Modeling of radio-frequency induced currents on lead wires during MR imaging using a modified transmission line method
Modeling of radio-frequency induced currents on lead wires during MR imaging using a modified transmission line method
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DOI：
10.1118/1.3662865
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发表时间：
2011-12-01
期刊：
影响因子：
3.8
通讯作者：
Atalar, Ergin
中科院分区：
文献类型：
--
作者：
Acikel, Volkan;Atalar, Ergin
目的：在磁共振(MR)成像过程中，金属植入物可能会引起严重的组织发热。这种加热是由于射频(RF)场产生的感应电流引起的。到目前为止，人们已经做了大量的工作来了解射频场和感应电流之间的关系。然而，这些研究大多纯粹基于实验或数值方法。本研究有三个主要目的：(1)使用两个参数定义植入体导线的射频加热特性；(2)建立直接解释射频场与感应电流之间关系的解析公式；(3)形成复杂情况分析的基础。方法：在本研究中，将传输线的集总单元模型修改为体内植入体引线的模型。使用该模型，导线使用两个参数来定义：单位长度阻抗Z和沿导线的有效波数k(T)。这两个参数是用类似于传输线理论的方法得到的。只要导线的这些参数是已知的，无论导线的几何形状多么复杂，都可以获得导线中感应的电流。用改进的传输线方法(MoTLiM)计算了裸线、有损导线和绝缘导线中的感应电流。首先，计算了均匀电场分布下的感应电流，并与矩量法的计算结果进行了比较。此外，还将MoTLiM结果与体模实验结果进行了比较。为了进行实验验证，采用了感应电流引起的翻转角失真。利用翻转角成像方法测量了导线周围的翻转角分布，并利用MoTLiM获得的电流分布计算了翻转角分布。结果：用MoTLiM和MOM分别求解了1.5T和3T扫描仪在均匀电场和线性变化电场照射下的裸露理想电导线、裸露有耗绝缘理想导体导线。测定结果的均方误差在10%以内。比较了实验得到的翻转角在与导线不同距离的方位向路径上的分布。结论：提出了一种新的方法来定义双参数的植入导线的射频加热特性，并分析在磁共振成像(MRI)扫描过程中暴露在有损介质中的植入导线上的感应电流。通过一些简单的案例来解释MOTLiM，为复杂案例的分析奠定了基础。该方法显示了入射射频场与感应电流之间的直接关系。此外，MoTLiM还根据导线的物理特征和介质的电学性质揭示了植入导线的射频加热特性。(C)2011年美国医学物理学家协会。[DOI：10.1118/1.3662865]
Purpose: 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.Methods: 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.Results: 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.Conclusions: 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. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3662865]