Associations of local white matter geometry with network efficiency, macrostructural abnormalities, and clinical severity in behavioural variant frontotemporal dementia.

Behavioural variant frontotemporal dementia (bvFTD), marked by profound changes in behaviour and personality, is the most common subtype of frontotemporal dementia, driven by neurodegeneration in frontotemporal regions. This neurodegeneration pattern is partially shaped by white matter abnormalities arising from the spread of protein aggregates along axonal pathways. While prior studies mainly focused on diffusion tensor imaging metrics such as fractional anisotropy and mean diffusivity, the alteration in local white matter geometry remains largely unexplored. Using a novel Director Field Analysis (DFA) method, 51 patients with bvFTD and 51 healthy controls were studied to examine alterations in the local geometry of white matter fibres in bvFTD, and their associations with macrostructural morphology, global network parameters, and clinical manifestations. Unlike the unidirectional decrease in fractional anisotropy and increase in mean diffusivity, we identified significant bidirectional alterations in white matter local geometry, characterized by increased geometric distortion in the forceps minor and dorsal cingulum and decreased distortion in widespread frontotemporal association tracts, including the inferior fronto-occipital fasciculus, superior longitudinal fasciculus, uncinate fasciculus, frontal aslant tract, and arcuate fasciculus. Patients with bvFTD also showed reduced cerebral white and grey matter volumes (both