Iterative correction of RF envelope distortion with GRATER-measured waveforms

Area of Science:

• Magnetic Resonance Imaging
• Pulse Sequence Development
• RF Engineering

Background:

• Radiofrequency (RF) pulse accuracy is critical for Magnetic Resonance Imaging (MRI), especially for advanced techniques like simultaneous multi-slice (SMS) imaging.
• Existing methods for RF envelope correction may require additional hardware or synchronization, limiting their practicality.

Purpose of the Study:

• To develop and evaluate a novel, hardware-free method for RF envelope correction.
• To improve the performance of RF pulses used in SMS imaging without extra equipment or synchronization.

Main Methods:

• The Gradient Reversal Approach to Evaluate RF (GRATER) pulse sequence was employed to measure transmitted RF waveforms.
• Measured waveforms were used iteratively to compute predistorted RF waveforms until a stopping criterion was met.
• Excitation profiles and SMS image quality were assessed before and after RF predistortion.

Main Results:

• The proposed method achieved a >12-fold reduction in normalized root mean squared error (NRMSE) for multiband RF pulses.
• Spurious side-lobe excitation was reduced by >6-fold, demonstrated in SMS bSSFP imaging at 3T.
• Improvements were validated in phantom and cardiac imaging, showing reduced unwanted side-lobe signal.

Conclusions:

• Iterative GRATER-based predistortion offers a practical, hardware-free solution for enhancing RF pulse performance.
• This technique is particularly beneficial for short-duration, low flip angle RF pulses used in SMS bSSFP imaging.
• The method's efficiency allows for integration into initial scan setups or subsequent scan adjustments.