Origin of B0 orientation dependent R2* (=Â 1/T2*) in white matter

Recent MRI studies have demonstrated that the relative orientation of white matter fibers to the B0 field significantly affects R2⁎ measurement. In this work, the origin of this effect was investigated by measuring R2 and R2⁎ in multiple orientations and fitting the results to magnetic susceptibility-based models and magic angle-based models. To further explore the source of magnetic susceptibility effect, the contribution of tissue iron to the orientation dependent R2⁎ contrast was investigated. Additionally, the effects of temperature on R2⁎ and orientation dependent R2⁎ contrasts were studied to understand the differences reported between a fixed specimen at room temperature and in vivo at body temperature. The results suggest that the B0 dependent R2⁎ variation is better explained by the magnetic susceptibility-based model with susceptibility anisotropy. However, extracting tissue iron did not reduce the orientation dependent R2⁎ contrast, suggesting iron is not the origin of the contrast. This leaves susceptibility effects from myelin as the most probable origin of the contrast. Temperature showed large contribution on both R2⁎ and orientation dependent R2⁎ contrasts, explaining a portion of the contrast difference between the in-vivo and in-vitro conditions.