Singerman R W, Denison T J, Wen H, Balaban R S
Laboratory of Cardiac Energetics, National Heart Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892-1061, USA.
J Magn Reson. 1997 Mar;125(1):72-83. doi: 10.1006/jmre.1996.1073.
Two issues that pertain to the optimal static magnetic field for cardiac MRI were addressed: intrinsic signal-to-noise ratio (ISNR) and radiofrequency power deposition. From 1.5 to 9.5 T, proton Larmor frequencies of 63 to 400 MHz, numerical simulations were performed of the RF fields from a surface coil and a body coil loaded by a heterogeneous, three-dimensional, symmetric model of the human chest. The RF field distribution, the power required to produce the RF field, and the ISNR at the center of the heart were computed. The model was validated by comparison with experimental data up to 4 T. The RF field distortion was quantified and found to increase linearly up to 6 T due mostly to dielectric resonance modes. Body coil simulations beyond 6 T showed the onset of higher-order modes at the center of the heart. A range of expected RF power requirements was constructed as a function of field up to 9.5 T for surface coils and up to 6.8 T for body coils. Over this range of static field, ISNR for a constant coil geometry was bracketed by an upper limit that was slightly greater than linear with field and a lower limit that was slightly less than linear with field. The RF power and ISNR showed a strong dependence on chest thickness at 1.5 and 4.0 T. Additionally, independent of chest thickness, the model predicts a lower limit of a factor of 5 increase in RF power as the static field is increased from 1.5 to 4 T. Implications for imaging with other nuclei are discussed. Methods for checking the self-consistency of electrodynamic simulations are presented.
探讨了与心脏磁共振成像（MRI）最佳静磁场相关的两个问题：固有信噪比（ISNR）和射频功率沉积。在1.5至9.5T（质子拉莫尔频率为63至400MHz）范围内，利用人体胸部的非均匀三维对称模型加载表面线圈和体线圈，对射频场进行了数值模拟。计算了射频场分布、产生射频场所需的功率以及心脏中心处的ISNR。通过与高达4T的实验数据进行比较，验证了该模型。对射频场畸变进行了量化，发现其在6T之前呈线性增加，主要归因于介电共振模式。超过6T的体线圈模拟显示心脏中心出现高阶模式。针对表面线圈构建了高达9.5T、针对体线圈构建了高达6.8T的一系列预期射频功率需求与场强的函数关系。在这个静磁场范围内，对于恒定的线圈几何形状，ISNR被一个上限和一个下限所界定，上限略高于与场强呈线性关系，下限略低于与场强呈线性关系。在1.5T和4.0T时，射频功率和ISNR对胸部厚度有很强的依赖性。此外，与胸部厚度无关，该模型预测当静磁场从1.5T增加到4T时，射频功率下限会增加5倍。讨论了对其他原子核成像的影响。还介绍了检查电动力学模拟自洽性的方法。
