Microstructure Imaging by Diffusion MRI

Diffusion weighted imaging provides a unique tool to interrogate the microstructure of living tissue without the need for an invasive procedure. For timescales applicable in a clinical MRI setting, the diffusion process is sensitive to the structural configuration of tissue on the micrometer scale, i.e., the size of cells. Features such as the packing density of the cell matrix affect the overall rate of diffusion, and anisotropic structures impose barriers on the diffusion such that they appear to move faster or slower along certain directions, for example, along and across axonal bundles, respectively. In this chapter we survey the foundation for the diffusion MRI contrast, and discuss how it assumes features depending on the tissue microstructure and give examples of how these can be quantified. Due to their prevalence in neuroimaging, we focus on diffusion weighted imaging (DWI) and diffusion tensor imaging (DTI). To emphasize that diffusion MRI is evolving rapidly, we also look ahead to more advanced methods for analysis and alternatives to the conventional experimental design.