Bernard and Irene Schwartz Center for Biomedical Imaging, Department of Radiology, New York University School of Medicine, New York, New York 10016, USA.
Magn Reson Med. 2013 May;69(5):1379-88. doi: 10.1002/mrm.24377. Epub 2012 Jun 19.
Radiofrequency shimming with multiple channel excitation has been proposed to increase the transverse magnetic field uniformity and reduce specific absorption rate at high magnetic field strengths (≥7 T) where high-frequency effects can make traditional single channel volume coils unsuitable for transmission. In the case of deep anatomic regions and power-demanding pulse sequences, optimization of transmit efficiency may be a more critical requirement than homogeneity per se. This work introduces a novel method to maximize transmit efficiency using multiple channel excitation and radiofrequency shimming. Shimming weights are calculated in order to obtain the lowest possible net radiofrequency power deposition into the subject for a given transverse magnetic field strength. The method was demonstrated in imaging studies of articular cartilage of the hip joint at 7 T. We show that the new radiofrequency shimming method can enable reduction in power deposition while maintaining an average flip angle or adiabatic condition in the hip cartilage. Building upon the improved shimming, we further show that the signal-to-noise ratio in hip cartilage at 7 T can be substantially greater than that at 3 T, illustrating the potential benefits of high field hip imaging.
已提出使用多通道激发的射频调谐来提高横向磁场均匀性并降低高磁场强度（≥7 T）下的比吸收率，在该场强下高频效应会使传统的单通道容积线圈不适合用于传输。在深部解剖区域和需要大功率脉冲序列的情况下，与均匀性本身相比，传输效率的优化可能是更关键的要求。本工作介绍了一种使用多通道激发和射频调谐来最大化传输效率的新方法。为了在给定的横向磁场强度下将射频功率沉积到受试者中的量最小化，计算调谐权重。该方法在 7 T 髋关节关节软骨的成像研究中得到了验证。我们表明，新的射频调谐方法可以在保持髋关节软骨中的平均翻转角或绝热条件的同时，降低功率沉积。基于改进的调谐，我们进一步表明，7 T 髋关节软骨的信噪比可以大大高于 3 T 的信噪比，这说明了高磁场髋关节成像的潜在优势。
