高场和超高场MR下人体内B1场均匀性及SAR随场强变化规律的研究

摘要

高场和超高场磁共振成像(MRI)具有高的信噪比,可获得高分辨率的图像,成为磁共振的发展趋势.在磁共振成像系统中,射频线圈产生的射频磁场(B1场)均匀性将直接影响图像质量,且人体组织对射频能量的过多吸收会给受检者带来安全问题,因此研究高场和超高场MR下人体组织B1场均匀性和特定能量吸收率(SAR)随场强变化的关系意义重大.为了准确分析不同场强下人体组织B1场均匀性和SAR值分布,采用时域有限差分法,以真实女性盆腔电磁模型作为磁共振系统中TEN体线圈的负载,在磁通密度为1.5、3、7T下分别对盆腔内部B1场和SAR进行数值计算,比较分析不同场强下人体内部B1场分布均匀性和SAR的变化规律.数值计算结果表明:磁通密度由1.5T分别增加到3和7T时,B1场的均匀性分别降低11.54％和49.97％,盆腔各组织SAR的平均值最大提高4.7和24.14倍,局部最大值最大提高5.5和22倍；而且随着场强增高,人体内深层组织吸收的射频能量显著增加,7T时皮肤、肌肉和组织液的SAR已经超出了安全阈值.可见,B1场均匀性和SAR值随场强增高变化显著,需要根据实际需要对TEM线圈进行优化,以提高成像效果并确保受检者安全.%High and ultra-high field MRI are receiving more concerns in current clinical applications due to substantial enhancement of the signal-to-noise ratio and spatial resolution. The uniformity of B, field directly affects the quality of the image and the RF energy absorbed inside the tissues relates to the safety directly. Therefore, there is a great significance to study on the relationship between magnetic field strength and the B, field uniformity, specific energy absorption rate (SAR) at high and ultra-high field MR. The finite difference time domain (FDTD) was utilized to accurately analyze the performance and safety of high and ultra-high field MR system loaded with the real female pelvis electromagnetic model using the TEM body coil. The B, field and the SAR inside the pelvic region were calculated and analyzed at 1. 5T, 3T and 7T magnetic flux densities respectively. The results indicated that the uniformity of B, field was reduced by 11. 54% and 49. 97% , the average SAR of various pelvic tissues was maximally increased by 4.7 times and 24. 14 times, local maximum SAR were maximally increased by 5. 5 times and 22 times and RF energy trended to focus on internal tissues, with the increase of the magnetic flux density from 1. 5T to 3T and 7T. The SAR of skin, muscle and body fluid at 7T exceeds the safety threshold. Therefore, it is necessary to correspondingly optimize the TEM coil to improve the B1 field homogeneity and ensure the qualified SAR at high and ultra-high field MR, according to the different main magnetic field strength.