Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
Department of Radiology, University Medical Center Utrecht, Utrecht, The Netherlands.
Magn Reson Med. 2018 Dec;80(6):2771-2784. doi: 10.1002/mrm.27218. Epub 2018 Apr 24.
We introduce the transfer matrix (TM) that makes MR-based wireless determination of transfer functions (TFs) possible. TFs are implant specific measures for RF-safety assessment of linear implants. The TF relates an incident tangential electric field on an implant to a scattered electric field at its tip that generally governs local heating. The TM extends this concept and relates an incident tangential electric field to a current distribution in the implant therewith characterizing the RF response along the entire implant. The TM is exploited to measure TFs with MRI without hardware alterations.
A model of rightward and leftward propagating attenuated waves undergoing multiple reflections is used to derive an analytical expression for the TM. This allows parameterization of the TM of generic implants, e.g., (partially) insulated single wires, in a homogeneous medium in a few unknowns that simultaneously describe the TF. These unknowns can be determined with MRI making it possible to measure the TM and, therefore, also the TF.
The TM is able to predict an induced current due to an incident electric field and can be accurately parameterized with a limited number of unknowns. Using this description the TF is determined accurately (with a Pearson correlation coefficient R ≥ 0.9 between measurements and simulations) from MRI acquisitions.
The TM enables measuring of TFs with MRI of the tested generic implant models. The MR-based method does not need hardware alterations and is wireless hence making TF determination in more realistic scenarios conceivable.
我们引入转移矩阵（TM），使基于磁共振的传递函数（TFs）的无线测定成为可能。TFs 是用于线性植入物射频安全评估的植入物特异性措施。TF 将入射切向电场与位于其尖端的散射电场相关联，而后者通常决定局部加热。TM 扩展了这一概念，并将入射切向电场与植入体中的电流分布相关联，从而描述了整个植入体的射频响应。TM 被用于在不进行硬件改动的情况下通过 MRI 测量 TFs。
采用右行和左行衰减波经历多次反射的模型，推导出 TM 的解析表达式。这允许对通用植入物（例如，部分绝缘的单根线）的 TM 进行参数化，在均匀介质中用几个未知数同时描述 TF。这些未知数可以通过 MRI 确定，从而可以测量 TM，因此也可以测量 TF。
TM 能够预测由于入射电场引起的感应电流，并且可以用有限数量的未知数进行精确参数化。使用此描述，可以从 MRI 采集准确地确定 TF（测量值与模拟值之间的 Pearson 相关系数 R≥0.9）。
TM 使我们能够通过 MRI 对测试的通用植入模型进行 TFs 的测量。基于 MRI 的方法不需要硬件改动，并且是无线的，因此可以设想在更现实的场景中进行 TF 测定。
