Local Specific Absorption Rate Control for Parallel Transmission by Virtual Observation Points
Local Specific Absorption Rate Control for Parallel Transmission by Virtual Observation Points
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DOI：
10.1002/mrm.22927
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发表时间：
2011-11-01
影响因子：
3.3
通讯作者：
Gebhardt, Matthias
中科院分区：
文献类型：
--
作者：
Eichfelder, Gabriele;Gebhardt, Matthias
在MRI的并行传输应用中，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。这些可以叠加后，相对于某些组合激励下的调查，并可以评估当地的SAR分布。局部SAR最大值可以通过对身体模型的所有研究子体积进行穷举搜索来获得。只要预期的组合射频激励仍然不确定，就出现了充分处理和比较预先计算的场分布的实际挑战。局部SAR的最差情况近似值导致显著的射频脉冲性能限制。在射频脉冲设计中使用每个子体积的约束条件优化局部SAR是不切实际的。提出了一种方法，以显着降低复杂性，而不限于特定的射频激励。通过构造几个矩阵，只考虑这些所谓的虚拟观测点就足以对最大局部SAR进行适当、保守的估计。所应用的技术涉及向量优化以及半定规划的概念。Magn Reson Med 66：1468-1476，2011. (C)2011 Wiley Periodicals，Inc.
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. Magn Reson Med 66:1468-1476, 2011. (C) 2011 Wiley Periodicals, Inc.