiform nucleus Tectum
Monro
Optic chiasm
Internal capsule Cerebral aqueduct
Thalamus

Pituitary
Fourth ventricle
gland



Fig. 7.15. T1 weighted axial MRI. The basal ganglia.

Sphenoid sinus Mamillary body

Planum sphenoidale
Optic foramen
Fig. 7.17. T1 weighted sagittal MRI. Showing the major midline structures.
Lesser wing
of sphenoid
Optic groove Tuberculum sellae




Anterior clinoid
process
Internal
Foramen rotundum
carotid artery
Internal carotid
artery within the
Greater wing
sphenoid sinus
of sphenoid

Foramen
ovale Pituitary gland
Middle clinoid
process



Foramen
spinosum
Posterior clinoid process
Dorsum sellae Foramen of Vesalius

Fig. 7.18. T1 weighted axial MRI after intravenous gadolinium DTPA. The
cavernous sinuses.
Fig. 7.16. The bony anatomy of the sellar region.


The hypothalamus forms the ¬‚oor and part of the walls of the The cavernous sinuses lie lateral to the sella on either side (Fig. 7.18).
third ventricle. Posterior to the optic chiasm the pituitary stalk or These are extradural venous spaces through which the internal
infundibulum desscends to the pituitary gland. The tuber cinereum carotid arteries pass, and damage to the artery here, due to trauma,
extends posteriorly from the stalk to the mamillary bodies, thence can result in a carotico-cavernous ¬stula.
to the midbrain The third, fourth, branches of the ¬fth and the sixth cranial nerves
pass through the cavernous sinus to the orbit. The cavernous sinuses
receive blood from a number of facial veins and venous plexuses pro-
The pituitary gland and perisellar region
viding a potential route for sepsis to spread intracranially (Fig. 7.27).
The pituitary gland and perisellar region are frequently imaged in Above the pituitary gland is the appropriately named suprasellar or
cases of endocrine disturbance, or in visual failure. chiasmatic CSF cistern, which contains the circle of Willis and the
The pituitary gland lies in the sella turcica (“Turk™s saddle”) on top optic chiasm (Fig. 7.19). The basal ganglia are part of the extrapyrami-
of the body of the sphenoid bone (Fig. 7.16). The body of the sphenoid dal system and consist of the caudate and lentiform nuclei, together
bone contains the sphenoid air sinus, which provides a route for surgi- known as the corpus striatum, the amygdala, and claustrum.
cal access to the pituitary gland via the nose. The sella is roofed by the The caudate nucleus is C-shaped with a head indenting the frontal
dural diaphragma sellae, which is pierced by the pituitary stalk horn of the lateral ventricle, a body running alongside the body of the
leading to the hypothalamus (Fig. 7.17). lateral ventricle and a tail lying just above the temporal horn of the
There is no blood“brain barrier around the pituitary gland, which lateral ventricle.
therefore takes up intravenous contrast media avidly, either the iodi- The lentiform nucleus is divided in the parasagittal plane into the
nated agents used for CT or gadolinium DTPA used in MRI. medial globus pallidus and larger lateral putamen.
The pituitary gland should be no more than 9 mm in height,
although it varies in size. In some normal individuals it appears as a
The motor and sensory tracts
thin rim of tissue at the base of the sella. Its upper margin is usually
concave, although it is often convex in children and in females of The upper motor neurones controlling voluntary movement are found
reproductive age. in the precentral gyrus of the frontal lobe. Axons pass from the cell

73
The skull and brain paul butler


(a)
Cingulate gyrus

Paracentral lobule
Corpus callosum s Me
eu dia
n
cu l fro
re nta
P l gy
rus
Septum pellucidum
Frontal horn of lateral ventricle




eus
Head of caudate nucleus




Cun
Anterior cerebral artery

e sulcus
Middle cerebral artery Calcarin
Pa r a
hipp
Optic chiasm ocam
Med
ial o pal g
ccip yrus
Internal carotid artery itote
Late mpo
ral o ral g
ccip yrus
itote
mpo
ral g
Pituitary gland yru s
Uncus


(b)
Fig. 7.19. T2 weighted coronal MRI. The pituitary gland and suprasellar cistern.
us
gyr
tal
on
r fr Su
erio pe




us
Parieto-
Sup rio




yr
lob r pa
us




lg
occipital
gyr




lg s ru
ule rie




us
bodies via the corona radiata to the internal capsule to the motor




tra
l
nta ta sulcus




nt sulc
yr
fro l




en
Inf
le eri




ec
idd
nuclei in the brainstem and to the anterior horns of the spinal cord.




