Characterization of white matter myelinated and unmyelinated axons from diffusion MRI perspective

We segment all myelinated and unmyelinated axons from electron microscopy (EM) images of brain white matter (WM) of a mouse and a rat and quantify their cross-sectional morphology. We provide the exact relation between the long-time limit of the along-axon diffusion coefficients and the geometry of segmented intra-axonal spaces (IAS). By performing Monte Carlo (MC) simulations of diffusion in the IASs at varying diffusion times, we establish the time-dependence of diffusion along segmented myelinated and unmyelinated axons and validate the relation between axonal geometry and the long-time behavior of the longitudinal diffusion coefficient.