The effect of inter-individual differences and number of fiber directions on possible orientation dependence of myelin water imaging

Previous work suggests myelin water fraction (MWF) may be fibre orientation dependent. We compared MWF to fiber angle (from diffusion tensor imaging (DTI)) in white matter (WM) from 16 healthy participants using BedpostX to estimate the number of fiber directions. MWF vs. fiber angle graphs showed trends which varied between participants, suggesting that real myelination differences may be at least partly responsible for apparent orientation dependence of MWF. There were little obvious differences between trends with 1, 2, or 3+ fiber orientations in WM, suggesting number of fibers directions within a voxel does not impact orientation dependence.