Purpose: Parallel transmit technology for MRI at 7 tesla will significantly benefit from high performance transmit arrays that offer high transmit efficiency and low mutual coupling between the individual array elements. A novel dual-mode transmit array with nested array elements has been developed to support imaging the human brain in both the single-channel (sTx) and parallel-transmit (pTx) excitation modes of a 7 tesla MRI scanner. In this work, the design, implementation, validation, specific absorption rate (SAR) management, and performance of the head coil is presented. Methods: The transmit array consisted of a nested arrangement to improve decoupling between the second-neighboring elements. Two large cut-outs were introduced in the RF shield for an open-face design to reduce claustrophobia and to allow patient monitoring. A hardware interface allows the coil to be used in both the sTx and pTx modes. SAR monitoring is done with virtual observation points (VOP) derived from human body models. The transmit efficiency and coverage is compared with the commercial single-channel and parallel-transmit head coils. Results: Decoupling inductors between the second-neighboring coil elements reduced the coupling to less than -20dB. Local SAR estimates from the electromagnetic (EM) simulations were always less than the EM-based VOPs, which in turn were always less than scanner predictions and measurements for static and dynamic pTx waveforms. In sTx mode, we demonstrate improved coverage of the brain compared to the commercial sTx coil. The transmit efficiency is within 10% of the commercial pTx despite the two large cut-outs in the RF shield. In pTx mode, improved signal homogeneity was shown when the Universal Pulse was used for acquisition in vivo. Conclusion: A novel head coil which includes a nested eight-channel transmit array has been presented. The large cut-outs improve patient monitoring and reduces claustrophobia. For pTx mode, the EM simulation and VOP-based SAR management provided us with greater flexibility to apply pTx methods without the limitations of SAR constraints. For scanning in vivo, the coil was shown to provide an improved coverage in sTx mode compared to a standard commercial head coil.

中文翻译：

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具有开放式概念的嵌套八通道发射阵列，用于 7 特斯拉的人脑成像

目的：用于 7 特斯拉 MRI 的并行传输技术将显着受益于高性能传输阵列，这些阵列提供高传输效率和单个阵列元件之间的低互耦合。已开发出一种具有嵌套阵列元件的新型双模发射阵列，以支持在 7 特斯拉 MRI 扫描仪的单通道 (sTx) 和并行发射 (pTx) 激励模式下对人脑进行成像。在这项工作中，介绍了头部线圈的设计、实施、验证、比吸收率 (SAR) 管理和性能。方法：发射阵列由嵌套排列组成，以改善第二个相邻元件之间的去耦。射频屏蔽中引入了两个大切口，用于开放式设计，以减少幽闭恐惧症并允许对患者进行监测。硬件接口允许在 sTx 和 pTx 模式下使用线圈。SAR 监测是使用源自人体模型的虚拟观察点 (VOP) 完成的。传输效率和覆盖范围与商用单通道和并行传输头部线圈进行比较。结果：第二个相邻线圈元件之间的去耦电感器将耦合降低到小于 -20dB。来自电磁 (EM) 模拟的局部 SAR 估计值始终小于基于 EM 的 VOP，而后者又始终小于扫描仪对静态和动态 pTx 波形的预测和测量。在 sTx 模式下，与商业 sTx 线圈相比，我们展示了更好的大脑覆盖范围。尽管射频屏蔽中有两个大的切口，但发射效率在商用 pTx 的 10% 以内。在 pTx 模式下，当 Universal Pulse 用于体内采集时，信号均匀性得到改善。结论：已经提出了一种包括嵌套八通道发射阵列的新型头部线圈。大切口改善了对患者的监测并减少了幽闭恐惧症。对于 pTx 模式，EM 模拟和基于 VOP 的 SAR 管理为我们提供了更大的灵活性来应用 pTx 方法，而不受 SAR 约束的限制。对于体内扫描，与标准商业头部线圈相比，该线圈在 sTx 模式下提供了更好的覆盖范围。EM 模拟和基于 VOP 的 SAR 管理为我们提供了更大的灵活性来应用 pTx 方法，而不受 SAR 约束的限制。对于体内扫描，与标准商业头部线圈相比，该线圈在 sTx 模式下提供了更好的覆盖范围。EM 模拟和基于 VOP 的 SAR 管理为我们提供了更大的灵活性来应用 pTx 方法，而不受 SAR 约束的限制。对于体内扫描，与标准商业头部线圈相比，该线圈在 sTx 模式下提供了更好的覆盖范围。