Objective: The radiofrequency specific energy absorption rates are the key to evaluate the safety of patients under MR scan, In order to protect patients against excessive radio frequency energy absorption, the understanding of SAR distribution in different tissues is essential. Methods: Based on the CT scanning images of a volunteer, the numerical model of female pelvic cavity was established from the original CT scanning data. Numerical 3D models of skin (a), fat (b), muscle(c), bone(d), pelvic(e),uterine cavity(f) were obtained based on different gray values after the optimized segmentations. Combining with the different electromagnetic dielectric parameters of different tissues, the electromagnetic model of female pelvis was finally established by comprising of six different tissues. The FDTD simulation was used to calculate the SAR distribution in the different tissues at 64 MHz using the local SAR calculation formula from standard NEMA MS-10. Results: Among the six tissues, the values of SARavg are highest in skin and lowest in uterus at 64 MHz; the values of SAR1g are highest in skin and lowest in bone at the same conditions. Conclusions: Local SARs distribution in different tissues is different. The values of SARs are determined by the different dielectric parameters, the different sizes of various tissues, and the source-object distances.%目的:磁共振(MR)扫描中,特定吸收率(specific absorption rate,SAR)是衡量被检者安全的最重要指标.为了确保被检者的安全,需了解不同人体组织在MR射频作用下SAR分布规律.方法:采集女性志愿者原始X射线断层成像数据,重建女性盆腔数值断层图像;根据断层图像灰度值分割出皮肤、脂肪、肌肉、骨骼、子宫、子宫腔等不同人体组织原始蒙罩,对分割的断层图像进行网格优化和三角面片优化后,结合不同组织电磁参数,建立女性盆腔电磁数值模型.依据NEMA MS- 10标准中的局部SAR计算原则,运用时域有限差分(FDTD)法计算64MHz下盆腔局部不同组织SAR分布.结果:64 MHz下,女性盆腔电磁模型分割出的六种不同组织中,对SARavg值而言,皮肤处最大,子宫处最小;对SAR1g值而言,皮肤处最大,骨骼处最小.结论:MR扫描下不同人体组织内的SAR值分布各不相同.不同组织内SAR值分布与组织的电磁特性和几何形状,以及组织和射频场源距离等因素相关.研究MR扫描下不同人体组织中SAR值的分布规律,掌握影响SAR值分布的因素,对保证被检者的安全具有重要参考价值.
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