The Ritz adjoint method for optimization of arbitrary waveform excitation pulses: Demonstration in slice-selective parallel transmission

Motivation: Optimization of arbitrary RF and gradient waveforms provides considerable degrees-of-freedom to mitigate excitation inhomogeneity at 7 T; however, waveform optimization is a computationally difficult non-convex optimization problem. Goal(s): We introduce an arbitrary waveform pulse optimization method to enable real-time (~3 min) tailored slice-selective pulse optimization. Approach: We employ the Ritz adjoint method to efficiently calculate the optimization gradients to optimize waveforms parameterized by global polynomials, while respecting SAR and system constraints. Results: A 3.5 ms slice-selective, tailored parallel transmission pulse can be designed in 3 minutes with in-plane NRMSE 50% better than an optimized 3-spoke design. Impact: The Ritz adjoint method efficiently optimized arbitrary waveform universal and tailored parallel transmission slice-selective pulses that respect SAR constraints and system limitations while correcting the flip angle inhomogeneities for high-field excitations.