Abstract
Diffusion MRI is a technique that enables the evaluation of white matter microstructure by utilizing water molecule diffusion, and its utility as an early diagnostic and prognostic marker in various neuropsychiatric disorders has been suggested. However, conventional white matter microstructure models are known to be unable to accurately estimate fiber structures when multiple fiber bundles exist within a single voxel. Recently developed fibre orientation distribution (FOD) has gained attention as a model that can distinguish multiple fiber directions within a single voxel using spherical deconvolution methods, overcoming this limitation. This paper provides an overview of the basic principles and limitations of conventional microstructure models using diffusion MRI, and the development of FOD. We also introduce Fixel‐based analysis, a white matter structure evaluation method based on FOD, and discuss its utility in neuropsychiatric disorders. Looking ahead, we anticipate that the application of diffusion MRI quantitative techniques to large‐scale cohort data of thousands of cases, such as Brain/MINDS Beyond, will accelerate our understanding of the pathophysiology of neuropsychiatric disorders and the development of novel biomarkers.
