Department of Radiotherapy, University Medical Center Utrecht, P.O. Box 85500, 3508 GA Utrecht, The Netherlands.
Med Phys. 2010 Feb;37(2):814-21. doi: 10.1118/1.3298006.
Serious tissue heating may occur at the tips of elongated metallic structures used in MRI-guided interventions, such as vascular guidewires, catheters, biopsy needles, and brachytherapy needles. This heating is due to resonating electromagnetic radiofrequency (RF) waves along the structure. Since it is hard to predict the exact length at which resonance occurs under in vivo conditions, there is a need for methods to monitor this resonance behavior. In this study, the authors propose a method based on the RF induced image artefacts and demonstrate its applicability in two phantom experiments.
The authors developed an analytical model that describes the RF induced image artefacts as a function of the induced current in an elongated metallic structure placed parallel to the static magnetic field. It describes the total RF field as a sum of the RF fields produced by the transmit coil of the MR scanner and by the elongated metallic structure. Several spoiled gradient echo images with different nominal flip angle settings were acquired to map the B1+ field, which is a quantitative measure for the RF distortion around the structure. From this map, the current was extracted by fitting the analytical model. To investigate the sensitivity of our method we performed two phantom experiments with different setup parameters: One that mimics a brachytherapy needle insertion and one that resembles a guidewire intervention. In the first experiment, a short needle was placed centrally in the MR bore to ensure that the induced currents would be small. In the second experiment, a longer wire was placed in an off-center position to mimic a worst case scenario for the patient. In both experiments, a Luxtron (Santa Clara, CA) fiberoptic temperature sensor was positioned at the structure tip to record the temperature.
In the first experiment, no significant temperature increases were measured, while the RF image artefacts and the induced currents in the needle increased with the applied insertion depth. The maximum induced current in the needle was 44 mA. Furthermore, a standing wave pattern became clearly visible for larger insertion depths. In the second experiment, significant temperature increases up to 2.4 degrees C in 1 min were recorded during the image acquisitions. The maximum current value was 1.4 A. In both experiments, a proper estimation of the current in the metallic structure could be made using our analytical model.
The authors have developed a method to quantitatively determine the induced current in an elongated metallic structure from its RF distortion. This creates a powerful and sensitive method to investigate the resonant behavior of RF waves along elongated metallic structures used for MRI-guided interventions, for example, to monitor the RF safety or to inspect the influence of coating on the resonance length. Principally, it can be applied under in vivo conditions and for noncylindrical metallic structures such as hip implants by taking their geometry into account.
在 MRI 引导介入中使用的细长金属结构（如血管导丝、导管、活检针和近距离放射治疗针）的尖端可能会发生严重的组织加热。这种加热是由于沿着结构的共振电磁射频（RF）波引起的。由于很难预测在体内条件下发生共振的确切长度，因此需要一种监测这种共振行为的方法。在这项研究中，作者提出了一种基于 RF 感应图像伪影的方法，并在两个体模实验中证明了其适用性。
作者开发了一种分析模型，该模型将放置在平行于静磁场的细长金属结构中的感应电流描述为 RF 感应图像伪影的函数。它将总 RF 场描述为磁共振扫描仪的发射线圈和细长金属结构产生的 RF 场的总和。采集了几个具有不同标称翻转角设置的扰动脉冲梯度回波图像，以绘制 B1+场图，B1+场图是结构周围 RF 失真的定量测量。从该图中，通过拟合分析模型提取电流。为了研究我们方法的灵敏度，我们进行了两个具有不同设置参数的体模实验：一个模拟近距离放射治疗针插入，另一个模拟导丝介入。在第一个实验中，将一根短针放置在 MR 孔的中心，以确保感应电流较小。在第二个实验中，将一根较长的金属丝放置在偏离中心的位置，以模拟对患者最不利的情况。在两个实验中，都将 Luxtron（加利福尼亚州圣克拉拉）光纤温度传感器放置在结构尖端以记录温度。
在第一个实验中，当施加的插入深度增加时，未测量到明显的温度升高，而针中的 RF 图像伪影和感应电流增加。针中的最大感应电流为 44 mA。此外，对于较大的插入深度，明显出现驻波模式。在第二个实验中，在图像采集过程中记录到 1 分钟内高达 2.4°C 的明显温升。最大电流值为 1.4 A。在两个实验中，都可以使用我们的分析模型对金属结构中的电流进行适当估计。
作者已经开发出一种从 RF 失真定量确定细长金属结构中感应电流的方法。这为研究 MRI 引导介入中使用的细长金属结构中的 RF 波的共振行为提供了一种强大而敏感的方法，例如，用于监测 RF 安全性或检查涂层对共振长度的影响。原则上，它可以在体内条件下以及髋关节植入物等非圆柱形金属结构下应用，同时考虑其几何形状。
