Longitudinally hindered diffusion of in vivo human white matter at long diffusion time

Introduction Conventional diffusion MRI provides exquisite sensitivity to tissue microstructure through models of restricted and hindered diffusion within and around axons, respectively. These models often idealize axons as parallel, infinite and impermeable cylinders, where diffusion is often assumed to be free along the direction of axons. However, quantification of the degree of diffusion hindrance parallel to axon bundles, or the permeability of axon walls, may be possible with measurements at a range of diffusion times longer than those in typical experiments. At longer diffusion times, spins have an opportunity to probe longer length/time scales. Here, we present long diffusion time measurements of in vivo white matter using stimulated echoes and compare the fitting quality of successively simpler models of the diffusion attenuation accounting for overfitting.