Tier-based formalism for safety assessment of custom-built radio-frequency transmit coils.
Tier-based formalism for safety assessment of custom-built radio-frequency transmit coils.
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这项工作的目的是为定制射频(RF)线圈的安全评估提出一种基于层次的形式化方法，以平衡验证工作与确定安全系数的工作。形式主义有三个层次。在验证电磁模拟结果时，较高的级别需要更多的工作，但允许较不保守的安全系数。此外，我们提出了一种新的方法来计算模拟和测量之间的建模不确定性，以及一种新的方法来将模拟中的不确定性传播到一个安全系数，从而最小化低估峰比吸收率(SAR)的风险。使用来自两个不同研究地点的数据，完成了对所有级别的新的安全性评估程序，其中8通道偶极阵列用于7T的前列腺成像，8通道偶极阵列用于10.5T的头部成像。对于7T车体阵列，验证过程导致了测量和模拟的局部SAR分布之间的建模不确定度为77%。对于对前列腺进行射频垫片的情况，第2层和第3层的平均功率限制分别为2.4W/通道和4.5W/通道。当计算所有相位设置中最差情况的峰值SAR时，第二层和第三层的功率极限分别为1.4 W/通道和2.7 W/通道。对于10.5T头阵，基于映射的建模不确定度为21%。对于第2层验证，计算的功率限制为2.6 W/通道。所展示的层级系统提供了用于评估建模不准确性的策略，允许快速转换具有保守安全系数的新型线圈设计，并且以增加验证工作为代价，实现用于频繁使用的线圈阵列的较不保守的安全系数。
The purpose of this work is to propose a tier-based formalism for safety assessment of custom-built radio-frequency (RF) coils that balances validation effort with the effort put in determinating the safety factor. The formalism has three tier levels. Higher tiers require increased effort when validating electromagnetic simulation results but allow for less conservative safety factors. In addition, we propose a new method to calculate modeling uncertainty between simulations and measurements and a new method to propagate uncertainties in the simulation into a safety factor that minimizes the risk of underestimating the peak specific absorption rate (SAR). The new safety assessment procedure was completed for all tier levels for an eight-channel dipole array for prostate imaging at 7 T and an eight-channel dipole array for head imaging at 10.5 T, using data from two different research sites. For the 7 T body array, the validation procedure resulted in a modeling uncertainty of 77% between measured and simulated local SAR distributions. For a situation where RF shimming is performed on the prostate, average power limits of 2.4 and 4.5 W/channel were found for tiers 2 and 3, respectively. When the worst-case peak SAR among all phase settings was calculated, power limits of 1.4 and 2.7 W/channel were found for tiers 2 and 3, respectively. For the 10.5 T head array, a modeling uncertainty of 21% was found based on mapping. For the tier 2 validation, a power limit of 2.6 W/channel was calculated. The demonstrated tier system provides a strategy for evaluating modeling inaccuracy, allowing for the rapid translation of novel coil designs with conservative safety factors and the implementation of less conservative safety factors for frequently used coil arrays at the expense of increased validation effort.
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