The presence of micrometer-level restrictions leads to a decrease of diffusion coefficient with
diffusion time. Here we investigate this effect in human white matter in vivo. We focus on a
broad range of diffusion times, up to 600 ms, covering diffusion length scales up to about 30
μm. We perform stimulated echo diffusion tensor imaging on 5 healthy volunteers and
observe a relatively weak time-dependence in diffusion transverse to major fiber tracts.
Remarkably, we also find notable time-dependence in the longitudinal direction. Comparing …

The dependence of the diffusion MRI signal on the diffusion time t is a hallmark of tissue
microstructure at the scale of the diffusion length. Here we measure the time-dependence of
the mean diffusivity D (t) and mean kurtosis K (t) in cortical gray matter and in 25​ gray
matter sub-regions, in 10 healthy subjects. Significant diffusivity and kurtosis time-
dependence is observed for t= 21.2-100​ ms, and is characterized by a power-law tail∼ t−
ϑ with dynamical exponent ϑ. To interpret our measurements, we systematize the relevant …