Adiabatic null passage for on-resonance magnetization transfer preparation

Area of Science:

• Magnetic Resonance Imaging
• Biophysics

Background:

• Magnetization transfer (MT) imaging is crucial for characterizing tissue properties.
• Existing MT techniques are sensitive to radiofrequency (RF) field and B0 inhomogeneities, and T2 relaxation effects.
• Improving the robustness and accuracy of MT imaging is essential for reliable clinical applications.

Purpose of the Study:

• To introduce and evaluate a novel adiabatic null passage (ANP) radiofrequency (RF) pulse for magnetization transfer (MT) preparation.
• To assess the ANP pulse's improved insensitivity to B0 and radiofrequency (B1) inhomogeneities.
• To mitigate direct saturation and T2 effects in MT imaging.

Main Methods:

• Modified a 6-ms time-resampled frequency offset-corrected pulse to achieve a zero flip angle at the end.
• Simulated the spectral response and investigated insensitivity to B0 and B1, comparing it with a binomial pulse.
• Implemented the ANP pulse in a 2D-EPI sequence for MT contrast and MT ratio (MTR) mapping, conducting in vivo experiments on healthy participants.

Main Results:

• In vivo experiments showed MTR decreased from 61% (64%) to 38% (42%) with ANP, mitigating T2/direct effects by 22% (22%).
• Bloch-McConnell simulations corroborated these findings.
• ANP demonstrated significant MTR improvements in areas with high B0 inhomogeneity.

Conclusions:

• The ANP pulse is advantageous over binomial pulses for MT contrast generation.
• ANP effectively mitigates T2 effects and direct saturation of the liquid pool.
• The ANP pulse exhibits reduced sensitivity to B1 and B0 inhomogeneity, enhancing MT imaging robustness.