Figure 1.

Schematic drawing of PVS around basal and cortical perforating arterioles. The perivascular spaces surround the cerebral small perforating vessels as they pass through the brain parenchyma from the subarachnoid space (SAS). The outer limits of PVS are the glia limitans of the underlying brain, and inner limits are the endothelial basement membranes of the vessel. The basal perforating arterioles are covered by two layers of leptomeninges, with the inner membrane tightly adhering to the arteriolar wall, and the outer layer extending from the pia mater (left). The spaces between these two meningeal membranes connect to the SAS directly (left). Cortical arterioles are surrounded by only one leptomeningeal layer, closely applied to the vessel wall, so the cortical perivascular spaces connect directly to the subpial space and indirectly to the SAS in some way (right).