Avdievich N I, Pfrommer A, Giapitzakis I A, Henning A
High-Field MR Center, Max Planck Institute for Biological Cybernetics, Tübingen, Germany.
Institute of Physics, Ernst-Moritz-Arndt University Greifswald, Greifswald, Germany.
NMR Biomed. 2017 Oct;30(10). doi: 10.1002/nbm.3759. Epub 2017 Jun 20.
Ultrahigh-field (UHF) (≥7 T) transmit (Tx) human head surface loop phased arrays improve both the Tx efficiency (B /√P) and homogeneity in comparison with single-channel quadrature Tx volume coils. For multi-channel arrays, decoupling becomes one of the major problems during the design process. Further insight into the coupling between array elements and its dependence on various factors can facilitate array development. The evaluation of the entire impedance matrix Z for an array loaded with a realistic voxel model or phantom is a time-consuming procedure when performed using electromagnetic (EM) solvers. This motivates the development of an analytical model, which could provide a quick assessment of the Z-matrix. In this work, an analytical model based on dyadic Green's functions was developed and validated using an EM solver and bench measurements. The model evaluates the complex coupling, including both the electric (mutual resistance) and magnetic (mutual inductance) coupling. Validation demonstrated that the model does well to describe the coupling at lower fields (≤3 T). At UHFs, the model also performs well for a practical case of low magnetic coupling. Based on the modeling, the geometry of a 400-MHz, two-loop transceiver array was optimized, such that, by simply overlapping the loops, both the mutual inductance and the mutual resistance were compensated at the same time. As a result, excellent decoupling (below -40 dB) was obtained without any additional decoupling circuits. An overlapped array prototype was compared (signal-to-noise ratio, Tx efficiency) favorably to a gapped array, a geometry which has been utilized previously in designs of UHF Tx arrays.
与单通道正交发射体线圈相比，超高场（UHF）（≥7T）发射（Tx）人体头部表面环形相控阵既能提高发射效率（B/√P），又能改善均匀性。对于多通道阵列而言，去耦成为设计过程中的主要问题之一。深入了解阵列元件之间的耦合及其对各种因素的依赖性有助于阵列的开发。当使用电磁（EM）求解器对阵列加载实际体素模型或体模时，评估整个阻抗矩阵Z是一个耗时的过程。这促使人们开发一种解析模型，该模型可以快速评估Z矩阵。在这项工作中，基于并矢格林函数开发了一种解析模型，并使用EM求解器和实验台测量进行了验证。该模型评估了包括电耦合（互电阻）和磁耦合（互感）在内的复杂耦合。验证表明，该模型在较低场强（≤3T）下能很好地描述耦合情况。在超高场强下，该模型对于低磁耦合的实际情况也表现良好。基于该模型，对一个400MHz的双环收发阵列的几何结构进行了优化，使得通过简单地重叠环路，互感和互电阻能够同时得到补偿。结果，无需任何额外的去耦电路就获得了出色的去耦效果（低于-40dB）。将一个重叠阵列原型与一个间隙阵列在信噪比、发射效率方面进行了比较，间隙阵列是一种先前在超高场发射阵列设计中使用过的几何结构。
