Quantifying brain microstructure with diffusion MRI: Theory and parameter estimation

TL;DR

This paper reviews models of diffusion MRI in brain tissue, emphasizing three main approaches to quantify microstructure, including transient effects, long-time limit modeling, and advanced encoding techniques, to improve tissue characterization.

Contribution

It provides a comprehensive framework unifying diffusion MRI models in brain tissue and discusses future directions for quantitative microstructure markers.

Findings

Transient diffusion effects reveal microstructural length scales.

Parameter estimation faces degeneracies in multi-Gaussian models.

Multiple diffusion encoding techniques access additional tissue information.

Abstract

We review, systematize and discuss models of diffusion in neuronal tissue, by putting them into an overarching physical context of coarse-graining over an increasing diffusion length scale. From this perspective, we view research on quantifying brain microstructure as occurring along the three major avenues. The first avenue focusses on the transient, or time-dependent, effects in diffusion. These effects signify the gradual coarse-graining of tissue structure, which occurs qualitatively differently in different brain tissue compartments. We show that studying the transient effects has the potential to quantify the relevant length scales for neuronal tissue, such as the packing correlation length for neuronal fibers, the degree of neuronal beading, and compartment sizes. The second avenue corresponds to the long-time limit, when the observed signal can be approximated as a sum of multiple non-exchanging anisotropic Gaussian components. Here the challenge lies in parameter estimation and in resolving its hidden degeneracies. The third avenue employs multiple diffusion encoding techniques, able to access information not contained in the conventional diffusion propagator. We conclude with our outlook on the future directions which can open exciting possibilities for designing quantitative markers of tissue physiology and pathology, based on methods of studying mesoscopic transport in disordered systems.