Fig. 12.

Illustration of diffusion models and key concepts. The concept of compartmentalization was initially proposed by Stanisz et al. (a), following which the AxCaliber aims to estimate the diameter distribution of the restricted water compartment (b). Variations of the AxCaliber model was proposed, such as modifications to account for fiber dispersions (c) and crossings (d). Realistic axons are very different from ideal cylinders, instead, there are plenty of beadings and undulations (f) and structural disorder are commonly seen along neurites (g). In gray matter, a sphere (e) or dot (h) compartment is incorporated into the physical model to account for additional signal component present in the measurement. Figure adapted from the papers labeled therein.