Multiphoton parallel transmission (MP‐pTx): Pulse design methods and numerical validation

Abstract Purpose We propose and evaluate multiphoton parallel transmission (MP‐pTx) to mitigate flip angle inhomogeneities in high‐field MRI. MP‐pTx is an excitation method that utilizes a single, conventional birdcage coil supplemented with low‐frequency (kHz) irradiation from a multichannel shim array and/or gradient channels. SAR analysis is simplified to that of a conventional birdcage coil, because only the radiofrequency (RF) field from the birdcage coil produces significant SAR. Methods MP‐pTx employs an off‐resonance RF pulse from a conventional birdcage coil supplemented with oscillating ‐directed fields from a multichannel shim array and/or the gradient coils. We simulate the ability of MP‐pTx to create uniform nonselective brain excitations at 7 T using realistic and field maps. The RF, shim array, and gradient waveform's amplitudes and phases are optimized using a genetic algorithm followed by sequential quadratic programming. Results A 1 ms MP‐pTx excitation using a 32‐channel shim array with current constrained to less than 50 Amp‐turns reduced the transverse magnetization's normalized root‐mean‐squared error from 29% for a conventional birdcage excitation to 6.6% and was nearly 40% better than a 1 ms birdcage coil 5 kT‐point excitation with optimized kT‐point locations and comparable pulse power. Conclusion The MP‐pTx method resembles conventional pTx in its goals and approach but replaces the parallel RF channels with cheaper, low‐frequency shim channels. The method mitigates high‐field flip angle inhomogeneities to a level better than 3 T CP‐mode and comparable to 7 T pTx while retaining the straightforward SAR characteristics of conventional birdcage excitations, as low‐frequency shim array fields produce negligible SAR.