l
or




ra
ra
M




Pr
lob parie




nt
ce
Ce
ule tal
The internal capsule is a V-shaped myelinated tract with the genu




st
us




Po
gyr
l
nta
(bend) pointing medially separating the anterior and larger posterior fro
ior
f e r t r i a n Pa r s
limbs. In gul
rus
a ris
o ra l g y
temp
From the various cutaneous receptors, sensory neurones with cell perior
Su
bodies in the dorsal root ganglia synapse in the thalami. Axons of us
ra l g y r
tempo
Middle
second-order neurones synapse in the thalami. Third-order neurons yrus
ra l g
po
r tem Temporo-occipital
io
pass from thalami to sensory cortex. Infer incisure



The cerebral hemispheres Fig. 7.20. The cortical gyri: (a) medial and (b) lateral aspects.

The cerebral hemispheres lie above the tentorium and are divided by
¬ssures and sulci into frontal, parietal, temporal, and occipital lobes. to the central sulcus. In common with the temporal lobe, the frontal
The limbic system (see below) is also considered to be a lobe. lobe has three major gyri, superior, middle, and inferior, which are ori-
The hemispheres are linked by the corpus callosum, the largest of ented horizontally. The temporal lobe occupies the middle cranial fossa
the commissural tracts, which interconnect paired structures across The anterior limit of the parietal lobe is the central sulcus, which,
the midline. Other examples of commissural tracts are the anterior, running in the coronal plane, separates the precentral (motor) gyrus of
posterior, and habenular commissures. The anterior and posterior the frontal lobe from the postcentral (sensory) gyrus. The boundary
commissures are landmarks used in image-guided neurosurgical pro- between the parietal and temporal lobes laterally is indistinct but the
cedures. parieto-occipital incisure medially de¬nes the two lobes. The main cor-
The corpus callosum is a myelinated tract and appears curved in tical supply of the occipital lobe relates to vision. The calcarine (visual)
sagittal images. The anterior rostrum blends with the anterior com- cortex can be seen to indent the posterior (occipital) horns of the lateral
missure inferiorly. The curved genu (knee) leads posteriorly to the ventricles. The cortex here is deeply infolded with little intervening
body thence the largest and most posterior part, the splenium. Fibers white matter. Inferiorly and laterally the temporo-occipital ¬ssure
from the corpus callosum sweep anteriorly into the frontal white marks the division between the two lobes.
matter as the forceps minor and posteriorly into the occipital white The Sylvian or lateral ¬ssure separates the superior surface of the
matter as the forceps major. temporal lobe from the inferior frontal lobe and the anterior parietal
There is considerable individual variation in gyral anatomy but the lobe. During development, the cortex overlying the basal ganglia is
more constant gyri are shown in Fig. 7.20. It is also important to invaginated to form the insula (Fig. 7.21). The cortex in front of, above,
appreciate that the relationship of function to structure may be vari- and below this depression expands to form covering folds termed the
able and that speech, for example, may be represented over a number operculum.
of gyri with intervening white matter. Equally, it may be dif¬cult to The Sylvian ¬ssure is formed between these folds. On axial imaging
identify the central sulcus and adjacent motor strip accurately. In it runs in the coronal plane on the lower cuts and in the sagittal
specialized centers, functional MRI is carried out with patients per- plane on the higher slices. On coronal MRI, it resembles the shape of
forming appropriate intellectual, motor, or sensory tasks. Regional a T lying on its side.
variations in cerebral oxygen utilization can then be registered and
the eloquent area identi¬ed.
The limbic system
The anatomical boundaries of the individual lobes may be indistinct,
depending on the aspect. The frontal lobe is the largest of the anatomi- The anatomy of the limbic system is complex, its many components
cal lobes occupying the anterior cranial fossa and extending posteriorly retaining their descriptive, classical names, unfortunately with some

74
The skull and brain paul butler


(a)




Amygdala
Insula




(b)

Insula
Fig. 7.21. T1 weighted parasagittal MRI. The insula. Globus pallidus
Putamen


Anterior
Cingulate gyrus and cingulum Termination of commissure
basilar artery
Indusium griseum
Dorsal fornix

