Universal Design of Multiphoton Parallel Transmission (MP-pTx) Pulses for Uniform, High-Flip Angle Excitations

Motivation: Contrast in high-field MRI is complicated by the spatially non-uniform transmission profile of birdcage coils. Goal(s): We create “universal” pulses for spatially-uniform excitations using multiphoton parallel transmission (MP-pTx). Approach: MP-pTx operates a $$$B_z$$$ shim array in the kHz range to supplement birdcage excitation. We extend this framework to arbitrary flip angles and a universal design (a population of $$$B_1^+$$$ and $$$\Delta B_0$$$ maps) using spinor-domain, Bloch dynamics representation. Results: Universal MP-pTx pulses have 11.3%, and 16.3% flip angle NRMSE for 90º and 180º pulses played on test subjects not used for training, compared to 24.5% and 25.3% with conventional birdcage. Impact: Universal MP-pTx pulses will allow users to mitigate flip angle inhomogeneity present in the brain at 7 T using precomputed pulses without SAR concerns beyond that of a conventional birdcage transmit coil or subject-specific calculations.