It is well established that white matter diffusion fractional anisotropy (FA) is sensitive to age, but interpreting this effect in terms of cellular microstructure has posed a challenge. Here we investigate how age-related differences in FA relate to measures of crossing and dispersing fibres derived from advanced diffusion MRI models. We find that, in aging, decreased FA is associated with increased fibre dispersion, and increased FA is associated with selective degeneration of crossing fibres. Importantly, we show that age-related differences in FA more closely reflect age-related differences in fibre architecture when applying free water elimination.
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