Multishell models

Multishell acquisitions can accommodate a broader range of models that relate the underlying white matter fibers to the diffusion-weighted signal. Broadly speaking, these models can be categorized into models of diffusion and models of fibrous tissue. A diffusion model merely describes the diffusion phenomenon within an MRI voxel without attributing the MRI signal to a particular substrate. While this relieves the user of deciding on a particular microstructural model, it makes these approaches highly unspecific. Models of fibrous tissue, on the other hand, break down the MRI signal in each imaging voxel into a sum of contributions from several compartments, where each compartment is assumed to correspond to a certain biological substrate. While these methods have to make potentially inaccurate assumptions about the nature and number of these compartments, they can provide much more specific information about the tissue under investigation.