The apparent orientation dependence of MTR, ihMTR and MWF is affected by local microstructural properties in white matter

We measured MTR, ihMTR, MWF and fibre orientation from DTI in 17 white matter regions in 17 healthy adults at 3 T. All three metrics showed an apparent orientation dependence: MWF and ihMTR were lower in fibres perpendicular to B0 by 6% and 1% respectively compared to those parallel, while MTR was lower by 0.5% at ~40°, with the highest values in fibres perpendicular to B0. However, separating the apparent orientation dependence by region revealed large variation in the trends, suggesting that real differences in myelination and other microstructural properties are confounding the apparent orientation dependence measured using this method.