Liu Feng, Beck Barbara L, Fitzsimmons Jeffrey R, Blackband Stephen J, Crozier Stuart
The School of Information Technology and Electrical Engineering, The University of Queensland, St Lucia, Brisbane, Queensland 4072, Australia.
Phys Med Biol. 2005 Nov 21;50(22):5281-91. doi: 10.1088/0031-9155/50/22/005. Epub 2005 Oct 24.
In this paper, numerical simulations are used in an attempt to find optimal source profiles for high frequency radiofrequency (RF) volume coils. Biologically loaded, shielded/unshielded circular and elliptical birdcage coils operating at 170 MHz, 300 MHz and 470 MHz are modelled using the FDTD method for both 2D and 3D cases. Taking advantage of the fact that some aspects of the electromagnetic system are linear, two approaches have been proposed for the determination of the drives for individual elements in the RF resonator. The first method is an iterative optimization technique with a kernel for the evaluation of RF fields inside an imaging plane of a human head model using pre-characterized sensitivity profiles of the individual rungs of a resonator; the second method is a regularization-based technique. In the second approach, a sensitivity matrix is explicitly constructed and a regularization procedure is employed to solve the ill-posed problem. Test simulations show that both methods can improve the B(1)-field homogeneity in both focused and non-focused scenarios. While the regularization-based method is more efficient, the first optimization method is more flexible as it can take into account other issues such as controlling SAR or reshaping the resonator structures. It is hoped that these schemes and their extensions will be useful for the determination of multi-element RF drives in a variety of applications.
在本文中，通过数值模拟来尝试寻找高频射频（RF）容积线圈的最佳源分布。使用时域有限差分（FDTD）方法对工作在170MHz、300MHz和470MHz的加载生物模型的屏蔽/非屏蔽圆形和椭圆形鸟笼线圈进行二维和三维建模。利用电磁系统某些方面具有线性这一事实，提出了两种确定RF谐振器中单个元件驱动的方法。第一种方法是一种迭代优化技术，其核心是利用谐振器各个梯级的预先表征的灵敏度分布来评估人头模型成像平面内的RF场；第二种方法是基于正则化的技术。在第二种方法中，明确构建灵敏度矩阵并采用正则化程序来解决不适定问题。测试模拟表明，这两种方法在聚焦和非聚焦情况下都能提高B(1)场均匀性。虽然基于正则化的方法效率更高，但第一种优化方法更灵活，因为它可以考虑其他问题，如控制比吸收率（SAR）或重塑谐振器结构。希望这些方案及其扩展将有助于确定各种应用中的多元件RF驱动。
