Compartments of the diffusion model. We use a three compartment model that can describe diffusion MRI signals from various brain tissues namely CSF, white matter and gray matter. The isotropic compartment models unrestricted diffusion of water molecules outside of tissue (CSF) with a single free parameter . The intra-axonal compartment models the diffusion of water within axons as several sticks with identical parallel diffusivity parameter , and zero radial diffusivity, that are dispersed by a Watson distribution with orientation dispersion index . The extra-axonal compartment is also a Watson dispersed zeppelin with parallel diffusivity and perpendicular diffusivity . Including the signal fraction parameters () this model has 8 free parameters, which are more than that can be fitted to a conventional dMRI data.