头颈部多通道B1 +标测和颈动脉RF匀场（使用并行发射头部线圈）
Head-and-neck multi-channel B1+ mapping and carotid arteries RF shimming using a parallel transmit head coil
摘要：
目的：当使用7 T发射头线圈时，神经血管MRI遭受B1+快速下降到颈部。改善颈部主要供血动脉中B1+幅度的一种解决方案是使用定制的RF射频同轴并行发射（pTx）头部线圈。然而，计算这种B1+标测需要头部和颈部的稳健的多通道B1+标测图，这是具有挑战性的，因为射频穿透到颈部的程度较低，多通道B1+标测技术的动态范围有限，以及B 0灵敏度。因此，我们寻求一种强大的大动态范围pTx场标测方案，并测试了RF匀场是否可以在实践中改善颈动脉B1+。方法：提出了一种管道，该管道将使用圆极化（CP-）和CP 2模式RF闪烁器在多个电压下采集的B1+标测数据相结合。通过比较多个随机发射脉冲的预测和测量B1+，并通过评估RF匀场增加颈动脉中B1+的能力，对管道进行了评价。结果：该方法在头部和颈部的B1+预测和测量之间取得了良好的一致性。与CP模式相比，使用定制射频消融仪，颈动脉中的B1+幅度可增加42%，使用通用射频消融仪可增加37%，同时还改善了射频均匀性。结论：使用从头部和颈部的多通道B1+图计算的RF能量可以增加颈部的B1+。这可以通过使用通用的纯相位RF PLL来实现，从而便于在现有序列中轻松实现。
英文摘要：
Purpose: Neurovascular MRI suffers from a rapid drop in B1+ into the neck when using transmit head coils at 7T. One solution to improving B1+ magnitude in the major feeding arteries in the neck is to use custom RF shims on parallel transmit (pTx) head coils. However, calculating such shims requires robust multi-channel B1+ maps in both the head and the neck, which is challenging due to low RF penetration into the neck, limited dynamic range of multi-channel B1+ mapping techniques, and B0 sensitivity. We therefore sought a robust large-dynamic-range pTx field mapping protocol, and tested whether RF shimming can improve carotid artery B1+ in practice. Methods: A pipeline is presented that combines B1+ mapping data acquired using circularly polarized (CP-) and CP2-mode RF shims at multiple voltages. The pipeline was evaluated by comparing the predicted and measured B1+ for multiple random transmit shims, and by assessing the ability of RF shimming to increase the B1+ in the carotid arteries. Results: The proposed method achieved good agreement between predicted and measured B1+ in both the head and the neck. The B1+ magnitude in the carotid arteries can be increased by 42% using tailored RF shims or by 37% using universal RF shims, while also improving the RF homogeneity compared to CP mode. Conclusion: B1+ in the neck can be increased using RF shims calculated from multi-channel B1+ maps in both the head and the neck. This can be achieved using universal phase-only RF shims, facilitating easy implementation in existing sequences.
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