Septum pellucidum
(c)
Column
Fornices
of fornix




Hippocampus

Parahippocampal
gyrus



Isthmus (d)
Olfactory
bulb
Fimbria of fornix
Amygdala nuclear
complex
Hippocampus
Uncus Temporal horn of
Dentate gyrus Mamillary body lateral ventricle

Parahippocampal gyrus


Fig. 7.22. Medial aspect of cerebral hemisphere showing the limbic system.
(e)
Fornix
synonyms. The limbic system is also classi¬ed as one of the cerebral
Third ventricle
lobes (the limbic lobe).
The limbic system can be regarded as two C-shaped gyral arches in Temporal horn of
lateral ventricle
each hemisphere, running from near to the midline in the frontal Hippocampus
lobes to the medial part of the temporal lobe (Fig. 7.22). Their course
mirrors the curved relationship of the frontal and temporal lobes and
the various components can be identi¬ed in Fig. 7.23.
(f)
The arches comprise the following.
Fornix

Thalamus
The outer limbic gyrus (the larger arc)
subcallosal area, cingulate gyrus, parahippocampal gyrus, subiculum,
Hippocampus
uncus,
Tectum

The inner limbic gyrus (the smaller arc) Cerebellum Tentorium cerebelli

supracallosal and paraterminal gyri, hippocampus, The outer and
inner limbic gyri are separated by the hippocampal sulcus and its con- (g)
tinuation, the callosal sulcus.
Fornix
The hippocampus (sea horse or monster), consists of a head, body, Internal
and tail, and is the ¬rst part of the cerebral cortex to form (Figs. 7.24, cerebral
veins
7.25). The broadest part is the head anteriorly. More posteriorly, the Hippocampus
body of the hippocampus forms the ¬‚oor of the temporal horn of the
lateral ventricle. The tail extends around the splenium of the corpus
callosum and is continuous with the supracallosal indusium griseum.
The indusium griseum is closely applied to the surface of the corpus Fig. 7.23. T2 weighted extended coronal MRI. Obtained perpendicular to the long
callosum and anteriorly it merges with the paraterminal gyrus. axes of the temporal lobes, (a) to (h), anterior to posterior.


75
The skull and brain paul butler


(h) Fig. 7.26. Diagram of
Tail of caudate nucleus components of limbic
lobe.
Temporal horn of Alveus
Splenium of corpus
Choroid plexus
lateral ventricle
callosum
Ammon™s horn
Fimbria of
Quadrigeminal fornix
plate cistern




Dentate gyrus
Fig. 7.23. Continued
Subiculum




Hippocampus




Axons from the subiculum and hippocampus form the alveus
(white matter) and converge as the ¬mbria, which leads into the
fornix (arch) at the posterior hippocampus. The two fornices converge
7.24. T1 weighted parasagittal MRI. The hippocampus.
near to the foramen of Monro. The uncus is formed anteriorly from
the parahippocampal gyrus and posteriorly from the medial part of
Fig. 7.25. T1 weighted
the hippocampal head.
Temporal horn
axial image in the plane
The subcortical structures of the limbic system comprise the amyg-
of the temporal lobe.
dala, habenula, mamillary body, and septal nuclei.
The hippocampus.
The septal area is in the medial part of the frontal lobes and
includes the subcallosal area and paraterminal gyri, from the outer
and inner limbic gyri, respectively. There are also limbic connections
with the thalamus and hypothalamus.


