Vanderbilt University Institute of Imaging Science, 1161 21st Avenue South, Nashville, TN 37232, USA; Department of Physics and Astronomy, Vanderbilt University, 6301 Stevenson Science Center, Nashville, TN 37232, USA.
Vanderbilt University Institute of Imaging Science, 1161 21st Avenue South, Nashville, TN 37232, USA; Department of Biomedical Engineering, Vanderbilt University, PMB 351631, 2301 Vanderbilt Place, Nashville, TN 37235, USA.
Magn Reson Imaging. 2022 Oct;92:187-196. doi: 10.1016/j.mri.2022.07.009. Epub 2022 Jul 13.
This study shows how inter-subject variation over a dataset of 72 head models results in specific absorption rate (SAR) and B field homogeneity differences using common shim scenarios.
MR-CT datasets were used to segment 71 head models into 10 tissue compartments. These head models were affixed to the shoulders and neck of the virtual family Duke model and placed within an 8 channel transmit surface-loop array to simulate the electromagnetic fields of a 7T imaging experiment. Radio frequency (RF) shimming using the Gerchberg-Saxton algorithm and Circularly Polarized shim weights over the entire brain and select slices of each model was simulated. Various SAR metrics and B maps were calculated to demonstrate the contribution of head variation to transmit inhomogeneity and SAR variability.
With varying head geometries the loading for each transmit loop changes as evidenced by changes in S-parameters. The varying shim conditions and head geometries are shown to affect excitation uniformity, spatial distributions of local SAR, and SAR averaging over different pulse sequences. The Gerchberg-Saxton RF shimming algorithm outperforms circularly polarized shimming for all head models. Peak local SAR within the coil most often occurs nearest the coil on the periphery of the body. Shim conditions vary the spatial distribution of SAR.
The work gives further support to the need for fast and more subject specific SAR calculations to maintain safety. Local SAR is shown to vary spatially given shim conditions, subject geometry and composition, and position within the coil.
本研究展示了在 72 个头模型数据集上的受试者间变异性如何导致特定吸收率 (SAR) 和 B 场均匀性差异，使用常见的匀场方案。
MR-CT 数据集用于将 71 个头模型分割成 10 个组织隔室。这些头模型被固定在虚拟家族 Duke 模型的肩部和颈部，并放置在 8 通道发射表面-环形阵列内，以模拟 7T 成像实验的电磁场。使用 Gerchberg-Saxton 算法和全脑以及每个模型的选定切片的圆极化匀场权重模拟射频 (RF) 匀场。计算了各种 SAR 指标和 B 图，以证明头部变化对发射不均匀性和 SAR 可变性的贡献。
由于每个发射环的负载随头型的变化而变化，因此 S 参数的变化证明了这一点。不同的匀场条件和头型被证明会影响激励均匀性、局部 SAR 的空间分布以及不同脉冲序列的 SAR 平均值。对于所有头模型，Gerchberg-Saxton RF 匀场算法的性能均优于圆极化匀场。线圈内的局部 SAR 峰值通常最靠近身体边缘的线圈。匀场条件改变了 SAR 的空间分布。
本工作进一步支持需要快速且更具个体特异性的 SAR 计算以维持安全性。局部 SAR 被证明在给定匀场条件、受试者几何形状和组成以及在线圈内的位置时会发生空间变化。
