Origin of B-0 orientation dependent R*(2) (=1/T*(2)) in white matter

Abstract

Recent MRI studies have demonstrated that the relative orientation of white matter fibers to the B-0 field significantly affects R*(2) measurement. In this work, the origin of this effect was investigated by measuring R-2 and R*(2) 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 R*(2) contrast was investigated. Additionally, the effects of temperature on R*(2) and orientation dependent R*(2) 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 B-0 dependent R*(2) variation is better explained by the magnetic susceptibility-based model with susceptibility anisotropy. However, extracting tissue iron did not reduce the orientation dependent R*(2) 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 R*(2) and orientation dependent R*(2) contrasts, explaining a portion of the contrast difference between the in-vivo and in-vitro conditions. (C) 2013 Elsevier Inc. All rights reserved.