中文摘要：

在确实需要的前提下，孕期满3个月的孕妇可行MRI检查，相关研究已经证实1．5T胎儿磁共振成像（MRI）检查相对安全，但3T胎儿MRI检查的安全性相关研究还很不够，随着3TMRI设备在临床应用数量快速增加，加强相关研究非常有必要。为了分析3T胎儿MRI检查的安全问题和成像质量，根据医学图像和国际标准建立了孕妇盆腔电磁模型，参照临床常规，分别在1．5T和3T鸟笼发射线圈激励下，采用时域有限差分法对用于衡量MRI安全问题的特定能量吸收率（SAR）和影响图像质量的射频B．场进行数值计算和分析。数值结果表明，3T下孕妇盆腔各组织SAR的平均值和局部最大值分别比1．5T最多提高了2．79倍和3．96倍。特别地，3T下皮肤、羊水SAR的局部最大值超出了安全阈值；3T下过子宫颈、子宫体和子宫底的3个横断面上子宫区域内B1场的均匀性比1．5T分别降低了19％，25％和21％，而且3T下子宫内B1，场均匀性低于67％。结果表明，过高的局部SAR值和曰，场的不均匀性将成为3T磁共振应用于胎儿成像的主要限制因素。

英文摘要：

The fetus magnetic resonance imaging （MRI） at 1.5 T is permitted at clinic when the MRI scan is really necessary for the pregnant women with more than three months pregnancy. However, the relevant studies about the safety of MRI scan for pregnant women at 3 T have not been thoroughly implemented. With the rapid increase of 3 T MRI system at clinic, the related studies are necessary to exploit the advantage of 3 T MRI system. In this paper, the finite difference time domain method was utilized to calculate the specific energy absorption rate （SAR） and the B1 field within pregnant woman pelvis. The electromagnetic model of pelvis was built in accordance with medical image and international standard. The BI field homogeneity and the SAR distribution were respectively analyzed using birdcage coil at 1.5T and 3T systems. The nu- merical results indicate that the average SAR and the maximum local SAR were maximally increased 2.79 times and 3.96 times respectively from 1.5T to 3T and the maximum local SAR of skin and amniotie fluid at 3T exceeds the safety threshold. The uniformity of BI field was reduced by 19% , 25% , and 21% within the pregnant woman uterus region on three selected sections. The homogeneity of the B1 field inside the uterus at 3 T is less than 67%. The results indicate that the local SAR and the inhomogeneity of B1 field are the main constraints for the application in the fetus MRI scan at 3T.