Medical Physics in Radiology, German Cancer Research Center (DKFZ), Heidelberg, Germany.
Erwin L. Hahn Institute for MRI, University Duisburg-Essen, Essen, Germany.
Magn Reson Med. 2025 Feb;93(2):842-849. doi: 10.1002/mrm.30306. Epub 2024 Sep 20.
Transmit arrays for body imaging have characteristics of both volume and local transmit coils. This study evaluates two specific absorption rate (SAR) aspects, local and whole-body SAR, of arrays for body imaging at 7 T and also for a 3 T birdcage.
Simulations were performed for six antenna arrays at 7 T and one 3 T birdcage. Local SAR matrices and the whole-body SAR matrix were computed and evaluated with random shims. A set of reduced local SAR matrices was determined by removing all matrices dominated by the whole-body SAR matrix.
The results indicate that all RF transmit coils for body imaging in this study are constrained by the local SAR limit. The ratio between local and whole-body SAR is nevertheless smaller for arrays with large FOV, as these arrays also expose a larger part of the human body. By using the whole-body SAR matrix, the number of local SAR matrices can be reduced (e.g., 33.3% matrices remained for an 8-channel local array and 89.7% for a 30-channel remote array; 12.1% for the 3 T birdcage).
For transmit antenna arrays used for body imaging at 7 T as well as for the 3 T birdcage, all evaluated cases show that the local SAR limit was reached before reaching the whole-body SAR limit. Nevertheless, the whole-body SAR matrix can be used to reduce the number of local SAR matrices, which is important to reduce memory and computing time for a virtual observation points (VOP) compression. This step can be included as a pre-compression prior to a VOP compression.
体部成像发射天线兼具容积式发射线圈和局部发射线圈的特点。本研究评估了 7T 体部成像专用发射天线阵和 3T 鸟笼式线圈的两个特定吸收率（SAR）方面，即局部 SAR 和全身 SAR。
在 7T 下对 6 个天线阵和 1 个 3T 鸟笼进行了模拟。使用随机匀场片计算并评估了局部 SAR 矩阵和全身 SAR 矩阵。通过去除由全身 SAR 矩阵主导的所有矩阵，确定了一组较小的局部 SAR 矩阵。
结果表明，本研究中所有用于体部成像的射频发射线圈均受到局部 SAR 限制的约束。然而，具有较大视野的阵列的局部 SAR 与全身 SAR 的比值更小，因为这些阵列也会暴露更大的人体部分。通过使用全身 SAR 矩阵，可以减少局部 SAR 矩阵的数量（例如，对于 8 通道局部阵列，保留了 33.3%的矩阵，对于 30 通道远程阵列，保留了 89.7%的矩阵；对于 3T 鸟笼，保留了 12.1%的矩阵）。
对于在 7T 下用于体部成像的发射天线阵列以及 3T 鸟笼式线圈，所有评估案例均表明，在达到全身 SAR 限制之前，局部 SAR 限制已达到。然而，全身 SAR 矩阵可用于减少局部 SAR 矩阵的数量，这对于减少虚拟观察点（VOP）压缩的内存和计算时间非常重要。此步骤可以作为 VOP 压缩之前的预压缩步骤。
