Avdievich Nikolai I, Giapitzakis Ioannis A, Pfrommer Andreas, Shajan Gunamony, Scheffler Klaus, Henning Anke
High-Field MR Center, Max Planck Institute for Biological Cybernetics, 72076, Tübingen, Germany.
Institute of Physics, Ernst-Moritz-Arndt University, Greifswald, Germany.
NMR Biomed. 2018 Sep;31(9):e3964. doi: 10.1002/nbm.3964. Epub 2018 Jul 5.
One of the major challenges in constructing multi-channel and multi-row transmit (Tx) or transceiver (TxRx) arrays is the decoupling of the array's loop elements. Overlapping of the surface loops allows the decoupling of adjacent elements and also helps to improve the radiofrequency field profile by increasing the penetration depth and eliminating voids between the loops. This also simplifies the design by reducing the number of decoupling circuits. At the same time, overlapping may compromise decoupling by generating high resistive (electric) coupling near the overlap, which cannot be compensated for by common decoupling techniques. Previously, based on analytical modeling, we demonstrated that electric coupling has strong frequency and loading dependence, and, at 9.4 T, both the magnetic and electric coupling between two heavily loaded loops can be compensated at the same time simply by overlapping the loops. As a result, excellent decoupling was obtained between adjacent loops of an eight-loop single-row (1 × 8) human head tight-fit TxRx array. In this work, we designed and constructed a 9.4-T (400-MHz) 16-loop double-row (2 × 8) overlapped TxRx head array based on the results of the analytical and numerical electromagnetic modeling. We demonstrated that, simply by the optimal overlap of array loops, a very good decoupling can be obtained without additional decoupling strategies. The constructed TxRx array provides whole-brain coverage and approximately 1.5 times greater Tx efficiency relative to a transmit-only/receive-only (ToRo) array, which consists of a larger Tx-only array and a nested tight-fit 31-loop receive (Rx)-only array. At the same time, the ToRo array provides greater peripheral signal-to-noise ratio (SNR) and better Rx parallel performance in the head-feet direction. Overall, our work provides a recipe for a simple, robust and very Tx-efficient design suitable for parallel transmission and whole-brain imaging at ultra-high fields.
构建多通道、多行发射（Tx）或收发（TxRx）阵列的主要挑战之一是阵列环形元件的去耦。表面环形结构的重叠能够实现相邻元件的去耦，还可通过增加穿透深度和消除环形结构之间的空隙来改善射频场分布。这也通过减少去耦电路的数量简化了设计。同时，重叠可能会因在重叠处附近产生高电阻（电）耦合而损害去耦效果，而这是常见去耦技术无法补偿的。此前，基于分析建模，我们证明电耦合具有很强的频率和负载依赖性，并且在9.4T时，只需使两个重载环形结构重叠，就能同时补偿它们之间的磁耦合和电耦合。结果，在一个八环单行（1×8）人体头部紧密贴合TxRx阵列的相邻环之间实现了出色的去耦。在这项工作中，我们基于分析和数值电磁建模的结果，设计并构建了一个9.4T（400MHz）16环双行（2×8）重叠TxRx头部阵列。我们证明，仅通过阵列环的最佳重叠，无需额外的去耦策略就能实现非常好的去耦。所构建的TxRx阵列提供全脑覆盖，相对于由一个更大的仅发射阵列和一个嵌套的紧密贴合31环仅接收（Rx）阵列组成的仅发射/仅接收（ToRo）阵列，其发射效率提高了约1.5倍。同时，ToRo阵列在头脚方向提供更高的周边信噪比（SNR）和更好的Rx并行性能。总体而言，我们的工作为一种简单、稳健且发射效率极高的设计提供了方法，适用于超高场下的并行传输和全脑成像。
