Diffusion MRI is a unique imaging modality that enables us to probe the microstructural properties of tissues at orders of magnitude below nominal image resolution. Especially, biophysical modeling approaches are now attracting a lot of attention, as they provide quantities that are seemingly specific to microstructural properties, such as axon density. Among several models, neurite orientation dispersion and density imaging (NODDI) has become very popular and is extensively used in clinical research. However, to date, no definite consensus has been reached on a method to parameterize the complex microstructure of the brain white matter. Because diffusion biophysical modeling in clinical studies often lacks means of direct validation, we need to interpret the results from a critical viewpoint, with understanding of the behavior of the model when its constraints/assumptions are violated. This article aims to provide a brief overview of the models in current use, with focus on the model constraints/assumptions and their backgrounds.
Kamiya et al. (Mon,) studied this question.