Cerebrospinal Fluid
Section IV. Perivascular Spaces (PVSs) and Arachnoid Granulations (AGs)
PVSs can be a route of pathologic spread, can be confused with pathology such a lacunar infarct, can become prominent in some conditions, and have functional significance for dynamic fluid transfer via the glymphatic system. The glymphatic system has analogous function of the lymphatic system of the body with increasing evidence of relevance to pathologic conditions such as Alzheimer’s disease.
Some authors categorize perivascular spaces as Type 1 (basal perforating arteries), Type 2 (perforating medullary arteries), or Type 3 (midbrain arteries). Occasionally, PVSs can have some surrounding T2 hyperintense signal, which could be confused for other pathology, including lacunar infarct or toxic/metabolic signal change of the globus pallidi.
Prominent PVSs
In some articles/texts, perivascular spaces in this location are categorized as type II, associated with perforating medullary arteries. Such prominence of PVSs has been proposed as a marker of glymphatic system dysfunction.
Prominent PVSs
Prominent perivascular spaces throughout the basal ganglia. In this location, these can be categorized as type I, associated with lenticulostriate arteries.
Artery in Large PVS
Example of a large PVS in the right basal ganglia. Well-circumscribed rounded CSF density area in the inferior 1/3 of the basal ganglia without surrounding T2 hyperintensity supports a PVS rather than chronic lacunar infarct (although these features are not perfect discriminators, surrounding gliosis becomes more likely with increasing PVS size). Note the vascular flow void centrally.
Artery in Large PVS
Coronal MRA in the same patient shows the perforating artery within the PVS. Usually the artery is not well-visualized.
Special Case
Prominent perivascular spaces in the subcortical space can be seen and can have associated surrounding T2 hyperintensity,. This may be particularly prominent in the anterior temporal lobe and could be confused with pathology.
Rawal S. et al. Subcortical Cystic Lesions within the Anterior Superior Temporal Gyrus: A Newly Recognized Characteristic Location for Dilated Perivascular Spaces. AJNR 2014;35(2):317-22
Prominent PVSs with Tuberous Sclerosis
Prominent PVSs can be associated with a variety of conditions. In this case, there are scattered prominent white matter PVSs in a patient with tuberous sclerosis. A broadened gyrus with T2 hyperintense signal consistent with a cortical tuber is seen (red arrow).
There are also a variety of normal variant cysts that are useful to recognize that are not PVSs.
Choroidal Fissure Cyst
The choroidal fissure at this level is located superior to the hippocampus, spanning between the temporal horn of the lateral ventricle and the basilar cisterns.
Arachnoid Granulations
AGs at the Vertex
Bilateral AGs along the vertex near the superior sagittal sinus. There is some intermediate signal within the AGs that are not contiguous with adjacent brain parenchyma.
AGs of the Spine
AGs are not only found intracranially, they also exist in the spine. There, they are most numerous in the thoracic levels extending into adjacent veins, but are too small too visualize on imaging. A substantial portion of CSF resorption seems to occur via AGs in the spine rather than solely the intracranial routes.
Brain Herniation into AGs (BHAGs)
BHAG
A prominent AG of the posterior fossa demonstrates brain herniation into the AG. In our experience, this is most common in the posterior fossa and can be associated with encephalomalacia of the herniated cerebellum and adjacent parenchyma. This could be confused with dural venous sinus thrombosis with prior associated infarct if involving the transverse sinus.
For more information, see:
Liebo G et al. Brain herniation into arachnoid granulations: clinical and neuroimaging findings. J Neuroimaging 2016;26(6):592-598
Spinal AGs
AGs also occur in the spine along the nerve root sheaths, most commonly in the thoracic spine. These presumably facilitate CSF resorption similar to intracranial AGs. The presence of normal spinal AGs could potentially help account for the development of pathologic CSF-venous fistulas in some patients with spontaneous intracranial hypotension.