Ventricles, cisterns, CSF production and flow ventricles

the cerebellar veins drain to the nearby dural sinuses as follows: • The superior and posterior parts to the straight and transverse sinuses; and • The inferior aspect to the inferior petrosal, sigmoid and occipital sinuses.

Radiological features of the cerebellum

CT and MRI On axial CT and MR sections taken through the pons (see Fig 2. 3a), the cerebellum is seen to be separated from the pons by the fourth ventricle and connected to the pons on each side of this by the middle cerebellar peduncles. At this level the cerebellum is bounded anteriorly by the petrous temporal bones.

On higher slices (see Fig. 2. 3b) the cerebellum is separated from the temporal and occipital lobes anterolaterally the tentorial margins. Close to its bony attachment the tentorium can be easily seen on contrast-enhanced studies owing to the contained superior petrosal sinus.

The superior vermis can be seen between the occipital lobes on sections through the thalamus.

The normal flocculus enhances more than the rest of the cerebellum and should not be mistaken for a more anteriorly located acoustic neuroma.

Fissures separating lobes of the cerebellum can be seen on sagittal MRI (see Fig. 2. 16).

VENTRICLES, CISTERNS, CSF PRODUCTION AND FLOW VENTRICLES (Figs 2. 17-2. 19) These are fluid-filled spaces within the brain related to the development of the nervous system as a tubular structure with a central canal.

Two lateral ventricles represent expansion of the most anterior part of the ventricular system into each cerebral hemisphere. The third ventricle, the aqueduct and the fourth ventricle are midline in position and are continuous w i th the central canal of the cord. The ventricular system is also continuous w i th the subarachnoid space around the brain via foramina in the fourth ventricle. Approximately 25 mL of cerebrospinal fluid (CSF) fills these spaces in an adult.

The ventricles are lined w i th ependyma, which is invaginated by plexuses of blood vessels called the choroid plexus. These vessels produce the cerebrospinal fluid.

The lateral ventricles (see Fig. 2. 17) There is a lateral ventricle within each cerebral hemisphere. Each is C-shaped, w i th the limbs of the C facing anteriorly and a variably developed posterior extension from its midpoint. Each is described as having a frontal (anterior) horn, a body (atrium), a temporal (inferior) horn and an occipital (posterior) horn. An interventricular foramen (of Monro) at the junction of the anterior horn and the body connects each lateral ventricle w i th the third ventricle.

Frontal (anterior) horn This extends into the frontal lobe. Its roof and anterior extremity are formed by the corpus callosum, its rostrum and genu, and fibres radiating from these, the tapetum. The head of the caudate nucleus makes a prominent impression in the floor and lateral wall of the anterior horn. The medial wall is formed by the septum pellucidum. There is no choroid plexus in the anterior horn.

Body This is within the parietal lobe. As w i th the anterior horn, the roof and lateral wall of the body of the ventricle are

Fig. 2. 17 The ventricular system.

formed by the corpus callosum and its fibres - the tapetum - and the medial wall is formed by the septum pellucidum. The thalamus lies in the floor medially, w i th the body of the caudate nucleus laterally and the thalamostriate groove and vein between. The body of the fornix lies above the thalamus. The choroid plexus is invaginated into the cavity of the ventricle in a groove - the choroidal fissure - between the fornix and the thalamus.

Temporal (inferior) horn This extends anteriorly into the temporal lobe. It curves down and laterally from the body of the ventricle and then somewhat medially towards the temporal pole. Its lateral wall is formed by the fibres of the tapetum. The caudate nucleus lies in the floor of the anterior horn, and the body curves around in the concavity of the ventricle so that its tail lies in the roof of the inferior horn w i th the amygdaloid nucleus at its anterior end. The hippocampus forms the floor of the inferior horn, w i th the pes hippocampi anteriorly and the crus of the fornix arising from this (cf. the fornix).

The choroid plexus of the body of the lateral ventricle is continuous w i th that of the inferior horn.

Occipital horn This is the posterior extension of the lateral ventricle that extends into the occipital lobe. The posterior convexity of the body from which it arises is called the trigone of the lateral ventricle. The occipital horn may be absent or poorly developed, or may extend the full depth of the lobe. The posterior horns of the two lateral ventricles are often asymmetrical and are bilaterally well developed in only 12% of subjects. If the posterior horn is present on one side only it is usually the left.

