Brainstem

Each crus extends posteriorly to below the splenium of the corpus callosum, where it connects w i th that of the other side as the commissure of the fornix. Together the two crura form the body of the fornix. The body of the fornix extends anteriorly around the upper surface of the thalami. It is attached posteriorly to the undersurface of the corpus callosum and anteriorly to the inferior border of the septum pellucidum.

Above the interventricular foramen the body divides into two columns, which pass inferiorly between the foramen and the anterior commissure and form the anterior border of the interventricular foramen. The columns then pass to the hypothalamus and the mamillary bodies, in which they terminate. Some fibres from the columns of the fornix, the habenular fibres, turn back over the thalamus to join posteriorly in the habenular commissure.

Radiological features of the limbic lobe

Magnetic resonance imaging The body of the fornix is seen in midline sagittal MRI (see Fig. 2. 5), extending from the undersurface of the corpus callosum posteriorly to the interventricular foramen anteriorly along the undersurface of the septum pellucidum.

Fine cut coronal MRI perpendicular to the axis of the hippocampus and angled axial and sagittal images along its long axis are used to image the hippocampus, especially in the evaluation of epilepsy (see Fig. 2. 13).

Ultrasound examination of the neonatal brain (see Fig. 2. 8) The cingulate gyrus can be identified surrounding the corpus callosum. This is absent if the corpus callosum is absent. The body of the fornix can be seen below the septum pellucidum on midline sagittal images. In the coronal plane at the level of the third ventricle the C-shaped echoes of the parahippocampal gyrus are used as a far-field landmark.

THE BRAINSTEM (see Fig. 2. 10) The brainstem connects the cerebral hemispheres w i th the spinal cord and extends from just above the tentorial hiatus to just below the foramen magnum. It is bounded anteriorly by the clivus-basisphenoid above and the basiocciput below. The pons, the widest part of the brainstem, also grooves the apex of the petrous temporal bone on each side. This bony anterior boundary can cause an artefact in CT scanning of the brainstem, although it does not interfere w i th MRI studies.

The brainstem has three parts: from superior to inferior, the midbrain, the pons and the medulla.

Midbrain

External features

Anteriorly two cerebral peduncles are seen separated by the interpeduncular fossa. These peduncles (crura cerebri) carry fibres from the internal capsule to the pons. The floor of the interpeduncular fossa is the posterior perforated substance.

The posterior surface of the midbrain presents four rounded prominences - the corpora quadrigemini or the superior and inferior colliculi. Each superior colliculus is joined by a superior brachium to the lateral geniculate body of the optic tract. Each inferior colliculus is joined by an inferior brachium to the medial geniculate body of the auditory system.

Below the colliculi, the superior cerebellar peduncles converge to form the superior boundary of the fourth ventricle. A thin sheet of white matter between these is the superior medullary velum.

Internal features (Fig. 2. 14) Cerebral peduncles have a ventral part, the crus cerebri, which contains motor corticospinal tracts, and a dorsal part, the tegmentum. These are separated by the substantia nigra. Dorsal to the substantia nigra at the level of the superior colliculi, are paired red nuclei, which lie close to the median plane. These are part of the extrapyramidal pathway. That part of the midbrain posterior to the aqueduct is called the tectum or quadrigeminal plate. Cranial nerves The third (III) cranial nerve emerges from between the cerebral peduncles anteriorly. It passes between the posterior cerebral and superior cerebellar arteries and then lies lateral to the posterior communicating artery, where an aneurysm may compress it.

The fourth (IV) cranial nerve emerges from the dorsal surface of the midbrain and winds around its lateral aspect. It is the only cranial nerve to emerge from the posterior surface of the midbrain. As it curves around the midbrain it also lies between the posterior cerebral and superior cerebellar arteries.

Blood supply Central branches of the superior cerebellar artery supply the midbrain.

Posterior mesencephalic veins curve around each side of the midbrain towards the great cerebral vein.

Radiological features of the midbrain

CT and MRI The midbrain is seen on axial CT and MRI at the level of the circle of Willis (see Fig. 2. 3b). The crura can be identified anteriorly and the colliculi posteriorly. It is not always possible to identify the cerebral aqueduct.

