引导超声聚焦热消融的超高场磁共振射频电磁场匀场技术及人体组织比吸收率安全分析

B_1 field homogeneity and specific absorption rate in ultra-high field magnetic resonance imaging for guiding high-intensity focused ultrasound

目的探究在超高场磁共振引导下的高强度聚焦超声手术中射频电磁场B1场匀场技术及人体组织比吸收率(SAR)安全性。方法建立包含温度梯度的女性盆腔模型,分别仿真计算在正常模型匀场系数激励以及在温度梯度模型的匀场系数激励下的射频电磁场B1场的均匀性和局部比吸收率。结果对包含温度梯度的女性盆腔模型采用正常模型的匀场系数激励,组织局部SAR最大值达到10.24 W/kg,超出了国际电工委员会(IEC)对局部SAR值规定的10 W/kg的安全阈值;对包含温度梯度的女性盆腔模型采用温度梯度模型的匀场系数激励,组织局部SAR最大值为9.65W/kg在IEC的安全阈值内。结论在进行超高场磁共振引导下的高强度聚焦超声手术时,需要考虑超声能量在人体形成的温度分布,在温度梯度的基础上重新进行匀场优化能将组织局部SAR值降低到安全阈值内。

Objective To study the efficacy of B1 field shimming technique and evaluate the specific absorption rate（SAR） in ultra-high field magnetic resonance imaging（MRI） for MRI-guided high-intensity focused ultrasound（HIFU）. Methods An electromagnetic model of the female pelvis with a temperature gradient was established. B1 field homogeneity and local SAR were simulated and calculated using regular and optimized B1 shimming coefficients. Results The maximum local SAR reached 10.24 W/kg, which exceeded the safe threshold of 10 W/kg, as calculated using regular B1 shimming coefficients in the normal model. Using the optimized B1 shimming coefficients, the maximum local SAR of the tissue was 9.65 W/kg, which was below the safe threshold. Conclusion The temperature distributions in the body generated by ultrasound energy need to be considered in ultra- high field MRI- guided HIFU surgery. The proposed optimized B1 shimming strategy based on the temperature gradient can be used to control the local SAR levels.