Abstract
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.
Highlights
Longitudinal relaxation is one of the major contrast mechanisms in the central nervous system and is widely used for the investigation of white matter (WM) morphology
Since the longitudinal relaxation is direct related to the molecular mobility, it is thought that these restrictions strongly affect the R1 rate of the myelin water associated molecules[13]
In this work we investigated the dipolar-induced R1 relaxation inside the myelin sheath at several clinical relevant field strengths of 1.5 T, 3 T and 7 T
Summary
Longitudinal relaxation is one of the major contrast mechanisms in the central nervous system and is widely used for the investigation of WM morphology. Even though the underlying effects causing this excellent contrast are a frequently investigated issue in the neuroimaging community, they are still not completely understood It is well known, for example, that the longitudinal relaxation rate in WM regions depends on local tissue concentration and is strongly correlated with the degree of myelinated axons[1,2]. This magnetization transfer (MT) is mainly governed by two effects, namely the through space dipole-dipole coupling between the hydrogen nuclei and the direct exchange of individual protons or hydroxide groups between water and macromolecules[5,6,7] These biophysical and biochemical processes between adjacent compartments are referred to as through space cross-relaxation and chemical exchange, respectively. The continuous improvement of molecular force field simulations and the variety of parameterized lipid and protein molecules allows to simulate complex biological structures, i.e. a myelin-alike environment on atomistic scale
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