Fig. 4.

Standard Model of diffusion in neuronal tissue (Novikov et al., 2019, Sec. 3). In the long time limit (iii), elementary fiber segments (fascicles), consisting of intra- and extra-neurite compartments, are described by at least 4 independent parameters: f, D_a, $D_e^{\parallel }$ and $D_e^{\perp }$, with a possible addition of the isotropic CSF compartment. Within a macroscopic imaging voxel, such segments contribute to the directional dMRI signal according to their orientational dispersion $\mathcal{P}(\mathrm{n})$.