Department of Radiotherapy, University Medical Center Utrecht, P.O. Box 85500, Utrecht, 3508 GA, Netherlands.
Department of Radiology, University Medical Center Utrecht, P.O. Box 85500, Utrecht, 3508 GA, Netherlands.
Med Phys. 2021 Jan;48(1):132-141. doi: 10.1002/mp.14225. Epub 2020 Nov 28.
In this work a simulation study is performed to gain insights in the patterns of induced radiofrequency (RF) currents for various implant-like structures at 1.5 T. The previously introduced transfer matrix (TM) is used to determine why certain current patterns have a tendency to naturally occur. This can benefit current safety assessment techniques and may enable the identification of critical exposure conditions.
The induced current on an elongated implant can be determined by multiplication of the incident electric field along the implant with its TM. The eigenmode spectrum of the TMs for various lengths and various types of implants are determined. The eigenvector with the highest eigenvalue describes the incident electric field pattern that induces the highest current which in turn will lead to highest heating. Subsequently, a statistical probability analysis is performed using a wide range of potential incident electric field distributions in a representative human subject model during a 1.5 T MR exam which are determined by means of electromagnetic FDTD simulations. These incident electric field distributions and the resulting induced current patterns are projected onto eigenvectors of the TM to determine which eigenmodes of the implant dominate the current patterns.
The eigenvectors of the TM of bare and insulated wires resemble sinusoidal harmonics of a string fixed at both ends similar to the natural-current distribution on thin antennas(1). The currents on implants shorter than 20 cm are generally dominated by the first harmonic (similar to half a sine wave). This is firstly because for these implant lengths (relative to the RF wavelength), the first eigenvalue is more than three times bigger than the second showing the ability of an implant to accommodate one eigenmode better than another. Secondly, the incident electric fields have a high likelihood (≳95,7%) to project predominantly on this first eigenmode.
The eigenmode spectrum of the TM of an implant provides insight into the expected shape of induced current distributions and worst-case exposure conditions. For short implants, the first eigenvector is dominant. In addition, realistic incident electric field distributions project more heavily on this eigenvector. Both effects together cause significant currents to always resemble the dominant eigenmode of the TM for short implants at 1.5 T.
本研究通过 1.5T 下各种植入式结构的感应射频（RF）电流模式的仿真研究，深入了解感应 RF 电流的模式。本文采用之前提出的传输矩阵（TM）来确定为什么某些电流模式会自然出现，并利用这一方法来辅助当前的安全评估技术，以便识别关键的暴露条件。
在 TM 中，将植入体上的感应电流乘以其 TM 中的入射电场即可确定。我们确定了各种长度和类型的植入体的 TM 的特征模式谱。具有最大特征值的特征向量描述了感应最大电流的入射电场模式，而最大电流又会导致最高的加热。随后，在代表 1.5T MRI 检查中的人体模型中，使用电磁场 FDTD 模拟确定广泛的潜在入射电场分布，对其进行统计概率分析。将这些入射电场分布和感应电流模式投影到 TM 的特征向量上，以确定植入体的特征模式对电流模式的主导作用。
裸线和绝缘线 TM 的特征向量类似于两端固定的弦的正弦谐波，类似于细天线的自然电流分布(1)。长度小于 20cm 的植入体的电流通常主要由第一谐波（类似于半正弦波）主导。这首先是因为对于这些植入体长度（相对于射频波长），第一特征值比第二特征值大 3 倍以上，这表明植入体更好地适应一个特征模式而不是另一个特征模式的能力。其次，入射电场很有可能（≳95.7%）主要投影到这个第一特征模式上。
植入体 TM 的特征模式谱提供了对感应电流分布和最坏暴露条件的预期形状的深入了解。对于短植入体，第一特征向量占主导地位。此外，实际的入射电场分布在这个特征向量上的投影更重。这两个因素共同导致在 1.5T 下，短植入体的感应电流总是类似于 TM 的主导特征模式。
