Venous drainage of the brain

pulsation of the middle cerebral vessels can be seen in the insula. The anterior cerebral artery can be seen in the interhemispheric fissure on coronal scanning and, on parasagittal scanning, the pericallosal and callosomarginal arteries may be identifiable.

Radionuclide cerebral angiography During the arterial phase in the AP position a five-pointed star pattern is formed by the two carotid arteries, the two middle cerebral arteries and the two anterior cerebral arteries, which are superimposed upon each other.

VENOUS DRAINAGE OF THE BRAIN

The veins draining the central nervous system do not follow the same courses as the arteries that supply it. Generally, venous blood drains to the nearest venous sinus, except in the case of that draining from the deepest structures, which drain to deep veins. These, in turn, drain to the venous sinuses. The intracerebral veins do not have valves.

Venous sinuses (Fig. 2. 26) These are large low-pressure veins within the folds of dura - between fibrous dura and endosteum, except for the inferior sagittal and the straight sinuses which are between two layers of fibrous dura. They receive blood from the brain and the skull (diploic veins) and communicate w i th veins of the scalp and face (emissary veins).

The superior sagittal sinus starts anteriorly and runs posteriorly in the midline to the internal occipital protuberance. Veins enter the sinus obliquely against the flow of blood. Three or four venous lakes project laterally from the sinus between the dura and the endosteum. Into these the arachnoid (pacchionian) granulations and v i l li project to return CSF to the blood. Posteriorly the sinus turns to one side - usually the right - to become the transverse sinus.

The inferior sagittal sinus runs in the lower free edge of the falx cerebri. Posteriorly it joins w i th the great cerebral vein to become the straight sinus.

The straight sinus runs in the tentorium, where the falx is attached to it to the internal occipital protuberance - the confluence of the sinuses (torcula herophili). Here it turns to one side - usually the left - to become the transverse sinus.

The transverse and sigmoid sinuses - the transverse sinuses run right and left from the confluence of the sinuses to the mastoid bone, where they turn inferiorly and become the sigmoid sinus (the transverse and sigmoid sinuses are sometimes referred to together as the lateral sinus), which continues at the jugular foramen as the internal jugular vein. A focal dilatation of the vein within the foramen is called the jugular bulb.

The cavernous sinus (see Fig. 2. 11) - this sinus is on either side of the pituitary gland and the body of the sphenoid bone connected across the midline by intercavernous sinuses. It lies between layers of dura mater. The internal carotid artery passes through this sinus w i th the sixth cranial nerve lateral and inferior to it. The third and fourth cranial nerves and the ophthalmic and maxillary divisions of the fifth pass along the lateral wall of the cavernous sinus. After emerging from the sinus the carotid artery folds back on itself so that it comes to lie on the roof of the sinus. Above the sinus lies the optic tract. The cavernous sinus receives the ophthalmic vein, the sphenoid sinus and the superficial middle cerebral vein, and drains via the petrosal sinuses to the sigmoid sinus and the beginning of the internal jugular vein.

The superior petrosal sinus runs from the cavernous to the sigmoid sinus in the attached margin of the tentorium on the superior border of the petrous part of the temporal bone.

The inferior petrosal sinus runs from the cavernous sinus to the internal jugular vein at the base of the petrous temporal bone.

The sphenoparietal sinus runs along the free edge of the lesser wing of the sphenoid bone to the cavernous sinus. It may drain the anterior temporal diploic vein, a large vein in the wall of the middle cranial fossa, or the latter may drain separately to the cavernous sinus.

Superficial cerebral veins (see Fig. 2. 26) These veins are very variable. They drain to the nearest dural sinus - thus the superolateral surface of the hemisphere drains to the superior sagittal sinus and the posteroinferior aspect drains to the transverse sinus. These named veins are variably seen: • The superior anastomostic vein (of Trolard), which runs from the posterior end of the lateral sulcus posterosuperiorly to the superior sagittal sinus in the parietal region;

• The inferior anastomostic vein (of Labbe), which runs posteroinferiorly from the posterior end of the lateral sulcus to the transverse sinus; and • The superficial middle cerebral vein, which runs anteriorly along the lateral sulcus to drain via the sphenoid sinus to the cavernous sinus.

Deep cerebral veins (Fig. 2. 27) Several veins unite just behind the interventricular foramen (of Monro) to form the internal cerebral vein. The largest are: • The choroid vein, which runs from the choroid plexus of the lateral ventricle; • The septal vein, which runs from the region of the septum pellucidum in the anterior horn of the lateral ventricle; and • The thalamostriate vein, which runs anteriorly in the floor of the lateral ventricle in the thalamostriate groove between the thalamus and the lentiform nucleus.

