Orientation-dependent biases in powder averaging caused by inhomogeneous distributions of magnetic susceptibility in white matter

Numerical experiments show orientation dependency of the powder averaged diffusion signal (commonly assumed orientation-invariant) when susceptibility-induced inhomogeneity in white matter is taken into account. This implies an axon-orientation-dependent bias of the diffusion signal from white matter regions containing non-uniformly dispersed myelinated axons, which would lead to an over-estimation of the anisotropy. This implies a potential bias between the interpretation of the signal from eg. the internal capsule (oriented inferior-superior) and corpus callosum (oriented left-right).