The lateral wall of the occipital horn is formed by tapetal fibres and the optic radiation. Its floor is formed by white matter that is indented by grey matter in the depth of the collateral sulcus, and white matter in its medial wall is indented by grey matter in the depth of the calcarine sulcus. This latter indentation forms a bulge called the calcar avis. There is no choroid plexus in the occipital horn.

The choroid plexus of the lateral ventricle

The choroid plexus of the lateral ventricle is responsible for the production of most of the CSF. It extends from the inferior horn through the body to the interventricular foramen, where it is continuous w i th that of the third ventricle. The choroid plexuses are invaginated into the lateral ventricles medially through the choroidal fissure. The fold of pia containing the plexus is called the tela choroidea. This fold has a narrow rounded apex anteriorly at the level of the interventricular foramen and is wider posteriorly, where it is continuous w i th the remainder of the subarachnoid space. If CSF accumulates in this fold it forms a flat triangular cavum (or cistern of the) velum interpositum that may be visible on axial CT or MRI (Figs 2. 19 and 2. 20).

The vessels supplying the choroid plexus are the anterior and posterior choroidal arteries. The anterior choroidal artery, a branch of the internal carotid artery, enters the choroidal plexus in the anterior part of the inferior horn. The posterior choroidal arteries are branches of the posterior cerebral artery; these are variable in number and pass into the body of lateral ventricle behind and above

the thalamus and into the posterior part of the inferior horn below it.

The veins draining the plexus unite to form the choroidal vein, which begins in the inferior horn and passes anteriorly to the interventricular foramen, where it joins the thalamostriate vein to form the internal cerebral vein on each side.

The third ventricle (see Figs 2. 17-2. 19) This is a slit-like space between the thalami. Its w i d th is between 2 and 10 mm and increases w i th age. Its lateral walls are formed by the thalami and are limited inferiorly by the subthalamic (hypothalamic) groove. The thin anterior wall of the third ventricle between the anterior commissure above to the optic chiasm below is called the lamina terminalis. The extension of the cavity inferiorly into the optic chiasm is called the supraoptic recess. The floor of the third ventricle is formed by the structures of the hypothalamus, including the pituitary gland whose hollow stalk is the infundibular recess of the ventricle. Posteriorly, the ventricle extends as a small pineal recess into the pineal stalk, and above this into a recess whose size varies from 1 to 3 cm known as the suprapineal recess. The roof of the third ventricle is formed anteriorly by the anterior commissure, the column of the fornix and the interventricular foramen. Behind this the roof is formed by the body of the fornix, w i th the choroid plexus invaginating below the fornix, in the choroidal fissure, into the upper part of the ventricle.

The thalami are connected across the ventricle in 60% of subjects by a non-neural connection, the massa intermedia or interthalamic adhesion.

The cerebral aqueduct (see Figs 2. 17 and 2. 18) This is a narrow channel connecting the posterior end of the third ventricle w i th the superior end of the fourth ventricle. Measuring 1. 5 cm in length and 1-2 mm in diameter, it passes through the brainstem w i th the tectum (the quadrigeminal plate) posterior to it and the tegmentum and cerebral peduncles anteriorly. The nuclei of the third, fourth and fifth cranial nerves surround the aqueduct and are called the periaqueductal grey matter.

The fourth ventricle (see Figs 2. 17-2. 19) Posterior to the pons the aqueduct widens as the fourth ventricle and narrows again in the inferior part of the medulla as the central canal of the medulla and of the spinal cord.

The floor of the fourth ventricle is diamond-shaped (the rhomboid fossa) and is formed by the posterior surface of the pons and of the upper part of the medulla. The roof is formed superiorly by the superior cerebellar peduncles, w i th the superior medullary velum between, and inferiorly by the inferior cerebellar peduncles and the inferior medullary velum. Over these lies the cerebellum.

