Your equation is not quite right. We implemented our multi-compartment in a completely hierarchical fashion:
At the top level, we split the total signal into two
1) isotropic fraction (Virfrac) + non isotropic fraction (1 - Virfrac)
Next, we split the isotropic fraction into two
2) free diffusion (Viso) + non free diffusion (1 - Viso)
Finally, we split the non free diffusion into two
3) intra-cellular diffusion (Vic) + extra-cellular diffusion (1 - Vic)
extra-cellular diffusion is modelled as the hindered diffusion (with a diffusion tensor); there isn't a separate V(hindered).
So theoretically, Virfrac, Viso, and Vic can be equal to 1 all at the same time, or all equal to 0.