On contrast-enhanced CT the posterior cerebral arteries or the posterior mesencephalic veins may be seen passing around the midbrain.

Ferritin deposition in the substantia nigra and red nuclei may lead to a lower signal intensity here than in the remainder of the crus cerebri on 'T1 and T2-weighted' MRI (see Fig. 2. 14).

Pons

External features The bulbous anterior part of the pons consists mainly of cerebellopontine fibres continuous on each side w i th the middle cerebellar peduncle. This part also transmits corticospinal fibres continuous w i th the cerebral peduncles of the midbrain. A shallow groove is seen in the midline. The basilar artery may lie in this groove, but often lies lateral to it.

The posterior surface of the pons forms the upper part of the floor of the fourth ventricle.

Cranial nerves

The fifth (V) cranial nerve emerges from the anterolateral surface of the pons. There are four trigeminal nuclei, two in the pons, a mesencephalic nucleus in the midbrain, and a spinal nucleus in the medulla and upper cervical cord.

The sixth (VI) cranial nerve emerges at the junction w i th the medulla, close to the midline anteriorly.

The seventh and eighth (VII and VIII) cranial nerves emerge at the junction w i th the medulla laterally, that is, at the cerebellopontine angle.

Blood supply The pons is supplied by pontine branches of the basilar artery.

Blood from the pons drains to the anterior pontomesencephalic vein and the inferior petrosal sinus.

Radiological features of the pons

CT and MRI (see Fig. 2. 3a) The bony anterior relations of the pons - the clivus centrally and the petrous temporal bones laterally - may cause considerable artefact on CT scanning of this area. For this reason MRI is superior.

On CT scans, the cerebellopontine fibres of the middle cerebellar peduncle can be seen as an area of low attenuation extending from the pons into each cerebellar hemisphere. On higher cuts the superior cerebellar peduncle forms the lateral border of the fourth ventricle.

The trigeminal nerves can be seen entering its anterior surface and the seventh and eighth cranial nerves can be seen exiting at the cerebellopontine angle.

The basilar artery may be seen anterior to the pons. A position to the right or left of the midline is not abnormal. Asymmetry of the cerebellopontine cisterns, however, is usually associated w i th pathology.

Medulla oblongata

External features

Anteriorly, the ventral median fissure is deep in its superior part. A ridge on each side of this fissure is formed by pyramidal fibres and is called the pyramid. These fibres decussate in the lower medulla, obliterating the median fissure here. Lateral to the pyramid in the upper medulla is an oval bulge called the olive. Lateral to the olive lies the inferior cerebellar peduncle - known as the restiform body - joining the medulla to the cerebellum.

Posteriorly, the upper part of the medulla is open as the floor of the fourth ventricle, whereas its lower part is closed around the central canal. Dorsal median and lateral sulci define four columns below the ventricle - gracile columns medially and cuneate columns laterally. A correspondingly named tubercle is found at the upper end of each column.

Cranial nerves

The ninth and tenth (IX and X) cranial nerves emerge posterior to the olive.

The eleventh (XI) cranial nerve has medullary rootlets that arise inferior to those of X, and cervical rootlets that arise from the upper spinal cord and enter the foramen magnum to unite w i th the medullary roots.

The twelfth (XII) cranial nerve arises from several rootlets anterior to the olive, in line w i th ventral spinal roots.

Blood supply The vertebral and basilar arteries supply the anterior part of the medulla, whereas the posterior inferior cerebellar artery supplies its posterior part.

Venous drainage is to the occipital sinus posteriorly and the inferior petrosal sinus anteriorly. Inferior medullary veins communicate w i th spinal veins.

Radiological features of the medulla oblongata

CT and MRI

Sections through the lower medulla are difficult to distinguish from those through the upper cervical cord unless the atlas or foramen magnum can be identified. The upper medulla can be identified easily by the presence of the fourth ventricle and cerebellum posteriorly.

Magnetic resonance imaging of the medulla gives superior images because of the lack of bony artefact (see Fig. 2. 5). The ventral sulcus can be seen anteriorly on axial sections w i th the pre- and postolivary sulci laterally. The