The cerebral envelope (Fig. 7.27)
The meninges invest the brain and spinal cord. The three constituent
parts are the outer, ¬brous dura mater, the avascular, lattice-like,
arachnoid mater, and the inner vascular layer, the pia mater.
The subarachnoid space contains the cerebrospinal ¬‚uid (CSF),
which surrounds the cerebral arteries and veins. It is situated between
the arachnoid, which bridges the sulci, and the pia, which is closely
applied to the cerebral surface.
The dura consists of two layers which separate to enclose the
venous sinuses (Fig. 7.27). The outer layer is the periosteum of the
inner table of the skull. The inner layer covers the brain and gives rise
to the falx and tentorium.
The falx cerebri is a sickle-shaped fold of dura, which forms an
The internal structure of the hippocampus is complex and beyond incomplete partition between the cerebral hemispheres. The superior
the resolution of MRI at the time of writing. The dentate gyrus and and inferior sagittal sinuses mark its upper and lower margins.
cornu Ammonis infold into one another in the form of interlocking The “point of the sickle” is anterior, the falx being broader posteri-
Us (Fig. 7.26). orly. When there is swelling of one hemisphere, subfalcine herniation
The parahippocampal gyrus is inferior to the hippocampus and “midline shift” will be more pronounced anteriorly as a result.
forms the inferomedial aspect of the temporal lobe. Superior to it is The tentorium cerebelli (“tent”) forms a roof over the contents of
the hippocampal ¬ssure and laterally, the collateral sulcus. The the posterior fossa. Anteriorly and superiorly, the tentorial hiatus con-
parahippocampal gyrus becomes continuous with the cingulate gyrus stitutes a gap in the tent through which the midbrain passes. The free
which continues anteriorly into the subcallosal area. medial edge of the tentorium extends anteriorly to form the lateral
The subiculum is a cortical layer of the parahippocampal gyrus and wall of the cavernous sinus on each side.
is separated by the hippocampal ¬ssure from the dentate gyrus. On axial CT, the anterior margin of the tentorium migrates medially
The most anterior part, the hippocampal head, is separated by the on the higher scans. Structures lateral to the line are supratentorial,
temporal horn of the lateral ventricle from the amydala (almond), structures medial to the line are infratentorial (or lie within the
which lies more anteriorly and a little superiorly. posterior fossa).

76
The skull and brain paul butler


Falx cerebri

Superior sagittal sinus


Inferior sagittal sinus


Inferior petrosal sinus


Internal carotid
Straight sinus
artery



Optic nerve
Transverse
sinus
Superior
ophthalmic vein


Great vein
of Galen




Cavernous sinus
Anterior
facial vein




Pterygoid venous plexus
Superior petrosal sinus
Fig. 7.27. The cranial dura.




Fig. 7.28. The cerebral
Foramen of Monro
ventricles. Parieto-
occipital
Cerebral
fissure
aqueduct

Lateral Quadrigeminal
Anterior plate cistern
cerebral
artery
Cerebral
Suprasellar aqueduct
(chiasmatic)
Vermis
cistern
Third
Fourth
Fourth
ventricle
Prepontine
cistern

Cisterna magna
The cerebral ventricular system cerebrospinal ¬‚uid
spaces (Fig. 7.28)
Fig. 7.29. T2 weighted sagittal MRI. The basal CSF cisterns.
The cerebral ventricular system consists of the paired lateral and
single third and fourth ventricles.
The cerebral blood circulation
Cerebrospinal ¬‚uid (CSF), is produced in the choroid plexuses,
and most of it is in the lateral ventricles, entering medially through the
Cerebral arteries
choroidal ¬ssures. It ¬‚ows from the lateral ventricles to the third ventri-
The brain is supplied with oxygenated blood by the paired internal
cle through the foramen of Monro, in the anterior portion of the roof of
carotid and vertebral arteries. The common carotid artery in the neck
the third and from the third to fourth via the cerebral aqueduct of the
divides at the approximate level of the upper border of the thyroid
midbrain. From the fourth ventricle, the CSF enters the subarachnoid
cartilage (C4) into its internal and external branches, the latter supply-
spaces, leaving through the paired foramina of Luschka, laterally and
ing the various craniofacial structures.
the midline, single foramen of Magendie. These foramina provide a
potential route of spread for intraventricular tumors.
The internal carotid artery
At the base of the brain, there are relatively large CSF spaces, the
basal CSF cisterns, which are important both anatomically and in CT The internal carotid artery is the larger of the two branches, receiving
or MRI diagnosis. Although named individually, according to adjacent 70% of the common carotid blood ¬‚ow. It lies posterolateral to the
structures, they interconnect freely with each other and with the CSF external carotid near to the bifurcation and neither common nor
spaces generally (Figs. 7.29, 7.3, 7.7(f )). internal carotid arteries have cervical branches.

