Fast Gradient Ascent Pulse Engineering for parallel transmission pulse design in the small and large tip angle regimes

Motivation: Dynamic RF shimming in pTX at UHF faces two limitations: lengthy computational time of the GRAPE method or lack of precision of parametric approaches (e.g. kT-points). Goal(s): To validate an alternative pulse parameterization, fastGRAPE, potentially offering a better trade-off between the performance and computational speed. Approach: We compare GRAPE, fastGRAPE, and kT-point online and offline pulse designs for a commercial and a custom pTX RF coils at 7T using Bloch simulation and in vivo acquisitions. Results: For short STA excitation, fastGRAPE yields comparable performances to GRAPE and outperforms kT-points. For inversion with the custom coil, online fastGRAPE provides the best flip angle accuracy. Impact: With a flip angle accuracy comparable to GRAPE and a computation time comparable to kT-points, fastGRAPE presents a promising strategy both for online and offline dynamic RF shimming.