Longitudinally Hindered Diffusion of In Vivo Human White Matter at Long Diffusion Time

Diffusion imaging has enormous potential for quantitative measurements of geometric properties that are relevant to brain anatomy. We present long diffusion time measurements of white matter in healthy volunteers showing that the apparent diffusion coefficient measured parallel to axons exhibits diffusion time dependence. We fit a nested series of models with varying complexity and characterized their dimensional appropriateness. A model representing axons as impermeable, parallel, prolate ellipsoids was robustly fit to the data. Although axons are known not to be cylindrical, the ellipsoids could represent microscopic wiggling of the axons along their trajectories or fanning around the (mean) longitudinal direction.

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