In the lower part of the roof of the fourth ventricle there are three openings, one median and two lateral. The median aperture (of Magendie) is a large opening in the inferior medullary velum beneath the cerebellum, which communicates w i th the cisterna magna. The cavity of the fourth ventricle is prolonged laterally under the inferior cerebellar peduncle on each side as the lateral recesses of the ventricle. The lateral apertures (of Luschka) are at the apex of these recesses and open anteriorly just behind the eighth cranial nerve into the pontine cistern.

The fourth ventricle tends to be symmetrical in its anatomy (as do the third ventricle and the aqueduct) and minor asymmetry may be a sign of pathology.

The choroid plexus of the fourth ventricle invaginates the lower part of its roof and is supplied by a branch of the inferior cerebellar artery.

Radiological features of the ventricular system

Skull radiographs The ventricular system cannot be seen by plain skull radiography but the position of the lateral ventricles is often indicated in adults by calcification of its choroid plexus. This is seen on a lateral view in the parietal area about 2. 5 cm above the pineal. On OF or FO views it is seen to be nearly always symmetrical and bilateral.

The normal position of the fourth ventricle is indicated by Twining's line, which runs from the tuberculum sellae to the internal occipital protuberance. The midpoint of this line lies within the fourth ventricle near its floor.

CT and MRI (see Figs 2. 3, 2. 7 and 2. 19) The ventricular system can be visualized on axial CT and MR scans. Starting on the lowest cuts (see Fig. 2. 3a), the fourth ventricle can be seen as a slit-like CSF-filled structure between the brainstem and the cerebellum. Sections taken through the midbrain (see Fig. 2. 3b) may show the aqueduct w i th high-attenuation periaqueductal grey matter. The third ventricle becomes visible on higher cuts (see Fig. 2. 3c) as a slit-like space between the thalami. At this level, the anterior horns of the lateral ventricles can be seen separated by the septum pellucidum, w i th the head of the caudate nucleus indenting their lateral wall.

The posterior horns are visible also on this section and calcified choroid plexus is commonly seen in the trigone of the lateral ventricles. The temporal horns are small or not visible unless they are dilated. Higher cuts show the bodies of the lateral ventricles separated by the septum pellucidum or by the corpus callosum. The body and tail of the caudate nucleus can be seen as a high-attenuation structure in its lateral wall:

Sagittal MR images show the third ventricle, aqueduct and fourth ventricle in continuity w i th each other and w i th the central canal of the medulla and of the spinal cord. The

communication of the fourth ventricle w i th the cisterna magna at the foramen of Magendie can also be seen. The recesses of the third ventricle can be seen particularly well on this view (see Fig. 2. 19).

Ultrasound examination of the neonatal brain (see Fig. 2. 8) In anterior coronal views, the frontal horns of the lateral ventricles are seen as triangular in cross-section, with the floor and lateral wall indented by the head of the caudate nucleus. The echogenic roof is formed by the corpus callosum.

The bodies of the lateral ventricles are also approximately triangular in cross-section, indented inferolaterally by the thalamus medially and the body of the caudate nucleus laterally. Scans through the posterior part of the body of the lateral ventricle show echogenic divergent bands of the glomus of the choroid plexus. The fluid-filled ventricle around the choroid plexus is not always seen. The calcar avis may be seen indenting the medial wall of the ventricle. The mean width of the lateral ventricles is measured at the level of the body and is 9 mm in a 30-week premature infant and 12 mm in the full-term infant.

Parasagittal scans show the full sweep of the lateral ventricle, w i th the choroid plexus in the floor of the body and in the roof of the inferior horn. The head of the caudate nucleus can be identified anteriorly, the thalamus posterior to this and the thalamocaudate notch between.

The third ventricle may be seen as a slit-like space between the thalami. If not enlarged, this space may not be visible on a normal scan. Superolateral communications w i th the lateral ventricles at the foramina of Monro can be identified. The echogenic choroid plexus of the third ventricle can be seen in the roof of the third ventricle. The interthalamic adhesion may be outlined by echo-free CSF.

Midline sagittal scanning identifies the cavum septi pellucidi and vergae above the third ventricle, between the frontal horns and bodies of the two lateral ventricles. These are, in fact, a single structure w i th that part posterior to the foramen of Monro being named the cavum vergae. They are not normally connected w i th the remainder of the ventricular system and the term ' f i f th ventricle', which is sometimes used, is therefore misleading. The cavum vergae begins to close from posterior to anterior in the sixth gestational month and progresses to complete obliteration of both spaces by 2 months of age in 85% of infants.

