University Duisburg-Essen, 45141 Duisburg, Germany.
IEEE Trans Biomed Eng. 2012 Sep;59(9):2549-57. doi: 10.1109/TBME.2012.2205926. Epub 2012 Jun 25.
This contribution discusses the utilization of RF power in an MRI system with RF mode shimming which enables the superposition of circularly polarized modes of a transmit RF coil array driven by a Butler matrix. Since the required power for the individual modes can vary widely, mode-shimming can result in a significant underutilization of the total available RF power. A variable power combiner (VPC) is proposed to improve the power utilization: it can be realized as a reconfiguration of the MRI transmit system by the inclusion of one additional matrix network which receives the power from all transmit amplifiers at its input ports and provides any desired (combined) power distribution at its output ports by controlling the phase and amplitude of the amplifiers' input signals. The power distribution at the output ports of the VPC is then fed into the "mode" ports of the coil array Butler matrix in order to superimpose the spatial modes at the highest achievable power utilization. The VPC configuration is compared to the standard configuration of the transmit chain of our MRI system with 8 transmit channels and 16 coils. In realistic scenarios, improved power utilization was achieved from 17% to 60% and from 14% to 55% for an elliptical phantom and a region of interest in the abdomen, respectively, and an increase of the power utilization of 1 dB for a region of interest in the upper leg. In general, it is found that the VPC allows significant improvement in power utilization when the shimming solution demands only a few modes to be energized, while the technique can yield loss in power utilization in cases with many modes required at high power level.
本研究探讨了磁共振成像（MRI）系统中射频（RF）功率的利用，该系统采用 RF 模式调谐，可实现由 Butler 矩阵驱动的发射 RF 线圈阵列的圆极化模式叠加。由于各个模式所需的功率差异很大，因此调谐可能导致总可用 RF 功率的严重未充分利用。提出了一种可变功率合成器（VPC）来提高功率利用率：通过在 MRI 发射系统中增加一个额外的矩阵网络，可以将其实现为一种重新配置，该网络从所有发射放大器的输入端口接收功率，并通过控制放大器输入信号的相位和幅度，在其输出端口提供任何所需的（组合）功率分配。然后，将 VPC 输出端口的功率分配馈送到线圈阵列 Butler 矩阵的“模式”端口，以在最高可达的功率利用率下叠加空间模式。将 VPC 配置与我们的 MRI 系统的 8 个发射通道和 16 个线圈的发射链的标准配置进行了比较。在实际场景中，对于椭圆形仿体和腹部感兴趣区域，功率利用率分别从 17%提高到 60%和从 14%提高到 55%，并且对于大腿上部感兴趣区域，功率利用率提高了 1dB。一般来说，发现当调谐解决方案只需要激励少数几个模式时，VPC 允许显著提高功率利用率，而在需要高功率水平的许多模式的情况下，该技术可能会导致功率利用率的损失。
