Sizes and Shapes of Fluid Channels in Brain Cortex

The flow of the cerebrospinal fluid (CSF) through perivascular spaces (PVSs) that surround vessels in the brain is a crucial part of the glymphatic system, which clears metabolic waste and might someday be used for drug delivery. The failure or abnormal functioning of the glymphatic system has been linked to neurodegenerative diseases like Alzheimer’s. The size and shape of the PVSs strongly affect the velocity and pressure of CSF, since hydraulic resistance scales inversely with PVS width to the fourth power. Existing descriptions of PVS shape use highly idealized geometries based on only a few in vivo observations. We characterized surface vessels and PVSs by quantitatively describing their sizes and shapes at 26 locations, in 15 different mice, from thousands of three-dimensional, in vivo images. We present a new idealized geometry for surface PVSs and validate the idealized geometry by comparing the PVS-vessel area ratio and hydraulic resistance for the idealized geometry and the original shape of the perivascular space obtained directly from the imaging data.