Diffusion MRI measures water diffusion in biological tissue, which can be used to probe its microstructure. The most common model for water diffusion in tissue is the diffusion tensor (DT), which assumes a Gaussian distribution. This assumption of Gaussian diffusion oversimplifies the diffusive behavior of water in complex media, and is known experimentally to break down for relatively large b-values. DT derived indices, such as mean diffusivity or fractional anisotropy, can correlate with major tissue damage, but lack sensitivity and specificity to subtle pathological changes. Microstructure Imaging of Crossing (MIX) is versatile and thus suitable to a broad range of generic multicompartment models, in particular for brain areas where axonal pathways cross.

Multicompartment models (assess the variability of diffusion in sub-voxel regions) enable the estimation of more specific indices and so potentially give much greater insight into tissue architecture.

Goal of Model Fitting: Identify which model compartments are essential to explain the data and parameters that are potentially estimable from a particular experiment and compare the models to each other using the BIC.