77
The skull and brain paul butler


The internal carotid artery enters the skull through the carotid Most cerebral arterial aneurysms are borne on the circle of Willis
canal and courses anteromedially and horizontally (the petrous and so their rupture results in hemorrhage into the subarachnoid
segment) before turning superiorly into the cavernous sinus (Fig. 7.5). space in the ¬rst instance. This includes an aneurysm at the origin of
In this position the artery forms the shape of a siphon. Emerging from the ophthalmic artery
the cavernous sinus, the artery enters the subarachnoid space and The circle of Willis is not circular in shape but rather a ¬ve- or six-
divides into its terminal branches, the anterior and middle cerebral pointed star, (Fig. 7.31). It is also complete in only a minority of indi-
arteries (Fig. 7.19). viduals. Indeed, the intracranial arterial anatomy is subject to so many
There are no angiographic markers of the position of the intracav- (usually minor) variations that a broad picture will be given here.
ernous portion of the internal carotid artery, but the origin of the oph- The terminal branches of the internal carotid artery are the ante-
thalmic artery is usually within the subarachnoid space (Fig. 7.30). rior and middle cerebral arteries. The anterior cerebral artery passes
The posterior communicating artery passes on each side from the horizontally towards the midline and links to the other anterior
internal carotid to the posterior cerebral arteries. cerebral by the anterior communicating artery, (Figs. 7.1(h), 7.2(e)).
The anterior choroidal artery arises from the internal carotid artery This ¬rst part is known as the A1 segment. From the origin of the
just above the posterior communicating artery anterior communicating artery both anterior cerebral arteries next
The circle of Willis is situated in the suprasellar cistern and links turn superiorly and run in close proximity, above the corpus callo-
the internal carotid arteries with each other and with the verte- sum, following a curving path posteriorly, again near the midline
brobasilar system, via the single anterior and paired posterior commu- (Fig. 7.29).
nicating arteries. It affords some protection in the event of occlusion The middle cerebral artery passes horizontally and laterally towards
of major arteries by facilitating “cross-¬‚ow.” the Sylvian ¬ssure. There is then a division into two or three branches
(middle cerebral artery bifurcation or trifurcation). These ascend
within the Sylvian ¬ssure and then loop infreolaterally over the oper-
cular cortex over the cerebral surface to supply the parietal and
(a)
temporal lobes.
Arising from the proximal anterior and middle cerebral
arteries, a leash of small, perforating arteries (the lenticulostriates)
supplies a variety of structures including the basal ganglia and
internal capsule.
Middle cerebral
Anterior cerebral
The vertebral arteries are the ¬rst branches of the subclavian arter-
artery branches
artery
ies. They ascend vertically within the foramina transversaria of the
Posterior cerebral
6th to the 2nd cervical vertebrae and posterolaterally through the
artery
foramen transversarium of the atlas, (¬rst cervical vertebra). The
Posterior
arteries then travel superomedially to pass into the skull through the
communicating
artery
foramen magnum, piercing the dura and entering the subarachnoid
Superior cerebellar
space (Figs. 7.7 (a, b)). At the level of the pontomedullary junction,
artery
Ophthalmic artery
the two arteries join to form the midline basilar artery (Figs. 7.1
Basilar artery
(a)“(f ), 7.2 (c)“(e)), which runs anterior to the brainstem. The cerebel-
lum is supplied by the posterior inferior cerebellar arteries, arising
(b)
from the vertebral arteries just before the con¬‚uence, and the ante-
rior inferior- and superior cerebellar arteries, arising from the basilar
artery.

Anterior cerebral artery

Anterior communicating
Posterior cerebral artery artery
Anterior cerebral artery

Middle cerebral artery
Middle
cerebral Basilar artery Internal carotid artery
artery
Posterior Posterior cerebral artery
Vertebral artery communicating
artery
Superior cerebellar artery
Anterior inferior
cerebellar artery



Vertebral artery
Fig. 7.30. Internal carotid angiograms: (a) lateral, (b) frontal projections. Although
only the internal carotid artery has been injected with contrast medium, the
vertebrobasilar system and contralateral middle cerebral artery are opaci¬ed
Posterior inferior cerebellar
due to the Circle of Willis. The triangle in (a) encloses the proximal middle artery
cerebral artery branches. Note that the anterior ceebral arteries are near to
Fig. 7.31. Magnetic resonance angiography. Circle of Willis.
the midline, whereas the middle cerebral artery branches are laterally situated.


78
The skull and brain paul butler


(a) (b)




ACA ACA


MCA MCA




AchA
A c hA
PCA PCA
MCA MCA

PCA
BA
PCA PCA

AICA AICA

SCA
PICA PICA




(c) (d)




ACA

ACA ACA
LS A
LS A




MCA MCA
MCA MCA
PCA
PCA




PCA PCA
PCA PCA



Fig. 7.32. The vascular
territories.

(e)



pass posteriorly to the brainstem. It is also the case that similar very
ACA ACA
small arteries arise from all of the major intracerebral arteries, includ-
ing the communicators.

