IEEE Trans Med Imaging. 2013 Jun;32(6):1058-67. doi: 10.1109/TMI.2013.2251653. Epub 2013 Mar 11.
Elevated specific absorption rate (SAR) associated with increased main magnetic field strength remains a major safety concern in ultra-high-field (UHF) magnetic resonance imaging (MRI) applications. The calculation of local SAR requires the knowledge of the electric field induced by radio-frequency (RF) excitation, and the local electrical properties of tissues. Since electric field distribution cannot be directly mapped in conventional MR measurements, SAR estimation is usually performed using numerical model-based electromagnetic simulations which, however, are highly time consuming and cannot account for the specific anatomy and tissue properties of the subject undergoing a scan. In the present study, starting from the measurable RF magnetic fields (B1) in MRI, we conducted a series of mathematical deduction to estimate the local, voxel-wise and subject-specific SAR for each single coil element using a multi-channel transceiver array coil. We first evaluated the feasibility of this approach in numerical simulations including two different human head models. We further conducted experimental study in a physical phantom and in two human subjects at 7T using a multi-channel transceiver head coil. Accuracy of the results is discussed in the context of predicting local SAR in the human brain at UHF MRI using multi-channel RF transmission.
与主磁场强度增加相关的特定吸收率 (SAR) 仍然是超高磁场 (UHF) 磁共振成像 (MRI) 应用中的一个主要安全问题。局部 SAR 的计算需要知道射频 (RF) 激发产生的电场，以及组织的局部电特性。由于在常规 MR 测量中不能直接映射电场分布，因此 SAR 估计通常使用基于数值模型的电磁模拟来进行，然而，这种方法非常耗时，并且不能考虑到正在进行扫描的对象的特定解剖结构和组织特性。在本研究中，我们从 MRI 中可测量的射频磁场 (B1) 出发，通过使用多通道收发器阵列线圈，对每个单线圈元件进行了一系列数学推导，以估计局部、体素和受试者特异性 SAR。我们首先在包括两个不同的人头模型的数值模拟中评估了这种方法的可行性。然后，我们在 7T 下的物理体模和两个人体受试者中进行了实验研究，使用了多通道收发器头部线圈。在使用多通道 RF 传输在 UHF MRI 中预测人脑的局部 SAR 的背景下，讨论了结果的准确性。
