Fig. 1.

Multiple scales of intra-voxel heterogeneity are responsible for the observed non-monoexponential decay (Ax: axons with various degrees of myelination; As: Astrocyte; O: Oligodendrocyte). (a): Large scale heterogeneity includes the mixing of large scale microstructural environments (LSME) such as the mixing of multiple WM fascicles with extra-cellular space. (b): Furthermore, each LSME may contain a complex varying microstructure such as axons with varying radii and degrees of myelination. (c): At an even smaller scale, other biophysical mechanisms such as intracellular heterogeneities and the proximity of cell membranes that locally restricts motion may contribute to the signal decay behavior. For example, Sehy et al. [8] observed a non-monoexponential decay within the intracellular space of a single cell, the frog oocyte.