The point of union of these veins is called the venous angle, which marks the posterior margin of the interventricular foramen.

The internal cerebral veins of each side run posteriorly in the roof of the third ventricle and unite beneath the sple¬ nium of the corpus callosum to form the great cerebral vein.

The great cerebral vein (of Galen) is a single short (1-2 cm), thick vein that passes posterosuperiorly behind the splenium of the corpus callosum in the quadrigeminal cistern. It receives the basal veins and posterior fossa veins and drains to the anterior end of the straight sinus where this unites w i th the inferior sagittal sinus.

The basal vein (of Rosenthal) begins at the anterior perforated substance by the union of three veins: • The anterior cerebral vein, which accompanies the anterior cerebral artery; • The deep middle cerebral vein from the insula; and • The striate veins from the inferior part of the basal ganglia via the anterior perforated substance.

The basal vein of each side passes around the midbrain to join the great cerebral vein.

Veins of the posterior fossa (Fig. 2. 27) The anterior pontomesencephalic vein runs on the anterior surface of the pons and midbrain in relation to the basilar artery. Inferiorly it drains via the petrosal veins to the superior petrosal sinuses. Superiorly this vein is connected to the posterior mesencephalic vein.

The posterior mesencephalic vein runs around the upper midbrain to drain into the great cerebral vein.

The precentral cerebellar vein arises between the cerebellum and the posterior midbrain in the midline and passes superiorly to drain to the great cerebral vein.

The superior vermian vein drains also to the great vein, and the inferior vermian veins (paired) drain to the straight sinus.

The pontine veins drain laterally to the petrosal sinuses. A midline pontine vein may exist which drains to the basal veins.

Inferior medullary veins drain to the sphenoid or petrosal sinuses.

Fig. 2. 28 Venous phase of internal carotid angiogram.

1. Sagittal sinus 2. Superficial cortical veins 3. Internal cerebral vein 4. Basal vein of Rosenthal 5. Great vein of Galen 6. Straight sinus 7. Torcula herophili (confluence of sinuses) 8. Transverse sinus 9. Inferior anastomotic vein of Labbe

Fig. 2. 29 MR venogram.

1. Superior sagittal sinus 2. Inferior sagittal sinus 3. Great vein of Galen 4. Straight sinus 5. Internal cerebral vein 6. Transverse sinus 7. Basal vein of Rosenthal 8. Sigmoid sinus 9. Internal jugular vein

Radiological features of the cerebral veins and venous sinuses

Skull radiographs The course of the venous sinuses can be seen where they groove the inner plate of bone. The arachnoid granulations may also indent the skull and show as lucencies on each side of the superior sagittal sinus on the skull radiograph ( see section on the radiology of the skull, page 5).

MR venography (see Fig. 2. 28) MR venography is the primary non-invasive method of demonstrating the cerebral veins and the venous sinuses. A two-dimensional time of flight technique is used, which is most sensitive to flow perpendicular to the imaging slice. Coronal scans are thus most useful for evaluation of the sagittal sinuses and the internal cerebral veins, but are l i mited for evaluation of the vertical posterior part of the superior sagittal sinuses and the sigmoid and petrous sinuses.

Angiography during the venous phase (Fig. 2. 29) Superficial veins are seen to f i ll before deep veins and frontal veins before posterior veins. Of the superficial veins the superior or inferior anastomotic vein are often visualized, but both are seldom seen on one angiogram. The deep veins are more constant than the superficial veins and before the widespread use of CT were used to visualize the ventricular system, w i th the thalamostriate vein indicating the size of the lateral ventricle and the internal cerebral veins indicating the position of the roof of the third ventricle. Similarly, a change in position of other deep veins may indicate the presence of a mass lesion nearby.

CT and MRI (see Fig. 2. 3) The tentorium and falx are seen as relatively hyperdense structures owing to the venous sinuses that they enclose. Venous sinuses may also be seen at the internal occipital protuberance. A high division of the superior sagittal sinus may occur as an unusual normal variant and should not be mistaken for sinus thrombosis. On axial imaging, at the level of the midbrain the posterior mesencephalic vein may be seen winding around the upper midbrain. In higher slices, the posterior end of the internal cerebral veins and the great cerebral vein may be seen in the quadrigeminal cistern.