Longitudinally Hindered Diffusion of In Vivo Human White Matter at Long Diffusion Time
Wilfred W Lam 1 , Karla L Miller 1 , Michiel Kleinnijenhuis 1 , and Saad Jbabdi 1
FMRIB Centre, University of Oxford, Oxford,
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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