T2*-based fiber orientation mapping

Recent MRI studies at high field have observed that, in certain white matter fiber bundles, the signal in T*(2)-weighted MRI (i.e. MRI sensitized to apparent transverse relaxivity) is dependent on fiber orientation theta relative to B-0. In this study, the characteristics of this dependency are quantitatively investigated at 7 T using ex-vivo brain specimens, which allowed a large range of rotation angles to be measured. The data confirm the previously suggested variation of R*(2) (=1/T-2*) with theta and also indicate that this dependency takes the shape of a combination of sin2 theta and sin4 theta functions, with modulation amplitudes (=Delta R*(2)) reaching 6.44 +/- 0.15 Hz (or Delta T*(2) = 2.91 +/- 0.33 ms) in the major fiber bundles of the corpus callosum. This particular dependency can be explained by a model of local, sub-voxel scale magnetic field changes resulting from magnetic susceptibility sources that are anisotropic. As an illustration of a potential use of the orientation dependence of R*(2), the feasibility of generating fiber orientation maps from R*(2) data is investigated. Published by Elsevier Inc.