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  • Title: Cardiac imaging at 7 Tesla: Single- and two-spoke radiofrequency pulse design with 16-channel parallel excitation.
    Author: Schmitter S, DelaBarre L, Wu X, Greiser A, Wang D, Auerbach EJ, Vaughan JT, Uğurbil K, Van de Moortele PF.
    Journal: Magn Reson Med; 2013 Nov; 70(5):1210-9. PubMed ID: 24038314.
    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.
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