Comparing orientation-dependent transverse relaxation at 3 T and 7 T: Deciphering anisotropic relaxation mechanisms...

Orientation-dependent transverse relaxation in human brain white matter (WM) has been widely reported, yet its biophysical origins remain debated. This study investigates the relative contributions of the magic angle effect (MAE) and susceptibility-based mechanisms at 3 T and 7 T. Publicly available diffusion tensor imaging (DTI) datasets from 25 young adults in the Human Connectome Project, acquired with b-values of [0,1000] and [0,2000] s/mm², were analyzed. Using a cone-based framework that incorporates a generalized MAE model, the magnitudes of orientation-dependent transverse relaxation rates (R₂^a) were derived from T₂-weighted images (b = 0) and compared across the two field strengths. Additionally, R₂^a values obtained from gradient-echo (GRE) signals reported in previous literature were evaluated at both 3 T and 7 T. Classical relaxation theory predicts a ratio η = R₂^a(7 T) / R₂^a(3 T) ≃ 1 if MAE dominates, or approximately η = 5.4 if susceptibility effects prevail. Model parameters were consistent across DTI datasets with different non-zero b-values. For b = 1000 s/mm², R₂^a increased from 4.0 ± 1.1 s^-1 at 3 T to 5.6 ± 1.6 s^-1 at 7 T, yielding a ratio η < 1.5. This increase suggests a partial contribution of susceptibility effects to the measured R₂^a, estimated at 8.3 ± 10.2% at 3 T and 34.5 ± 42.2% at 7 T. In contrast, GRE-based η values were close to unity. These findings suggest that MAE is the predominant mechanism underlying orientation-dependent transverse relaxation in WM at 3 T, offering a revised interpretation that contrasts with prior susceptibility-based explanations.