SAR Simulations & Safety
SAR Simulations & Safety
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DOI：
10.1016/j.neuroimage.2017.03.035
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发表时间：
2018-03-01
期刊：
影响因子：
5.7
通讯作者：
Bitz, Andreas K.
中科院分区：
文献类型：
--
作者：
Fiedler, Thomas M.;Ladd, Mark E.;Bitz, Andreas K.
与低场系统相比，在超高场中，射频 (RF) 安全性评估提出了一些新的挑战。多通道射频发射线圈与并行发射技术相结合，在组织中产生随时间变化且随空间变化的功率损耗密度。此外，在超高场系统中，由于从准稳态到电磁场状态的转变，局部场效应可能更加明显。因此，需要有关射频场的本地信息来进行可靠的射频安全评估以及在 MR 检查期间监测射频暴露。使用解剖人体模型对真实暴露场景进行数值射频和热模拟是目前获得有关磁场和电场分布以及组织温度的必要局部信息的唯一实用方法。在本文中，回顾了安全法规和超高场系统中射频场分布的基本特征。重点介绍了射频场计算的数值方法以及分析真实多通道射频暴露场景（包括人体解剖模型）的典型要求。近年来，局部组织温度的计算越来越受到人们的关注，因为温度与组织损伤直接相关，因此期望进行更准确的安全评估。关于热模拟，讨论了考虑人体对射频暴露的生理反应的生物传热模型和方法。此外，还提出了合适的方法来通过测量来验证计算的射频和热结果。最后，讨论了基于仿真的广义比吸收率（SAR）矩阵模型的概念。这些模型可以纳入多通道 MR 系统中的局部 SAR 监测，并允许在局部 SAR 约束下设计射频脉冲。
At ultra-high fields, the assessment of radiofrequency (RF) safety presents several new challenges compared to low-field systems. Multi-channel RF transmit coils in combination with parallel transmit techniques produce time-dependent and spatially varying power loss densities in the tissue. Further, in ultra-high-field systems, localized field effects can be more pronounced due to a transition from the quasi stationary to the electromagnetic field regime. Consequently, local information on the RF field is required for reliable RF safety assessment as well as for monitoring of RF exposure during MR examinations. Numerical RF and thermal simulations for realistic exposure scenarios with anatomical body models are currently the only practical way to obtain the requisite local information on magnetic and electric field distributions as well as tissue temperature.In this article, safety regulations and the fundamental characteristics of RF field distributions in ultra-highfield systems are reviewed. Numerical methods for computation of RF fields as well as typical requirements for the analysis of realistic multi-channel RF exposure scenarios including anatomical body models are highlighted. In recent years, computation of the local tissue temperature has become of increasing interest, since a more accurate safety assessment is expected because temperature is directly related to tissue damage. Regarding thermal simulation, bio-heat transfer models and approaches for taking into account the physiological response of the human body to RF exposure are discussed. In addition, suitable methods are presented to validate calculated RF and thermal results with measurements. Finally, the concept of generalized simulation-based specific absorption rate (SAR) matrix models is discussed. These models can be incorporated into local SAR monitoring in multi-channel MR systems and allow the design of RF pulses under constraints for local SAR.