Dipolar induced spin-lattice relaxation in the myelin sheath: A molecular dynamics study

Interactions between hydrogen protons of water molecules and macromolecules within the myelin sheath surrounding the axons are a major factor influencing the magnetic resonance (MR) contrast in white matter (WM) regions. In past decades, several studies have investigated the underlying effects and reported a wide range of R₁ rates for the myelin associated compartments at different field strengths. However, it was also shown that the experimental quantification of the compartment-specific R₁ rates is associated with large uncertainties. The current study therefore investigates the longitudinal relaxation rates within the myelin sheath using a molecular dynamic (MD) simulation. For this purpose, a realistic molecular model of the myelin sheath was employed to determine the dipole-dipole induced R₁ relaxation rate of the hydrogen protons at clinically relevant field strengths. The results obtained clearly reflect the spatial heterogeneity of R₁ with a increased relaxivity of myelin water due to a reduced molecular mobility near the membrane surface. Moreover, the calculated R₁ rates for both myelin water and macromolecules are in excellent agreement with experimental findings from the literature at different field strengths.