{Reference Type}: Journal Article
{Title}: Multiphoton parallel transmission (MP-pTx): Pulse design methods and numerical validation.
{Author}: Drago JM;Guerin B;Stockmann JP;Wald LL;
{Journal}: Magn Reson Med
{Volume}: 0
{Issue}: 0
{Year}: 2024 Jun 20
{Factor}: 3.737
{DOI}: 10.1002/mrm.30116
{Abstract}: <B>OBJECTIVE: </B>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.<BR><B>METHODS: </B>MP-pTx employs an off-resonance RF pulse from a conventional birdcage coil supplemented with oscillating  z $$ z $$  -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   B 1 +  $$ {\mathrm{B}}_1^{+} $$  and  Δ B 0  $$ \Delta {\mathrm{B}}_0 $$  field maps. The RF, shim array, and gradient waveform's amplitudes and phases are optimized using a genetic algorithm followed by sequential quadratic programming.<BR><B>RESULTS: </B>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.<BR><B>CONCLUSIONS: </B>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.