Probing gray matter microstructure differences in neurodegenerative dementia using b-tensor encoding diffusion MRI

Dementia is a major health concern in the aging population, with the number of people living with dementia expected to increase.Quantitative MRI techniques can offer information about microstructural changes in the brain, making them valuable tools to study and diagnose these diseases.This work studied the sensitivity of advanced diffusion MRI metrics obtained from b-tensor encoding (microscopic fractional anisotropy; isotropic kurtosis) to the microstructural changes of Alzheimer's disease, frontotemporal dementia, and Lewy body dementia.We complemented diffusion MRI with volumetric assessments and magnetization transfer MRI, which is sensitive to myelin, and focused on multiple gray matter regions known to be affected by these diseases.We performed an additional analysis in the hippocampus at the subfield level, given its central role in Alzheimer's disease, where certain subfields are known to be more strongly affected than others.We showed that the multidimensional space spanned by these metrics has distinct patterns for Alzheimer's disease and frontotemporal dementia compared to healthy controls, with microscopic fractional anisotropy contributing to differences between groups, particularly in the hippocampus.Accordingly, microscopic fractional anisotropy has the potential to complement other imaging markers of neurodegeneration in the study of dementia.