Radio frequency (RF) shimming is a widely used method to improve the MRI quality by adjusting the magnitude and phase of the transmit elements in the RF coil and eliminating the field inhomogeneity in 3T MRI system. This process alters the field distribution inside the MRI borehole and changes both the level and location of the absorbed RF energy in patients. It is necessary to assess the impact of RF shimming on the risk of RF-induced heating for patients especially those who have implantable longitude medical electrodes in place during the MRI procedure. This paper investigates the impacts of RF shimming on the RF-induced heating of a generic pacemaker electrode using a transfer function method. Two different shimming schemes were studied. The first excitation scheme was designed to obtain a homogeneous ${B_{1 + }}$ field in the image region-chest region. The second condition maximized the temperature rise around the generic pacemaker electrode with different implant trajectories. All the results were normalized to 2 W/kg average whole-body specific absorption rate (SAR) and a Q-matrices method was used to improve the SAR calculation efficiency. The average temperature rise at the electrode tip under quadrature excitation was 3.7 °C while under optimized excitation was 2.3 °C. The worst-case temperature rise for a selected trajectory was found to be 33.17 °C, which was almost 6 times of the temperature rise under quadrature excitation. Therefore, the RF-shimming condition can significantly change the RF-induced heating of electrodes. To ensure patient safety under MRI, RF-shimming condition should be considered and thoroughly integrated into the RF heating assessment.

中文翻译：

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射频匀场对 3T MRI 线圈中可植入医疗电极附近射频感应加热的影响

射频（RF）匀场是一种广泛使用的方法，通过调整射频线圈中发射元件的幅度和相位，消除 3T MRI 系统中的场不均匀性来提高 MRI 质量。这个过程改变了 MRI 钻孔内的场分布，并改变了患者吸收的射频能量的水平和位置。有必要评估 RF 匀场对患者的 RF 诱导加热风险的影响，尤其是那些在 MRI 过程中安装了可植入经度医疗电极的患者。本文使用传递函数方法研究了 RF 匀场对通用起搏器电极的 RF 感应加热的影响。研究了两种不同的匀场方案。第一个激发方案旨在在图像区域 - 胸部区域中获得均匀的 ${B_{1 + }}$ 场。第二个条件使具有不同植入轨迹的通用起搏器电极周围的温度升高最大化。所有结果均归一化为 2 W/kg 平均全身比吸收率 (SAR)，并使用 Q 矩阵方法提高 SAR 计算效率。正交激发下电极尖端的平均温升为 3.7 °C，而优化激发下的平均温升为 2.3 °C。发现所选轨迹的最坏情况温升为 33.17 °C，几乎是正交激励下温升的 6 倍。因此，射频匀场条件可以显着改变电极的射频感应加热。为确保 MRI 下患者的安全，