The aqueduct is seldom visible on the normal scan but the echogenic cerebellar vermis helps identify the roof and cavity of the fourth ventricle.

Pneumoencephalography and air or contrast ventriculography These have been replaced by CT, MRI and ultrasound scanning for visualization of the ventricles.

The subarachnoid cisterns (Fig. 2. 19) Because the brain and the internal contours of the skull show marked differences in shape, the subarachnoid space is deep in several places, particularly around the base of the

Fig. 2. 19 The subarachnoid cisterns.

1. Suprasellar cistern 2. Pontine cistern 3. Cavum velum interpositum 4. Quadrigeminal plate cistern 5. Interpeduncular cistern 6. Cisterna magna

brain. These deep spaces are called cisterns and are named according to nearby brain structures. They contain more CSF than the ventricles.

Cisterna magna (Cerebellomedullary cistern) This lies behind the medulla and below the cerebellar hemispheres and receives CSF from the median aperture of the fourth ventricle. It is continuous through the foramen magnum w i th the spinal subarachnoid space, and continues superiorly for a variable distance behind the cerebellum. On each side its lateral part contains the vertebral artery and its posterior inferior cerebellar branch.

Pontine cistern

This lies between the pons and the clivus and is continuous below with the cisterna magna and above w i th the interpeduncular cistern. It receives CSF from the lateral apertures of the fourth ventricle. The pontine cistern contains the basilar artery and its pontine and labyrinthine branches.

Interpeduncular cistern This cistern lies between the cerebral peduncles of the midbrain and the dorsum sellae. It is continuous below w i th the pontine cistern, laterally w i th the ambient cisterns and superiorly w i th the suprasellar cisterns. It contains the posterior part of the arterial circle of Willis.

Quadrigeminal cistern This cistern lies posterior to the quadrigeminal plate of the midbrain. It lies between the splenium above and the vermis below and is limited posteriorly by the tentorium and falx. Also called the cistern of the great vein, it contains the venous confluence of the internal cerebral veins and the basal veins to form the great cerebral vein (of Galen). This unites w i th the inferior sagittal sinus to form the straight sinus.

Cistern of the velum interpositum (Figs 2. 19 and 2. 20) The space between the layers of the tela choroidea as it passes beneath the splenium of the corpus callosum and the fornix may contain CSF and may be called the cistern (or cavum) of the velum interpositum. It is continuous posteriorly w i th the quadrigeminal plate cistern through a narrow slit under the splenium called the transverse fissure. It has a triangular shape on axial sections. The cistern of the velum interpositum contains the internal cerebral veins.

Ambient cisterns

These extend around both sides of the midbrain between the interpeduncular cistern anteriorly and the quadrigeminal cistern posteriorly. Lateral extensions of the superior part of the ambient cisterns around the posterior part of the thalamus are called the ambient wing cisterns. The posterior cerebral artery and the basal vein lie in the anterior part of each ambient cistern.

Suprasellar cisterns This cistern is above the pituitary fossa. It lies between the anterior part of the floor of the third ventricle above and the diaphragma sellae below. It is continuous posteriorly w i th the interpeduncular cistern and extends laterally into the sylvian cistern at the lower end of the sylvian fissure. The optic nerves pass to the chiasm in the anterior part of the suprasellar cistern, and the part of the cistern anterior to this is sometimes referred to as the chiasmatic cistern. The anterior part of the circle of Willis lies in the suprasellar cistern.

Pericallosal cistern

The suprasellar and chiasmatic cisterns continue superiorly as the cistern of the lamina terminalis and around the superior surface of the corpus callosum as the pericallosal cistern. This is in turn continuous posteriorly w i th the quadrigeminal cistern below the splenium. The pericallosal cistern contains branches of the anterior cerebral artery.

Radiological features of the subarachnoid cisterns

Pneumoencephalography In the past the brain was imaged by viewing air in the cisterns. This is now replaced by CT and MR scanning.