H H
Vascular territories
LSA LSA
Knowledge of the cerebral arterial territories can be of assistance in
Ach
A
PCA

PCA
Ach




the identi¬cation of a lesion as an infarct. These are illustrated in
A




Fig. 7.32.
MCA MCA


Cerebral venous drainage
A complex venous system drains blood from the brain into the inter-
PCA PCA
nal jugular veins in the neck (Fig. 7.33).
The super¬cial veins over the cerebral surface drain into the dural
venous sinuses, venous spaces within the dura (Fig. 7.27). There is
also a deep system draining into the paired internal cerebral veins
(Figs. 7.2(i), 7.8, 7.17, 7.23(g)). The internal ceebral veins lead into the
single great vein of Galen, thence into the straight sinus. This venous
The terminal branches of the basilar artery are the posterior cere- “con¬‚uence” is situated in the quadrigeminal plate cistern. Another
bral arteries, which supply the occipital (visual) cortex (Figs. 7.2 (g), con¬‚uence, this time, of the dural venous sinuses occurs at the inter-
7.23 (b)). Many smaller branches arise from the basilar arteries, which nal occipital protuberance or torcula, where the superior sagittal,
are too small to be shown at angiography. These “perforating” arteries transverse and straight sinuses converge.

79
The skull and brain paul butler


(c)
(a), (b) Superior sagittal sinus


Superior sagittal sinus
Inferior sagittal sinus
(a) (b)




Lateral sinus




Internal jugular vein


Fig. 7.33. The cerebral venous system: (a) T1 weighted sagittal MRI after
intravenous gadolinium DTPA; (b) carotid angiogram, venous phase, lateral view;
(c) carotid angiogram, venous phase, frontal view. Note that the lateral sinuses
are not seen on the MRI because it is a midline “slice.” The angiograms
Transverse sinus
Internal cerebral vein represent a 3-D arrangement displayed in 2-D.
Straight sinus
Great vein of Galen
Internal jugular vein
Sigmoid sinus




80
Section 4 The head, neck, and vertebral column

Chapter 8 The eye


C L AU D I A K I R S C H




Imaging considerations
The bony orbit is best examined with CT and images acquired in the
coronal plane are particularly useful to identify fractures. The radia-
tion dose to the lens is not insigni¬cant and cataract formation is a
potential hazard
Plain radiography of the orbit is largely reserved for identifying
metallic intraocular bodies prior to MRI scanning.
Intraorbital fat is hypodense (dark) on CT scans and provides a useful
contrast to the other soft tissue structures within the orbit. Conversely
fat is hyperintense (white)on both T1- and T2-weighted MRI. The relative
brightness of fat can obscure the orbital contents and, to counter this
“fat-suppressed” MR, pulse sequences are used, usually in combination
with intravenous gadolinium DTPA. These render fat hypointense (dark)
and thus improve visualization of the globe, extraocular muscles, and
lacrimal gland. Overall, the soft tissue detail with MRI is superior to CT.


Anatomy of the bony orbit
Nasociliary
The orbital cavity is shaped like a pyramid with its apex posteromedi- nerve
Superior rectus/levator
ally and base anterolaterally, opening onto the face. It is represented palpebrae superioris Ethmoid Frontal
Superior
diagrammatically in Fig. 8.1. The bony margins separate it from the muscles
Superior sinuses bone
oblique
ophthalmic muscle
anterior cranial fossa and frontal air sinus superiorly, the ethmoid and vein
Medial
sphenoid air sinuses medially, the maxillary sinus inferiorly, and the Crista
rectus
temporal fossa laterally (Fig. 8.2). galli
muscle Temporal
Lateral
fossa
rectus
muscle

Superior orbital Optic canal Greater wing Orbital plate
fissure of sphenoid of ethmoid bone
Lateral
Intraconal
wall of
fat
Roof orbit


Lacrimal bone Optic Lamina
Medial
Lateral
nerve papyracea
wall
wall
Nasal bone
Infraorbital
Inferior rectus Ostiomeatal
Floor
groove
muscle complex

Zygoma Lesser wing Orbital plate
of sphenoid of maxilla
Fig. 8.2. Coronal CT scan to show the orbital wall and extraocular muscles.
Fig. 8.1. Diagram of the bony anatomy of the orbit.


Applied Radiological Anatomy for Medical Students. Paul Butler, Adam Mitchell, and Harold Ellis (eds.) Published by Cambridge University Press. © P. Butler,
A. Mitchell, and H. Ellis 2007.

