Origin of B0 orientation dependent R2(*) (=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.