University of Pittsburgh, Bioengineering, Pittsburgh, PA, USA.
Carnegie Mellon University, Biomedical Engineering, Pittsburgh, PA, USA.
NMR Biomed. 2021 Mar;34(3):e4441. doi: 10.1002/nbm.4441. Epub 2020 Dec 22.
Electromagnetic simulations are an important tool for the safety assessment of RF coils. They are a useful resource for MRI RF coil designers, especially when complemented with experimental measurements and testing using physical phantoms. Regular-shaped (spherical/cylindrical) homogeneous phantoms are the MRI standard for RF testing but are somewhat inaccurate when compared with anthropomorphic anatomies, especially at high frequencies. In this work, using a recently developed anthropomorphic heterogeneous human head phantom, studies were performed to analyze the scattering parameters (S-parameters) and the electric and magnetic field distributions using (1) the B field mapping method on a 7 T human MRI scanner and (2) numerical full-wave electromagnetic simulations. All studies used the following: a recently developed six-compartment refillable 3D-printed anthropomorphic head phantom (developed from MRI scans obtained in vivo), where the phantom itself is filled in its entirety with either heterogeneous loading, or homogeneous brain or water loading, in vivo imaging, and a commercial homogeneous spherical water phantom. Our results determined that the calculated S-parameters for all the anthropomorphic head phantom models were comparable to the model that is based on the volunteer (within 17% difference of the reflection coefficient value) but differed for the commercial homogeneous spherical water phantom (within 45% difference). The experimentally measured B field maps of the anthropomorphic heterogeneous and homogeneous brain head phantoms were most comparable to the in vivo measured values. The numerical simulations also show that both the anthropomorphic homogeneous water and brain phantom models were less accurate in terms of electric field intensities/distributions when compared with the segmented in-vivo-based head model and the anthropomorphic heterogeneous head phantom model. The presented data highlights the differences between the physical phantoms/phantom models, and the in vivo measurements/segmented in-vivo-based head model. The results demonstrate the usefulness of 3D-printed anthropomorphic phantoms for RF coil evaluation and testing.
电磁仿真在射频线圈的安全评估中是一种重要的工具。对于 MRI 射频线圈设计师来说，它们是一种非常有用的资源，特别是在与物理体模的实验测量和测试相结合使用时。规则形状（球形/圆柱形）的均匀体模是 MRI 射频测试的标准，但与人体解剖结构相比，尤其是在高频下，它们的准确性有些不足。在这项工作中，使用最近开发的人体异质人头模型，进行了研究以分析散射参数（S 参数）以及使用（1）在 7 T 人体 MRI 扫描仪上进行 B 场映射方法和（2）数值全波电磁仿真的电场和磁场分布。所有研究都使用了以下内容：最近开发的六腔可填充 3D 打印人体异质人头模型（由体内获得的 MRI 扫描开发），其中整个模型要么充满不均匀的负载，要么充满均匀的大脑或水负载，进行体内成像，以及商业的均匀球形水体模。我们的结果确定，所有人体异质人头模型的计算 S 参数都与基于志愿者的模型（反射系数值的差异在 17%以内）相当，但与商业均匀球形水体模（差异在 45%以内）不同。人体异质和均匀脑头模型的实验测量 B 场图与体内测量值最接近。数值模拟还表明，与基于分割的体内头模型和人体异质头模型相比，人体异质均匀水和脑体模模型在电场强度/分布方面的准确性较差。所呈现的数据突出了物理体模/体模模型与体内测量值/基于分割的体内头模型之间的差异。结果证明了 3D 打印人体模型在射频线圈评估和测试中的有用性。
