Bundle-specific associations between white matter microstructure and Aβ and tau pathology at their connecting cortical endpoints in older adults at risk of Alzheimer’s disease

Beta-amyloid (Aβ) and tau proteins, the pathological hallmarks of Alzheimer’s disease (AD), are believed to spread through connected regions. Combining diffusion imaging and positron emission tomography, we investigated associations between Aβ, tau and white matter microstructure specifically in bundles connecting brain regions in which AD pathology accumulates. In 126 cognitively normal elderly at risk of AD, we focussed on free-water corrected diffusion measures in the cingulum, posterior cingulum, fornix and uncinate fasciculus. We found higher tissue fractional anisotropy and lower mean and radial diffusivity related to increased Aβ at the cortical endpoints of the cingulum and fornix. We observed similar but stronger associations in the uncinate fasciculus, but with increased Aβ and tau at the endpoints of this bundle. This consistent pattern of associations, with opposite directionality to the usual degeneration pattern in symptomatic individuals, suggests more restricted diffusion in bundles vulnerable to preclinical AD pathology.