Division of MR Research, Department of Radiology, Johns Hopkins School of Medicine, Baltimore, Maryland 21287 and Department of Electrical and Computer Engineering, Johns Hopkins University, Baltimore, Maryland 21218.
Med Phys. 2013 Dec;40(12):122303. doi: 10.1118/1.4829527.
The monitoring and management of radio frequency (RF) exposure is critical for ensuring magnetic resonance imaging (MRI) safety. Commercial MRI scanners can overestimate specific absorption rates (SAR) and improperly restrict clinical MRI scans or the application of new MRI sequences, while underestimation of SAR can lead to tissue heating and thermal injury. Accurate scanner-independent RF dosimetry is essential for measuring actual exposure when SAR is critical for ensuring regulatory compliance and MRI safety, for establishing RF exposure while evaluating interventional leads and devices, and for routine MRI quality assessment by medical physicists. However, at present there are no scanner-independent SAR dosimeters.
An SAR dosimeter with an RF transducer comprises two orthogonal, rectangular copper loops and a spherical MRI phantom. The transducer is placed in the magnet bore and calibrated to approximate the resistive loading of the scanner's whole-body birdcage RF coil for human subjects in Philips, GE and Siemens 3 tesla (3T) MRI scanners. The transducer loop reactances are adjusted to minimize interference with the transmit RF field (B1) at the MRI frequency. Power from the RF transducer is sampled with a high dynamic range power monitor and recorded on a computer. The deposited power is calibrated and tested on eight different MRI scanners. Whole-body absorbed power vs weight and body mass index (BMI) is measured directly on 26 subjects.
A single linear calibration curve sufficed for RF dosimetry at 127.8 MHz on three different Philips and three GE 3T MRI scanners. An RF dosimeter operating at 123.2 MHz on two Siemens 3T scanners required a separate transducer and a slightly different calibration curve. Measurement accuracy was ∼3%. With the torso landmarked at the xiphoid, human adult whole-body absorbed power varied approximately linearly with patient weight and BMI. This indicates that whole-body torso SAR is on average independent of the imaging subject, albeit with fluctuations.
Our 3T RF dosimeter and transducers accurately measure RF exposure in body-equivalent loads and provide scanner-independent assessments of whole-body RF power deposition for establishing safety compliance useful for MRI sequence and device testing.
监测和管理射频（RF）辐射对于确保磁共振成像（MRI）安全至关重要。商业 MRI 扫描仪可能会高估特定吸收率（SAR），从而错误地限制临床 MRI 扫描或新 MRI 序列的应用，而 SAR 的低估则可能导致组织加热和热损伤。准确的与扫描仪无关的 RF 剂量测定对于测量 SAR 至关重要的实际暴露非常重要，这对于确保监管合规性和 MRI 安全性、在评估介入性导联和设备时建立 RF 暴露以及通过医学物理学家进行常规 MRI 质量评估都至关重要。然而，目前尚无与扫描仪无关的 SAR 剂量计。
具有 RF 换能器的 SAR 剂量计包括两个正交的矩形铜环和一个球形 MRI 体模。将换能器放置在磁体孔中，并针对飞利浦、GE 和西门子 3 特斯拉（3T）MRI 扫描仪中的人体受试者对全身鸟笼 RF 线圈的电阻性负载进行校准。调整换能器环路电抗以最小化对 MRI 频率下的发射 RF 场（B1）的干扰。使用高动态范围功率监视器对来自 RF 换能器的功率进行采样，并记录在计算机上。在八台不同的 MRI 扫描仪上对沉积功率进行校准和测试。直接在 26 名受试者上测量全身吸收功率与体重和体重指数（BMI）的关系。
在三台不同的飞利浦和三台 GE 3T MRI 扫描仪上，127.8MHz 的 RF 剂量测定仅需一条单一的线性校准曲线即可。在两台西门子 3T 扫描仪上以 123.2MHz 运行的 RF 剂量计需要单独的换能器和略有不同的校准曲线。测量精度约为 3%。当以剑突标记躯干时，成年人体全身吸收功率与患者体重和 BMI 大致呈线性关系。这表明全身躯干 SAR 平均与成像受试者独立，尽管存在波动。
我们的 3T RF 剂量计和换能器可准确测量体等效负载中的 RF 暴露，并提供与扫描仪无关的全身 RF 功率沉积评估，可用于建立有助于 MRI 序列和设备测试的安全合规性。
