White matter (WM) microstructural aging is often studied with diffusion MRI via voxelwise metrics such as fractional anisotropy (FA). Decreasing FA with advancing age is frequently interpreted as WM microstructural degeneration, but relying on FA for this purpose only makes sense if a voxel contains a single fiber tract. In this work we use fixel-based analysis (FBA) to investigate the association of fiber-specific measures with age among each individual "fixel" within a voxel. We find intra-voxel differences in age associations of crossing fixels, demonstrating that traditional voxel-based analyses are inappropriate for probing age-related fiber degeneration in scenarios of multi-fiber crossings.
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