Neuroimaging aspects and clinical significance of giant perivascular spaces in the brain.

The cerebral vasculature represents a major component of the brain, constituting 25–30% of the total brain volume (1) and providing essential support to the cells of the nervous system. The blood vessels penetrating or leaving the brain parenchyma present unique anatomical properties, including the presence of a fluid-filled perivascular compartment, known as perivascular space (PVS) or Virchow-Robin space. PVS physiological function is still not completely understood, but is thought to be related to the cerebrospinal/interstitial fluid circulation and the clearance of cerebral waste products. In fact, in recent years, PVS have been described as main pathways in clearance system models of the brain (2), and numerous studies reported the association between PVS alterations and several neurological diseases. As PVS follow the course of the penetrating blood vessels and have a tubular shape, they appear on magnetic resonance imaging (MRI) as lines or dots, based on the orientation of the PVS with respect to the image acquisition plane (parallel or perpendicular, respectively), and have a signal intensity similar to the cerebrospinal fluid (i.e., hypointense on T1-weighted images and hyperintense on T2-weighted images). PVS cross-sectional diameter is usually less than 2 mm and on MRI they are most commonly seen in the centrum semiovale and basal ganglia (3,4). Thanks to the improved resolution and signal-to-noise ratio, modern MRI scanners and sequences currently allow to visualize PVS in practically all individuals, including healthy young adults