文摘
Recent MRI studies at high field have observed that, in certain white matter fiber bundles, the signal in T2-weighted MRI (i.e. MRI sensitized to apparent transverse relaxivity) is dependent on fiber orientation θ relative to B0. 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 R2 (= 1/T2*) with θ and also indicate that this dependency takes the shape of a combination of sin2θ and sin4θ functions, with modulation amplitudes (= ΔR2) reaching 6.44 ± 0.15 Hz (or ΔT2 = 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 R2, the feasibility of generating fiber orientation maps from R2 data is investigated.
