Department Mathematik, Universität Erlangen-Nürnberg, Erlangen, Germany.
Magn Reson Med. 2011 Nov;66(5):1468-76. doi: 10.1002/mrm.22927. Epub 2011 May 20.
The supervision of local specific absorption rate (SAR) in parallel transmission applications in MRI is crucial. One existing approach is to use electromagnetic simulations including human anatomical models and to precalculate the electric field distributions of each individual channel. These can be superposed later with respect to certain combined excitations under investigation, and the local SAR distribution can be evaluated. Local SAR maxima can be obtained by exhaustive search over all investigated subvolumes of the body model. Practical challenges arise for the adequate handling and comparing of precalculated field distributions as long as the expected combined radiofrequency excitations are still undetermined. Worst-case approximations for local SAR lead to significant radiofrequency pulse performance limitations. Optimizing local SAR in radiofrequency pulse design using constraints for each subvolume is impractical. A method is proposed to significantly reduce the complexity without restriction to particular radiofrequency excitations. By constructing several matrices, it becomes sufficient to consider only these so-called Virtual Observation Points for an adequate, conservative estimation of the maximum local SAR. The applied techniques involve concepts of vector optimization as well as semidefinite programming.
在 MRI 中并行传输应用中，局部比吸收率（SAR）的监督至关重要。一种现有的方法是使用包括人体解剖模型的电磁模拟，并预先计算每个单独通道的电场分布。然后可以根据特定的组合激励进行叠加，并评估局部 SAR 分布。通过对人体模型的所有研究子体积进行穷尽搜索，可以获得局部 SAR 的最大值。只要预期的组合射频激励仍然不确定，就会出现适当地处理和比较预先计算的场分布的实际挑战。局部 SAR 的最坏情况逼近导致显著的射频脉冲性能限制。使用每个子体积的约束来优化射频脉冲设计中的局部 SAR 是不切实际的。提出了一种方法，可以在不限制特定射频激励的情况下，显著降低复杂性。通过构建几个矩阵，仅考虑这些所谓的虚拟观测点，就足以对最大局部 SAR 进行充分保守的估计。所应用的技术涉及向量优化和半定规划的概念。
