Section on Experimental Radiology, University Hospital Tübingen, D-72076 Tübingen, Germany.
Med Phys. 2011 Oct;38(10):5522-9. doi: 10.1118/1.3637495.
State of the art to access radiofrequency (RF) heating near implants is computer modeling of the devices and solving Maxwell's equations for the specific setup. For a set of input parameters, a fixed result is obtained. This work presents a theoretical approach in the alternating current (ac) limit, which can potentially render closed formulas for the basic behavior of tissue heating near metallic structures. Dedicated experiments were performed to support the theory.
For the ac calculations, the implant was modeled as an RLC parallel circuit, with L being the secondary of a transformer and the RF transmission coil being its primary. Parameters influencing coupling, power matching, and specific absorption rate (SAR) were determined and formula relations were established. Experiments on a copper ring with a radial gap as capacitor for inductive coupling (at 1.5 T) and on needles for capacitive coupling (at 3 T) were carried out. The temperature rise in the embedding dielectric was observed as a function of its specific resistance using an infrared (IR) camera.
Closed formulas containing the parameters of the setup were obtained for the frequency dependence of the transmitted power at fixed load resistance, for the calculation of the resistance for optimum power transfer, and for the calculation of the transmitted power in dependence of the load resistance. Good qualitative agreement was found between the course of the experimentally obtained heating curves and the theoretically determined power curves. Power matching revealed as critical parameter especially if the sample was resonant close to the Larmor frequency.
The presented ac approach to RF heating near an implant, which mimics specific values for R, L, and C, allows for closed formulas to estimate the potential of RF energy transfer. A first reference point for worst-case determination in MR testing procedures can be obtained. Numerical approaches, necessary to determine spatially resolved heating maps, can be supported.
目前，对植入物附近射频 (RF) 加热的研究现状是对设备进行计算机建模，并针对特定设置求解麦克斯韦方程组。对于一组输入参数，会得到一个固定的结果。本工作提出了一种交流 (ac) 极限下的理论方法，该方法有可能为金属结构附近组织加热的基本行为提供封闭公式。专门的实验支持了该理论。
对于 ac 计算，植入物被建模为一个 RLC 并联电路，其中 L 是变压器的次级，RF 传输线圈是其初级。确定了影响耦合、功率匹配和比吸收率 (SAR) 的参数，并建立了公式关系。在 1.5 T 下进行了带有径向间隙的铜环作为电感耦合的电容器和 3 T 下的针作为电容耦合的实验。使用红外 (IR) 相机观察嵌入介电材料的温升与其比电阻的关系。
获得了在固定负载电阻下传输功率随频率变化的封闭公式，用于计算最佳功率传输的电阻，以及负载电阻变化时传输功率的计算。实验获得的加热曲线和理论确定的功率曲线之间存在很好的定性一致性。功率匹配被证明是一个关键参数，特别是当样品接近拉莫尔频率时发生谐振。
本文提出的植入物附近 RF 加热的 ac 方法模拟了 R、L 和 C 的特定值，可用于估计 RF 能量传输的潜力。可以获得磁共振测试程序中最坏情况确定的参考点。可以支持确定空间分辨加热图的数值方法。
