Abstract：
OBJECTIVE: 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 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.
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.
CONCLUSIONS: 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.
METHODS: 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.
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.
CONCLUSIONS: 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.
摘要：
目的：我们提出并评估了多光子并行传输（MP-pTx），以减轻高场MRI中的翻转角不均匀性。MP-pTx是一种利用单一激励方法，传统的鸟笼线圈补充了来自多通道匀场阵列和/或梯度通道的低频（kHz）辐射。SAR分析被简化为传统的鸟笼线圈，因为只有来自鸟笼线圈的射频(RF)场产生显著的SAR。
方法：MP-pTx采用来自传统鸟笼线圈的非共振RF脉冲，并补充了来自多通道匀场阵列和/或梯度线圈的振荡z$$z$$定向场。我们使用现实的B1$${\\mathrm{B}}_1^{}$和ΔB0$\\Delta{\mathrm{B}}_0$$场映射来模拟MP-pTx在7T时创建均匀的非选择性大脑兴奋的能力。RF,垫片阵列，和梯度波形的振幅和相位使用遗传算法进行优化，然后进行序列二次规划。
结果：使用32通道匀场阵列的1msMP-pTx激励，电流约束为小于50安培匝，将横向磁化强度的归一化均方根误差从传统鸟笼激励的29％降低到6.6％，比具有优化的kT点位置和可比的脉冲功率的1ms鸟笼线圈5kT点激励好了近40％。
结论：MP-pTx方法在其目标和方法上类似于传统的pTx，但以更便宜的价格取代了并行RF信道，低频垫片通道。该方法将高场翻转角不均匀性减轻到优于3TCP模式的水平，并与7TpTx相当，同时保留了常规鸟笼激发的简单SAR特性，作为低频垫片阵列场产生可忽略的SAR。
方法：MP-pTx采用来自传统鸟笼线圈的非共振RF脉冲，并补充了来自多通道匀场阵列和/或梯度线圈的振荡z$$z$$定向场。我们使用现实的B1$${\\mathrm{B}}_1^{}$和ΔB0$\\Delta{\mathrm{B}}_0$$场映射来模拟MP-pTx在7T时创建均匀的非选择性大脑兴奋的能力。RF,垫片阵列，和梯度波形的振幅和相位使用遗传算法进行优化，然后进行序列二次规划。
结果：使用32通道匀场阵列的1msMP-pTx激励，电流约束为小于50安培匝，将横向磁化强度的归一化均方根误差从传统鸟笼激励的29％降低到6.6％，比具有优化的kT点位置和可比的脉冲功率的1ms鸟笼线圈5kT点激励好了近40％。
结论：MP-pTx方法在其目标和方法上类似于传统的pTx，但以更便宜的价格取代了并行RF信道，低频垫片通道。该方法将高场翻转角不均匀性减轻到优于3TCP模式的水平，并与7TpTx相当，同时保留了常规鸟笼激发的简单SAR特性，作为低频垫片阵列场产生可忽略的SAR。
