基于磁共振信号幅值的射频场映像技术研究

摘要

MRI领域近年来的新兴研究热点之一——磁共振介电特性成像(MR EPT)技术能通过检测射频场分布,实现活体组织介电特性断层成像.作为MR EPT技术基础之一的磁共振射频场映像(B1 mapping)技术,其B1场成像的质量直接影响着MR EPT算法的精度,因而准确、稳健的B1 mapping技术对于后续的MR EPT算法至关重要.研究双角度法(DAM)和快速预饱和法(satTFL)两种基于磁共振信号幅值的B1 mapping技术的基本原理,以及其在体模和人体头部MR扫描中的应用,并利用FDTD仿真分析上述两种B1 mapping技术的B1场缩放系数(Ri)和平均相对差异系数(MRD),发现在低介电特性体模和脑白质B1场成像中,利用FDTD仿真获得的B1场缩放系数Ri与采用上述两种B1 mapping技术获得的实际B1场缩放系数Ri差异较小,它们的MRD在10％以内;然而在高介电特性体模和脑脊液B1场成像中,采用DAM方法获得的B1场缩放系数Ri要高于采用satTFL方法获得的B1场缩放系数Ri,且有DAM的MRD高达21％.研究的结果可为不同介电特性组织的B1场成像选取合适的B1 mapping技术提供参考,为推动MR EPT技术的实用化提供基础研究支持.%Recently developed technique of in vivo magnetic resonance electrical tomography (MR EPT) can extract the electrical properties of human tissues by using the information of B1 field.Thus,robust B1 mapping technique,regarded as the base of the technique of MR EPT,is critical for MR EPT,because the quality of B1 maps directly affects the accuracy of MR EPT.In this paper,the B1 field scale factor (Ri) for double angle methods (DAM),saturated turbo flash (satTFL) in the scan of the phantom and human head were calculated using the FDTD simulation,and then the performance of DAM and satTFL was evaluated based on the mean relative difference(MRD).The results indicated that the difference of Ri between above methods was less than 10％ in the tissue with low dielectric properties;on the contrary,the value of Ri of DAM was higher than that of satTFL,in the tissue with high dielectric properties such as cerebrospinal fluid,even up to 21％ for DAM.The research of this study can be used to select the appropriate B1 mapping technique for different dielectric property tissues and promote the practical application of MR EPT technology.