Ibrahim Tamer S, Mitchell Chad, Schmalbrock Petra, Lee Robert, Chakeres Donald W
School of Electrical and Computer Engineering and Center of BioEngineering, University of Oklahoma, Norman, Oklahoma 73019-1023, USA.
Magn Reson Med. 2005 Sep;54(3):683-90. doi: 10.1002/mrm.20596.
In this work experimental and numerical studies of the MR signal were performed at frequencies ranging from 64 MHz to 485 MHz, utilizing three different MRI coils: a single-strut transverse electromagnetic (TEM)-based coil, a TEM resonator, and a high-pass birdcage coil. The experimental analyses were conducted using 1.5 and 8 Tesla whole-body systems and volume RF head coils. The simulation data were obtained utilizing an in-house-developed finite difference time domain (FDTD) model. Pertinent data from the numerical and experimental setups were compared, and a remarkable agreement between the two methods was found that clearly demonstrates the effectiveness of the FDTD method when it is applied rigorously. The numerical and experimental studies demonstrate the complexity of the electromagnetic (EM) fields and their role in the MR signal. These studies also reveal unique similarities and differences between the transmit and receive field distributions at various field strengths. Finally, for ultra high-field operations, it was demonstrated mathematically, numerically, and experimentally that highly asymmetric inhomogeneous images can be acquired even for linear excitation, symmetrical load geometries, and symmetrical load positioning within the coil.
在这项工作中，利用三种不同的MRI线圈，在64 MHz至485 MHz的频率范围内对磁共振信号进行了实验和数值研究：一种基于单支柱横向电磁（TEM）的线圈、一个TEM谐振器和一个高通鸟笼线圈。实验分析使用1.5特斯拉和8特斯拉的全身系统以及容积射频头部线圈进行。模拟数据是利用自行开发的时域有限差分（FDTD）模型获得的。对数值和实验设置中的相关数据进行了比较，发现两种方法之间有显著的一致性，这清楚地证明了严格应用FDTD方法时的有效性。数值和实验研究证明了电磁场的复杂性及其在磁共振信号中的作用。这些研究还揭示了不同场强下发射和接收场分布之间独特的异同。最后，对于超高场操作，通过数学、数值和实验证明，即使对于线性激励、对称负载几何形状以及线圈内对称负载定位，也可以获取高度不对称的不均匀图像。
