Universal power-law scaling of water diffusion in human brain defines what we see with diffusion MRI

Here we identify a universal power-law scaling behavior of the diffusion MRI signal on a clinical scanner. This specific functional form provides a defining signature of water confined within narrow sticks establishing that exchange between intra- and extra-axonal water is not relevant, and the fraction of fully restricted water is negligible in the clinically accessible regime. The observed scaling for the first time in vivo validates the key ingredient specific to the microstructural models of MRI signal from neuronal tissue and enables the in vivo quantification of intra-axonal properties.