Ipek Ozlem, Raaijmakers Alexander J, Lagendijk Jan J, Luijten Peter R, van den Berg Cornelis A T
Department of Radiotherapy, University Medical Center Utrecht, Utrecht, the Netherlands.
Magn Reson Med. 2014 Apr;71(4):1559-67. doi: 10.1002/mrm.24794. Epub 2013 Jun 10.
Surface transmit arrays used in ultra-high field body MRI require local specific absorption rate (SAR) assessment. As local SAR cannot be measured directly, local SAR is determined by simulations using dielectric patient models. In this study, the inter-patient local SAR variation is investigated for 7T prostate imaging with the single-side adapted dipole antenna array.
Four-dedicated dielectric models were created by segmenting Dixon water-fat separated images that were obtained from four subjects with a 1.5T scanner and the surface array in place. Electromagnetic simulations were performed to calculate the SAR distribution for each model. Radio frequency (RF) exposure variations were determined by analyzing the SAR(10g) distributions (1) with one element active, (2) using a Q-matrix eigenvalue/eigenvector approach, (3) with the maximum potential SAR in each voxel, and (4) for a phase shimmed prostate measurement.
Maximum potential local SAR levels for 1 W time-averaged accepted power per transmit channel range from 4.1 to 7.1 W/kg.
These variations show that one model is not sufficient to determine safe scan settings. For the operation of the surface array conservative power settings were derived based on a worst-case SAR evaluation and the most SAR-sensitive body model.
超高场人体磁共振成像（MRI）中使用的表面发射阵列需要进行局部比吸收率（SAR）评估。由于无法直接测量局部SAR，因此通过使用介电患者模型进行模拟来确定局部SAR。在本研究中，使用单侧适配偶极天线阵列对7T前列腺成像的患者间局部SAR变化进行了研究。
通过分割从四名使用1.5T扫描仪和表面阵列的受试者获得的狄克逊水脂分离图像，创建了四个专用介电模型。进行电磁模拟以计算每个模型的SAR分布。通过分析SAR（10g）分布来确定射频（RF）暴露变化：（1）一个元件激活时；（2）使用Q矩阵特征值/特征向量方法；（3）每个体素中的最大潜在SAR；（4）对于相位匀场前列腺测量。
每个发射通道1W时间平均接受功率的最大潜在局部SAR水平范围为4.1至7.1W/kg。
这些变化表明，单一模型不足以确定安全的扫描设置。对于表面阵列的操作，基于最坏情况的SAR评估和最敏感的SAR人体模型得出了保守的功率设置。
