Cardiac imaging at 7 tesla: Single- and two-spoke radiofrequency pulse design with 16-channel parallel excitation

Abstract

Purpose Higher signal to noise ratio (SNR) and improved contrast have been demonstrated at ultra-high magnetic fields (≥7 Tesla [T]) in multiple targets, often with multi-channel transmit methods to address the deleterious impact on tissue contrast due to spatial variations in B1+ profiles. When imaging the heart at 7T, however, respiratory and cardiac motion, as well as B0 inhomogeneity, greatly increase the methodological challenge. In this study we compare two-spoke parallel transmit (pTX) RF pulses with static B1+ shimming in cardiac imaging at 7T. Methods Using a 16-channel pTX system, slice-selective two-spoke pTX pulses and static B1+ shimming were applied in cardiac CINE imaging. B1+ and B0 mapping required modified cardiac triggered sequences. Excitation homogeneity and RF energy were compared in different imaging orientations. Results Two-spoke pulses provide higher excitation homogeneity than B1+ shimming, especially in the more challenging posterior region of the heart. The peak value of channel-wise RF energy was reduced, allowing for a higher flip angle, hence increased tissue contrast. Image quality with two-spoke excitation proved to be stable throughout the entire cardiac cycle. Conclusion Two-spoke pTX excitation has been successfully demonstrated in the human heart at 7T, with improved image quality and reduced RF pulse energy when compared with B1+ shimming. Magn Reson Med 70:1210-1219, 2013. © 2013 Wiley Periodicals, Inc. Copyright © 2013 Wiley Periodicals, Inc.