Department of Electrical and Electronics Engineering, Bilkent University, Bilkent, Ankara 06800, Turkey.
Med Phys. 2011 Dec;38(12):6623-32. doi: 10.1118/1.3662865.
Metallic implants may cause serious tissue heating during magnetic resonance (MR) imaging. This heating occurs due to the induced currents caused by the radio-frequency (RF) field. Much work has been done to date to understand the relationship between the RF field and the induced currents. Most of these studies, however, were based purely on experimental or numerical methods. This study has three main purposes: (1) to define the RF heating properties of an implant lead using two parameters; (2) to develop an analytical formulation that directly explains the relationship between RF fields and induced currents; and (3) to form a basis for analysis of complex cases.
In this study, a lumped element model of the transmission line was modified to model leads of implants inside the body. Using this model, leads are defined using two parameters: impedance per unit length, Z, and effective wavenumber along the lead, k(t). These two parameters were obtained by using methods that are similar to the transmission line theory. As long as these parameters are known for a lead, currents induced in the lead can be obtained no matter how complex the lead geometry is. The currents induced in bare wire, lossy wire, and insulated wire were calculated using this new method which is called the modified transmission line method or MoTLiM. First, the calculated induced currents under uniform electric field distribution were solved and compared with method-of-moments (MoM) calculations. In addition, MoTLiM results were compared with those of phantom experiments. For experimental verification, the flip angle distortion due to the induced currents was used. The flip angle distribution around a wire was both measured by using flip angle imaging methods and calculated using current distribution obtained from the MoTLiM. Finally, these results were compared and an error analysis was carried out.
Bare perfect electric, bare lossy, and insulated perfect electric conductor wires under uniform and linearly varying electric field exposure were solved, both for 1.5 T and 3 T scanners, using both the MoTLiM and MoM. The results are in agreement within 10% mean-square error. The flip angle distribution that was obtained from experiments was compared along the azimuthal paths with different distances from the wire. The highest mean-square error was 20% among compared cases.
A novel method was developed to define the RF heating properties of implant leads with two parameters and analyze the induced currents on implant leads that are exposed to electromagnetic fields in a lossy medium during a magnetic resonance imaging (MRI) scan. Some simple cases are examined to explain the MoTLiM and a basis is formed for the analysis of complex cases. The method presented shows the direct relationship between the incident RF field and the induced currents. In addition, the MoTLiM reveals the RF heating properties of the implant leads in terms of the physical features of the lead and electrical properties of the medium.
金属植入物在磁共振（MR）成像期间可能会导致严重的组织加热。这种加热是由射频（RF）场引起的感应电流引起的。迄今为止，已经进行了大量工作来了解 RF 场与感应电流之间的关系。然而，这些研究中的大多数都是基于纯实验或数值方法。本研究有三个主要目的：（1）使用两个参数定义植入物引线的 RF 加热特性；（2）开发一种直接解释 RF 场与感应电流之间关系的解析公式；（3）为分析复杂情况奠定基础。
在这项研究中，修改了传输线的集总元件模型，以模拟体内植入物的引线。使用该模型，通过两种参数来定义引线：单位长度的阻抗，Z 和沿引线的有效波数，k（t）。通过类似于传输线理论的方法获得这两个参数。只要知道引线的这两个参数，无论引线几何形状多么复杂，都可以获得引线上感应的电流。使用称为改进传输线方法或 MoTLiM 的新方法计算了裸线、有损耗线和绝缘线中的感应电流。首先，求解在均匀电场分布下感应的电流，并将其与矩量法（MoM）的计算结果进行比较。此外，还比较了 MoTLiM 结果与幻影实验的结果。为了实验验证，使用感应电流引起的翻转角失真。使用翻转角成像方法测量了线周围的翻转角分布，并使用从 MoTLiM 获得的电流分布进行了计算。最后，对这些结果进行了比较，并进行了误差分析。
使用 MoTLiM 和 MoM，分别针对 1.5 T 和 3 T 扫描仪，解决了均匀和线性变化电场暴露下的裸完美电导体、裸有损和绝缘完美电导体线的问题。结果在均方误差 10%以内一致。从实验中获得的翻转角分布与距导线不同距离的沿方位路径进行了比较。比较案例中最高的均方误差为 20%。
开发了一种新方法，使用两个参数定义植入物引线的 RF 加热特性，并分析在磁共振成像（MRI）扫描期间，在有损耗介质中，电磁场所暴露的植入物引线上的感应电流。检查了一些简单的情况以解释 MoTLiM，并为复杂情况的分析奠定了基础。所提出的方法显示了入射 RF 场与感应电流之间的直接关系。此外，MoTLiM 以引线的物理特征和介质的电特性来表示植入物引线的 RF 加热特性。
