Anatomy

The superficial sinovenous system consists of cortical veins, the SSS, the inferior sagittal sinus, the torcular Herophili (also known as confluence of sinuses), and the transverse and sigmoid sinuses. The straight sinus is functionally and clinically included with the deep sinuses, as it serves as the main outlet for the deep cerebral veins. The dural sinuses contain arachnoid villi and granulations for CSF uptake and communicate via emissary veins with meningeal veins and with extracranial veins in the scalp.

Both superior and inferior sagittal sinuses run within the falx cerebri. The SSS terminates directly into the right transverse sinus, while the inferior terminates into the straight sinus. Occasionally, SSS enters the confluence located at the internal occipital protuberance rather than transverse sinus. Within the confluence, the SSS and the straight sinus join the 2 transverse sinuses. The SSS increases progressively in caliber. In anatomic variations, the rostral SSS might be absent and replaced by 2 superior cerebral veins. The inferior sagittal sinus receives flow from corpus collosum and medial cerebral hemispheres. It might be too narrow to be visualized on MRI.

The transverse sinuses run laterally and anteriorly within the attached margin of the tentorium cerebelli. They originate at the occipital protuberance of the torcular Herophili, and, at the base of the petrous portion of the temporal bone, transverse sinuses curve to form the sigmoid sinuses. The transverse sinuses are of unequal size—the larger has direct connection with the SSS. In 20% of people, one of the transverse sinuses, usually the left, is hypoplastic.

Cavernous sinuses lie on either side of the sphenoid bone, extending superior and lateral to the sphenoid sinuses from the superior orbital fissure to dorsum sellae. Blood flow from facial, ophthalmic, retinal, cerebral, and meningeal veins terminates in the cavernous sinuses, which in turn drain into the petrosal sinuses. The superior petrosal sinus empties into the transverse or sigmoid sinuses, while the inferior drains into the sigmoid sinus or the internal jugular vein.

The straight sinus runs from the splenium of the corpus collosum to the occipital protuberance within the junction of the falx cerebri and the tentorium cerebelli. It is really an extension of the Great vein of Galen as it joins the inferior sagittal sinus. The straight sinus receives cerebellar veins and is often visualized superior to the cerebellum on midsagittal MRI.

Superficial cerebral veins receive blood from superficial medullary veins that begin a few centimeters below the cortex and drain the cortex and adjacent white matter. The superficial cerebral veins are divided into 3 groups—dorsomedial, posterior-inferior, and anterior—based on the sinus into which they empty. The superficial venous system is variable; any of the 3 components might be absent or hypoplastic.

The deep venous system is less variable and includes internal cerebral veins, which receive flow from septal and thalamostriate veins, the basal veins of Rosenthal, the Great vein of Galen, and, functionally, the straight sinus. The deep venous system drains the deep white matter, periventricular regions, corpus collosum, and structures of the diencephalon. The basal veins of Rosenthal drain the inferior aspect of lentiform nuclei, midbrain, medial temporal, and occipital lobes. The internal cerebral veins course posteriorly in the roof of the third ventricle, beneath the corpus collosum, and around the splenium to join the basal veins of Rosenthal, forming the Great vein of Galen, which is often seen on midsagittal MRI. Blood flow within the vein of Galen runs posteriorly in the coronal plane, perpendicular to the flow in other veins, so it might not be well visualized on MRV. Venous drainage of posterior fossa structures is highly variable. The vein of Galen receives flow from dorsal cerebellum, vermis, and upper brainstem. The petrosal system and straight and transverse sinuses all receive drainage from the posterior fossa veins.

Cerebral veins and venous sinuses have no valves; therefore, blood can flow in either direction, depending on the pressure gradient. Extensive anastomosis exists among cortical veins, allowing for collateralization when a sinus is occluded. The vein of Trolad on the lateral surface of the hemisphere serves as a connection between the SSS and the superficial sylvian vein. The vein of Labbe connects the superficial sylvian vein to the transverse sinus.

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