Comparing Orientation-Dependent Transverse Relaxation at 3T and 7T: Deciphering Anisotropic Relaxation Mechanisms in White Matter

Purpose: This work aims to elucidate the mechanisms underlying orientation-dependent transverse relaxation in human brain white matter (WM), which have long been ambiguous. Methods: We analyzed publicly available 3T and 7T DTI datasets (b-values = 1000 and 2000 s/mm2) from 25 young adults participating in the Human Connectome Project. Orientation-dependent transverse relaxation R2 profiles from whole brain WM were generated from T2-weighted images (b-value = 0) and characterized using a previously developed cone model based on the generalized magic angle effect (MAE). Derived anisotropic R2 or R2a values were compared between different magnetic fields. Similar comparisons were also conducted for the derived R2a values from whole brain WM and two fiber tracts, based on gradient-echo signals reported in the literature. Classical relaxation theories predict that the ratio of (R2a (7T))/R2a (3T)) will be unity or (7/3)^2 (i.e., approximately 5.4) if the measured orientation-dependent R2 arises exclusively from MAE or from the previously proposed susceptibility effect, respectively. Results: Fitted model parameters were comparable for DTI datasets with b-values of 1000 and 2000 (s/mm2). The fitted R2a increased, on average, from 3.6±1.1 (1/s) at 3T to 5.4±1.5 (1/s) at 7T for DTI datasets with a b-value=1000 s/mm2. The measured ratio of (R2a (7T))/(R2a (3T)) was thus approximately 1.5. However, based on gradient-echo signals, this ratio essentially became unity within the measurement precision. Conclusion: This study suggests that MAE is the primary mechanism for the observed orientation-dependent transverse relaxation at 3T in human brain WM, offering a different perspective from previous literature.