81
The eye claudia kirsch


The triangular orbital ¬‚oor, which slants laterally, and the rectangu- The SOF communicates posteriorly with the cavernous sinus and
lar medial orbital wall, the descriptively named lamina papyracea, are the IOF with the pterygopalatine fossa, which leads to the infratempo-
both thin. The ¬‚oor also has a groove running anteriorly to a canal, ral fossa. The veins crossing these ¬ssures thus provide possible routes
the infraorbital foramen, transmitting the infraorbital nerve, con- for the spread of orbital infections both intracranially and into the
tributing further to its potential weakness. Predictably both the deep facial structures.
medial wall and ¬‚oor are prone to fractures and are demonstrated The periorbita is composed of the bony orbit periosteum and serves
optimally by coronal CT scans. as a protective barrier against spread of infection or neoplasms.
The lateral wall, also triangular is the thickest and is formed largely Posteriorly it merges with the optic nerve dura. Anteriorly, the perior-
from the zygomatic bone. bita continues as the orbital septum inserting on the tarsi within each
At the orbital apex, the optic canal, contained within the lesser of the eyelids. Each tarsus is a ¬brous structure, one in the upper, one
wing of the sphenoid bone, transmits the optic nerve, sympathetic in the lower eyelid. In the upper eyelid, the orbital septum also joins
¬bers and ophthalmic artery, opening posteriorly into the middle the tendon of the levator palpebrae muscle.
cranial fossa (Fig. 8.3). A preseptal orbital infection in front of the orbital septum may
The superior orbital ¬ssure (SOF), is located inferior and lateral be managed medically. A postseptal infection has spread behind
to the optic canal and is separated from the optic canal by the optic the septal barrier with loss of the normal orbital tissue planes and
strut (Fig. 8.4). The SOF is formed superiorly by the lesser wing of the is at risk for subperiosteal, intracavernous, and intracranial extension.
sphenoid bone and inferiorly by the greater wing. The SOF transmits
the oculomotor (IIIrd), trochlear (IVth), and abducent (VIth) cranial
Soft tissues of the orbit
nerves, the terminal ophthalmic nerve branches, and ophthalmic
veins. The soft tissues of the orbit are embedded in a fatty reticulum.
Seen from the front, the inferior orbital ¬ssure (IOF), forms a V-shape The globe is approximately 2.5 cm in diameter (Fig. 8.5). It is situated
with the SOF, its apex pointing medially.




Anterior lacrimal
Medial rectus muscle Lacrimal sac
crest (maxilla)
Orbital Cornea Ethmoid
Air beneath
Aqueous
septum
sinuses
lower lid
Lens
Insertion of
Inferior portion inferior oblique
Vitreous
Inferior pole of
of eye muscle
lacrimal gland
Outer coats
of eye
Zygoma
Orbital fat
Optic disc
(intraocular
Extraconal fat optic nerve)
Temporal
Inferior rectus
Lateral fossa
muscle
rectus
Intraconal fat
muscle
Greater wing
Superior
Ophthalmic of sphenoid
orbital
artery
fissure
Optic nerve
Superior Sphenoid Middle cranial fossa
(intraorbital)
Anterior
orbital sinus (temporal lobe)
Optic nerve Optic nerve in meningeal
clinoid Sphenoid Fat in
fissure
(intracranial) (intracanalicular) sheath
process sinus cavernous
Pituitary gland
sinus


Fig. 8.4. Axial CT scan (inferior to Fig. 8.3), at the level of the superior orbital
Fig. 8.3. Axial CT scan at the level of the optic canals.
¬ssure.


82
The eye claudia kirsch


anteriorly within the orbit and has three coats enclosing its contents.
From the outside in, there are the tough, ¬brous sclera, the vascular,
pigmented choroid, and the retina. These cannot be resolved sepa-
rately on routine CT and MRI. The vascular choroid can be identi¬ed
as a “blush” during carotid angiography.
Anteriorly within the globe, the circumferential ciliary body sup-
ports the lens and, anterior to the lens, the iris.
The lens demarcates two compartments, the anterior aqueous
and posterior vitreous. The iris further divides the aqueous
(incompletely because of the pupil), into anterior and posterior
chambers. The cornea forms the anterior boundary of the anterior
chamber.
The episcleral membrane, or Tenon™s capsule, encapsulates the
posterior four-¬fths of the globe, dividing it from the posterior
orbital fat.


