Revealing the nature of microstructural correlations along axons in human brain white matter with time-dependent diffusion

We explore axial diffusivity dependence on both diffusion time and gradient pulse width in major white matter tracts. This allows us to differentiate between two possible arrangements of restrictions (e.g. beads) along fibers: (1) short-range disorder, or (2) “hyperuniform” disorder (arrangement qualitatively closer to periodic). Unexpectedly, model prediction for hyperuniform disorder is more consistent with our data than for short-range disorder. If conformed histologically in human or animal studies, this would mean that restrictions along axons are not “purely” randomly distributed but rather spatially correlated − perhaps, for optimizing physiological constraints.