T1 relaxation and axon fibre configuration in human white matter.

Understanding the effects of white matter (WM) axon fibre microstructure on T1 relaxation is important for neuroimaging. Here, we have studied the interrelationship between T1 and axon fibre configurations at 3T and 7T. T1 and S0 (=signal intensity at zero TI) were computed from MP2RAGE images acquired with six inversion recovery times. Multishell diffusion MRI images were analysed for fractional anisotropy (FA); MD; V1; the volume fractions for the first (f), second (f) and third (f) fibre configuration; and fibre density cross-section images for the first (fdc), second (fdc) and third (fdc) fibres. T1 values were plotted as a function of FA, f, f, f, fdc, fdc and fdc to examine interrelationships between the longitudinal relaxation and the diffusion MRI microstructural measures. T1 values decreased with increasing FA, f and f in a nonlinear fashion. At low FA values (from 0.2 to 0.4), a steep shortening of T1 was followed by a shallow shortening by 6%-10% at both fields. The steep shortening was associated with decreasing S0 and MD. T1 also decreased with increasing fdc values in a nonlinear fashion. Instead, only a small T1 change as a function of either f or fdc was observed. In WM areas selected by fdc only masks, T1 was shorter than in those with fdc/fdc. In WM areas with high single fibre populations, as delineated by f/fdc masks, T1 was shorter than in tissue with high complex fibre configurations, as segmented by f/fdc or f/fdc masks. T1 differences between these WM areas are attributable to combined effects by T1 anisotropy and lowered FA. The current data show strong interrelationships between T1, axon fibre configuration and orientation in healthy WM. It is concluded that diffusion MRI microstructural measures are essential in the effort to interpret quantitative T1 images in terms of tissue state in health and disease.