FIG. 8. The Standard Model of diffusion in neuronal tissue, Eq. (3.4).

In the t → ∞ regime (iii), elementary fiber segments (fascicles), consisting of intra- and extra-neurite compartments, are described by at least 4 independent parameters: f, D_a, $D_e^{||}$ and $D_e^{\perp }$. CSF can be further added as the third compartment, cf. Eq. (3.5). Within a macroscopic voxel, such segments contribute to the directional dMRI signal according to their ODF $\mathcal{P}(\widehat{\mathbf{n}})$.