Given the heterogeneous nature of biological tissues which exhibit a high degree of structural heterogeneity and complexity, it is well known that water diffusion in tissues does not follow a Gaussian distribution. The FROC diffusion model captures this tissue complexity by for characterizing not only the diffusion process itself (D), but also the tissue structures (β and μ) through which water molecules diffuse. The CTRW diffusion model, which is an extension to the FROC model, recognizes intravoxel diffusion heterogeneity in both time (i.e., water molecules can take a variable amount of time to make a move, which we call “water trapping”) and space (i.e., water molecules can diffuse with drastically different free length, which we call “water jumping”). These diffusion heterogeneities (α and β) can directly reflect intravoxel structural heterogeneity, which is related to tissue complexity and microenvironment.
Our group has been investigating the feasibility of FROC and CTRW diffusion models in probing the underlying tissue characteristics in different disease conditions and organs, including adult and pediatric brain tumors, gastric cancer, and bladder cancer.