Fiber orientation-dependent T₁ angular features in human white matter at 1.5 T, 3 T, and 7 T

Purpose: White matter (WM) microstructure influences T₁ in vivo. Here, T₁ angular features were studied in human WM in vivo in quantitative terms at 1.5 T, 3 T and 7 T. Methods: MP2RAGE MRI was used to compute absolute T₁ images of the brain at three fields. Diffusion MRI images were acquired using the manufacturer's diffusion MRI (dMRI) protocol at 1.5 T and Human Connectome Project protocols at 3 T and 7 T to compute WM microstructural DTI indices. Axonal fiber-to-field dependency of T₁ relaxation was determined in WM and the quantitative characteristics of the angular features were measured in absolute terms at all three fields. Results: Two angular features in T₁ relaxation were present in WM with fractional anisotropy >0.5 at all three fields showing the characteristics: (1) increasing T₁ relaxation from parallel to perpendicular orientations of axonal fibers and (2) a broad long T₁ hump centered around 40° orientation in respect to the field. The former feature amounted to 4.5% to 4.9% of average T₁ at three fields, the latter was 4.2% to 3.4% of average T₁ at 1.5 T and 3 T, but only to 1.8% at 7 T. The angular plots of signals in TI images indicated much shorter T₁ in the proton species underpinning the 40° feature at 1.5 T than at 7 T, which may underpin the quantitative difference of the 40° hump between the fields. Conclusions: T₁ relaxation anisotropy is an inherent WM MRI contrast that is detectable at all MR fields most commonly used in human neuroimaging, and it should be considered in the evaluations of quantitative T₁ MRI.