Department of Radiology, Mayo Clinic, Rochester, MN, 55905, USA.
Med Phys. 2017 Nov;44(11):5749-5754. doi: 10.1002/mp.12567. Epub 2017 Oct 9.
To assess risks of RF-heating of a vagus nerve stimulator (VNS) during 1.5 T prostate MRI using body coil transmit and to compare these risks with those associated with MRI head exams using a transmit/receive head coil.
Spatial distributions of radio-frequency (RF) B1 fields generated by transmit/receive (T/R) body and head coils were empirically assessed along the long axis of a 1.5 T MRI scanner bore. Measurements were obtained along the center axis of the scanner and laterally offset by 15 cm (body coil) and 7 cm (head coil). RF-field measurements were supplemented with direct measurements of RF-heating of 15 cm long copper wires affixed to and submerged in the "neck" region of the gelled saline-filled (sodium chloride and polyacrylic acid) "head-and-torso" phantom. Temperature elevations at the lead tips were measured using fiber-optic thermometers with the phantom positioned at systematically increased distances from the scanner isocenter.
B1 field measurements demonstrated greater than 10 dB reduction in RF power at distances beyond 28 cm and 24 cm from isocenter for body and head coil, respectively. Moreover, RF power from body coil transmit at distances greater than 32 cm from isocenter was found to be lower than from the RF power from head coil transmit measured at locations adjacent to the coil array at its opening. Correspondingly, maximum temperature elevations at the tips of the copper wires decreased with increasing distance from isocenter - from 7.4°C at 0 cm to no appreciable heating at locations beyond 40 cm.
For the particular scanner model evaluated in this study, positioning an implanted VNS farther than 32 cm from isocenter (configuration achievable for prostate exams) can reduce risks of RF-heating resulting from the body coil transmit to those associated with using a T/R head coil.
在 1.5T 前列腺 MRI 中使用体线圈发射评估迷走神经刺激器（VNS）的射频（RF）加热风险，并将这些风险与使用发射/接收头线圈进行 MRI 头部检查相关联的风险进行比较。
通过经验评估体线圈和头线圈的射频（RF）B1 场在 1.5T MRI 扫描仪孔径长轴上的空间分布。测量结果是沿扫描仪的中心轴获得的，并在侧面偏移 15cm（体线圈）和 7cm（头线圈）处获得。RF 场测量结果补充了直接测量固定在凝胶盐水（氯化钠和聚丙烯酸）“头身”模型“颈部”区域内并浸没在其中的 15cm 长铜丝的 RF 加热。使用光纤温度计测量位于距扫描仪等中心系统增加距离处的模型中的温度升高。
B1 场测量结果表明，距离体线圈和头线圈等中心分别超过 28cm 和 24cm 处，RF 功率降低超过 10dB。此外，在距等中心大于 32cm 的距离处，体线圈发射的 RF 功率被发现低于在靠近线圈阵列开口处测量的头线圈发射的 RF 功率。相应地，随着与等中心的距离增加，铜丝尖端的最大温度升高降低 - 从 0cm 处的 7.4°C 到 40cm 以上的位置没有明显加热。
对于在这项研究中评估的特定扫描仪模型，将植入的 VNS 定位在距等中心大于 32cm（可用于前列腺检查的配置）的位置可以将体线圈发射引起的 RF 加热风险降低到与使用发射/接收头线圈相关联的风险。
