Inter-subject variability in fiber orientation density function increases with aging

The fiber orientation density function (fODF) estimated from diffusion MRI is typically used as an input for fiber tractography or microstructural modeling. However, since it encodes the fine details of the intra-voxel distribution of axon orientations, it can also be regarded as a quantity of physical interest that is potentially sensitive to subtle white matter changes. Here we demonstrate a method for quantifying inter-subject fODF differences in individual voxels within a white matter skeleton for a cohort of healthy older adults. We find that inter-subject fODF variability increases with age suggesting that diverse processes affect axons in healthy aging.