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Origin of orientation-dependent R1 (=1/T1 ) relaxation in white matter.

Felix Schyboll1, Uwe Jaekel1, Francesco Petruccione2

  • 1University of Applied Sciences Koblenz, RheinAhrCampus, Remagen, Germany.

Magnetic Resonance in Medicine
|April 11, 2020
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Summary

The longitudinal relaxation rate (R1) in white matter depends on nerve fiber orientation. This study shows that anisotropic dipole-dipole interactions within the myelin sheath are the primary cause of this MRI phenomenon.

Keywords:
longitudinal relaxationmagnetization transfermolecular dynamics simulationwhite matter

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Area of Science:

  • Biophysics
  • Neuroimaging
  • Magnetic Resonance Imaging

Background:

  • Longitudinal relaxation rate (R1) in white matter (WM) exhibits orientation dependency relative to the main magnetic field (B0).
  • The precise origin of this R1 orientation dependency is not fully understood but is hypothesized to involve the anisotropic molecular environment of the myelin sheath.

Purpose of the Study:

  • To theoretically investigate the contribution of dipolar-induced R1 relaxation of myelin-associated hydrogen nuclei.
  • To compare these theoretical findings with experimentally observed R1 orientation dependencies in WM.

Main Methods:

  • Utilized atomistic molecular dynamics simulations to calculate R1 relaxation rates for hydrogen nuclei in a myelin-like environment across various orientations relative to the B0 field.
  • Simulated observable R1 relaxation for different fiber orientations and fitted the results to experimental data using a realistic signal weighting scheme.

Main Results:

  • Demonstrated that the R1 relaxation rate for both solid myelin (SM) and myelin water (MW) is dependent on fiber orientation relative to the B0 field.
  • Achieved a good match between theoretically predicted and experimentally observed R1 orientation dependencies when employing realistic signal weighting and tissue characteristics.

Conclusions:

  • The R1 orientation dependency observed in white matter is primarily attributed to anisotropic dipole-dipole interactions among hydrogen nuclei within the myelin sheath.
  • This finding provides a theoretical basis for understanding the observed MRI signal variations related to white matter microstructure.