CT and MRI

The cisterns are visible on axial CT and MR scanning as CSF-filled spaces between parts of the brain and the skull. On slices through the midbrain (see Fig. 2. 3b) the interpeduncular, ambient and quadrigeminal cisterns are seen around the midbrain. Anterior to the interpeduncular cistern is the suprasellar cistern and posterior to the quadrigeminal cistern is the superior vermian cistern. On cuts at the level of the superior part of the lateral ventricles the pericallosal cistern can be seen between the falx and the genu of the corpus callosum anteriorly, and between the splenium and the falx posteriorly. The cistern of the velum interpositum may be seen as a triangular collection of fluid between the posterior parts of the ventricles inferior to the splenium.

The marked difference in relaxation constants in MR imaging between water and brain tissue results in particularly good demonstration of small structures in cisterns, where they are surrounded by CSF. Thus the optic nerve and chiasm in the suprasellar cistern and the acoustic nerve in the pontine cistern and vessels in all cisterns are seen well on T2-weighted MRI.

Cervical myelography Cisternal puncture may be used to introduce contrast medium into the cisterna magna and the spinal subarachnoid space. A needle is introduced above level C2 and directed upwards. At a depth of about 3 cm the needle is felt to penetrate the atlanto-occipital membrane and to enter the cisterna magna. Structures at risk of injury by this procedure include the spinal cord and the medulla oblongata anterior to the cistern, and the posterior inferior cerebellar artery and its branches within the cistern.

Cerebrospinal fluid production and flow

The total volume of CSF is about 150 mL, 25 mL of which is within and around the spinal cord. It is produced at 0. 4 mL/minute independent of CSF pressure. CSF is produced by the choroid plexuses of all ventricles, but principally by that of the lateral ventricles. It flows through the interventricular foramina into the third ventricle, through the cerebral aqueduct to the fourth ventricle, and from the ventricular system via the midline aperture into the cisterna magna and via the lateral apertures into the pontine cisterns. Some diffusion occurs w i th fluid from the spinal canal.

From the basal cisterns, some fluid flows down and bathes the spinal cord; the remainder passes upward through the tentorial hiatus and diffuses over the surface of the cerebral hemispheres. Pulsation of arteries within the cisterns may play a role in the directional flow of CSF.

CSF is absorbed through the arachnoid v i l l i. These are herniations of arachnoid through holes in the dura into the venous sinuses. They are most numerous in the superior sagittal sinus and in its laterally projecting blood lakes. In children these v i l li are discrete; w i th age they aggregate into visible clumps called arachnoid granulations (pacchionian granulations). These indent the inner table of the skull beside the dural venous sinuses.

About one-third of the CSF is either absorbed along similar spinal v i l li or escapes along nerve sheaths into the perineural lymphatics. This absorption is passive and dependent on hydrostatic pressure differences.

Radiological features of CSF production and flow

Knowledge of the circulatory pathway of CSF is important in diagnosis as ventricular dilation proximal to a point of obstruction of its flow indicates the site of the lesion. Thus a lesion obstructing the cerebral aqueduct causes dilatation of the lateral and third ventricles but not the fourth. Arachnoiditis blocking the exit foramina of the fourth ventricle causes dilatation of all four ventricles.

Skull radiographs Arachnoid (pacchionian) granulations are seen as relative translucencies or small bony defects. Tangential views show that they are indentations of the inner table only. They are most frequently found along the superior sagittal sinus but are also seen around the torcula (cf. venous sinuses). Calcium deposition can occur at the arachnoid granulations and may cause calcification, which is visible on plain films at these sites.

Radionuclide cisternography The radionuclide is injected into the CSF via the lumbar route. After 1-3 hours the isotope can be seen in the basal cisterns - the cisterna magna, the pontine and interpeduncular cisterns and the quadrigeminal cistern. After 3-6 hours the radionuclide is seen in the sylvian and interhemispheric fissures. By 24 hours activity surrounds the brain. In children the CSF flow is more rapid, w i th the basal cisterns being reached at 15-30 minutes and isotope surrounding the brain at 12 hours. At no time is activity normally seen within the ventricles.

Magnetic resonance CSF flow studies Phase contrast MR CSF flow studies w i th cardiac gating can be used for qualitative and quantitative assessment