Nuclear magnetic relaxation by the dipolar EMOR mechanism: Multi-spin systems

Area of Science:

• Magnetic Resonance
• Biophysics
• Physical Chemistry

Background:

• Longitudinal spin relaxation of water protons in biological tissues is crucial for MRI contrast.
• This relaxation is primarily driven by exchange-mediated orientational randomization (EMOR) of magnetic dipole-dipole couplings.
• Existing theories have limitations in fully capturing these complex interactions.

Purpose of the Study:

• To extend rigorous EMOR relaxation theory to larger spin systems.
• To develop an approximate multi-spin theory (GSRE) applicable to arbitrary spin system sizes.
• To provide a molecular-level interpretation for low-field MRI contrast in biological tissues.

Main Methods:

• Extension of stochastic Liouville equation theory to four-spin systems.
• Construction of a generalized stochastic Redfield equation (GSRE) for multi-spin systems.
• Computation of relaxation dispersion profiles for systems up to 16 protons.

Main Results:

• The GSRE theory incorporates longitudinal-transverse cross-mode relaxation and generalized spin diffusion.
• Computed relaxation dispersion profiles accurately model various motion regimes.
• The dipolar EMOR mechanism is confirmed as the primary cause of low-field longitudinal relaxation.

Conclusions:

• The developed GSRE theory offers a more comprehensive understanding of water proton relaxation.
• This provides a foundation for interpreting endogenous soft-tissue contrast in low-field MRI.
• The findings have implications for advancing emerging low-field MRI techniques.