Virchow-Robin spaces (VR spaces), also known as perivascular spaces, are spaces surrounding the walls of arteries, arterioles, veins, and venules as they course from the subarachnoid space through the brain parenchyma.

These spaces do not communicate directly with the subarachnoid space but they are filled with interstitial fluid which behaves similarly to CSF in MR.

Dilated perivascular spaces consist of regular cavities containing an artery. The mechanism that occurs is not well known but there are many hypotheses in study.

Perivascular spaces are considered to be dilated when the diameter exceeds 2 mm, and are well visualized in T2 weighted images. Rarely the size of the space is so large that it produces a mass effect on the adjacent brain parenchyma or causes hydrocephalus. The prevalence of VR spaces increases with aging. In some cases this perivascular atrophy of cerebral tissue generates many lacunae and is called "état criblé".

In imaging VR spaces are mostly seen as well-defined oval, rounded, or tubular structures, depending on the plane in which they are intersected. They have smooth margins, commonly appear bilaterally, and usually measure 5 mm or less. On CT they appear as small rounded hypodense lesions (< 1 cm). In MRI they behave similar to CSF in all sequences but when the signal strengths are measured these have a lower intensity than the CSF and closer to the interstitial fluid. They produce no flow restriction in diffusion-weighted images and they don't enhance after the administration of contrast material.

The key to differentiate VR spaces from other pathologies is that VR spaces are hypointense on FLAIR unlike other entities in the differential diagnosis.

According to the location of Virchow-Robin spaces can be classified into four types: ^1,4

• Type I: Appear along lenticulostriate arteries through the basal ganglia. The example shown is classified into this group

• Type II: They are in the path of the medullary perforating arteries when they enter the gray matter and heading toward the white matter

• Type III: Appears in the midbrain mainly in the cerebral peduncles

• Type IV: Anterior temporal ⁴

The VR spaces may show atypical dilatations, making it difficult to differentiate with others cysts by image alone.

In summary, widened VR spaces are a common aging phenomenon which should not be confused with other diseases. FLAIR images showing its hypointense appearance is useful for us in the differential diagnosis.