The optic nerve
The optic nerve is not a true cranial nerve. Rather, it is a cerebral
white matter tract. It arises from the posterior globe and pursues
an undulating course within the rectus muscle cone to pass
through the optic canal accompanied by the ophthalmic artery
(Fig. 8.6). Each optic nerve is about 4.5 mm in diameter and 5 cm
long. The distance from the posterior globe to the optic canal is
about 2 cm. This redundancy permits the nerve mobility with the
eye movements.
Belying its nature the optic nerve is surrounded by cerebrospinal
¬‚uid and encased in a meningeal sheath.


The extraocular muscles
Six striated extraocular muscles, four rectus muscles, and two oblique
muscles are responsible for the eye movements. The extraocular
muscles are arranged as a cone and de¬ne intra- and extraconal
compartments.
The four rectus muscles arise from the annulus of Zinn, a tendi-
Cornea
Aqueous
nous ring at the optic foramen. The annulus is composed of four
Ciliary body
Capsule and extraocular muscles: superior rectus, medial rectus, and inferior, and
nucleus of lens
lateral rectus muscles (Fig. 8.2). The oblique muscles have a more
complex course. The superior oblique muscle, the longest and
Vitreous
Choroid/sclera
thinnest of all orbital muscles, originates from the sphenoid bone
periosteum extending along the superior medial orbital wall as a
Lacrimal
Orbital fat slender tendon. The muscle enters the trochlea (L. pulley), a small
gland
¬brocartlaginous ring, sharply turning posterolaterally and inferiorly
behind the superior rectus muscle inserting on the lateral sclera. The
Medial rectus
muscle inferior oblique muscle originates from the medial portion of the
Optic disc
anterior orbital ¬‚oor and is inserted into the lateral aspect of the
Ophthalmic eyeball.
artery
The triangular levator palpebrae superioris muscle arises above and
Lateral rectus
muscle
in front of the optic canal to pass forwards above superior rectus to
insert into the upper eyelid.
Internal
carotid
artery

The nerves of the orbit
The superior oblique muscle is supplied exclusively by the trochlear
(IVth) cranial nerve and lateral rectus by the abducent (VIth) cranial
Fig. 8.5. Fat-suppressed T1W axial MRI to show the globe.




83
The eye claudia kirsch


nerve. The oculomotor (IIIrd) cranial nerve supplies the remaining,
striated, extraocular muscles. Sensory innervation is via the oph-
thalmic division and maxillary divisions of the trigeminal (Vth) cranial
nerve.


The lacrimal gland
The almond-shaped lacrimal gland is located anterolaterally in the
roof of the orbit in a small fossa (Fig. 8.7). It forms tears, which diffuse
to the conjunctiva and drain via the tear ducts running in the medial
portions of the margins of the upper and lower lids.


The orbital vasculature
The main arterial supply of the orbit is via the ophthalmic artery,
which arises directly from the internal carotid artery, in the majority
of cases just after it has exited the cavernous sinus (Fig. 8.8). It passes
forward to enter the orbit through the optic canal, accompanying the
optic nerve within the dural sheath. Initially inferior to the nerve, the
ophthalmic artery crosses the nerve to lie medial to it (Fig. 8.9). It
gives off numerous branches within the orbit including the central
artery of the retina. Further arterial supply is provided by branches
of the external carotid artery.




Cornea
Aqueous


Ciliary body



Optic disc



Region of
Orbital plate
Superior plate cribriform plate
of frontal bone
Malar process Superior oblique
of frontal bone
Optic nerve of frontal bone tendon
sheath
Nasociliary
Levator palpebrae
nerve
Lacrimal superioris muscle
gland
Optic nerve
Lamina papyracea
Ophthalmic
(ethmoid bone)
artery
Medial rectus
muscle Vitreous


Internal Orbital fat
External coats
carotid
of eye
artery

Inferior
Floor of orbit
oblique
muscle
Nasolacrimal Lacrimal Maxillary
duct bone antrim



Fig. 8.6. Fat-suppressed T1W axial MRI to show the optic nerve. Fig. 8.7. Coronal CT scan (anterior to Fig. 8.2), to show the lacrimal glands.



84
The eye claudia kirsch




Intracanalicular
Supraorbital artery segment of
ophthalmic artery
Lacrimal artery


Orbital veins
Upper lid Upper part of eye



Top of
lacrimal
Orbital fat gland

Superior
Internal carotid
ophthalmic
artery
vein

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