Iterative correction of RF envelope distortion with GRATER-measured waveforms

Abstract

Purpose: To develop and evaluate a method for RF envelope correction without extra hardware or synchronization. Methods: Transmitted RF waveforms are measured through a simple pulse sequence called the gradient reversal approach to evaluate RF (GRATER). The measured RF waveforms are used to compute predistorted RF waveforms. This process is repeated until a stopping criterion is met, for example, based on pulse performance or a maximum number of iterations. Excitation profiles and simultaneous multi-slice (SMS) image quality are compared before and after RF predistortion. Results: The proposed method improved the accuracy of multiband RF pulses, reducing normalized root mean squared error (NRMSE) by >12-fold, and reducing spurious side-lobe excitation by >6-fold. The reduction in unwanted side-lobe signal is demonstrated using SMS bSSFP imaging at 3T in phantoms and in the heart. Conclusion: Iterative GRATER-based predistortion is a practical, hardware-free way to boost performance of short duration, low flip angle RF pulses, such as those used in SMS bSSFP imaging. Because of its efficiency, this technique could be included as part of an initial scan setup or for use with subsequent scans.