Transverse signal decay under the weak field approximation: Theory and validation

Area of Science:

• Magnetic Resonance Imaging
• Biophysics

Background:

• Understanding transverse signal decay is crucial for interpreting magnetic resonance (MR) data, especially in complex biological systems like blood.
• The motional narrowing regime offers a simplified model for diffusion and relaxation processes.

Purpose of the Study:

• To derive an analytical expression for transverse signal decay in the motional narrowing regime.
• To validate this expression using in silico simulations and experimental ex vivo blood samples.

Main Methods:

• A closed-form solution (CFS) was derived using the weak field approximation for transverse signal decay under various refocusing pulse sequences.
• Simulations modeled water diffusion with spherical perturbers, and experiments used ex vivo blood with Carr-Purcell-Meiboom-Gill sequences.

Main Results:

• The CFS accurately predicted transverse signal decay within the motional narrowing and partially the intermediate dephasing regimes.
• Theoretical calculations showed excellent agreement with weak field approximation for R2 in Carr-Purcell-Meiboom-Gill sequences, confirmed by ex vivo analysis.

Conclusions:

• Transverse signal decay in the motional narrowing regime can be accurately described analytically.
• This theory has potential applications in tissue iron imaging, blood relaxometry, and contrast agent imaging.