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In general, a broad consensus exists that dipolar couplings between water protons and motional restricted macromolecules are prominent modulators of longitudinal relaxation in WM. Recent studies demonstrate anisotropic relaxation in human white matter (WM) in dependence of the fibre-to-field orientation. Our approach for further investigations of such effects overcomes previous limitations, in particular, sample reorientations for a direct demonstration of orientation effects, which were missing in earlier studies. Samples of ex-vivo WM tissue of pig brain yielded slightly reduced spin-lattice relaxation times by 1-2% around an angle of 40° between the main fibre direction and B0.
Wallstein et al. (Wed,) studied this question.