Design of parallel transmission radiofrequency pulses robust against respiration in cardiac MRI at 7 Tesla

• Published in: Magnetic resonance in medicine 2015-11, Vol.74 (5), p.1291-1305
• Main Authors: Schmitter, Sebastian, Wu, Xiaoping, Uğurbil, Kâmil, Van de Moortele, Pierre-François
• Format: Article
• Language: English
• Subjects: 7 Tesla; Breath Holding; breath-hold; Cardiac Imaging Techniques - methods; cardiac MRI; Heart Rate - physiology; Humans; Image Processing, Computer-Assisted - methods; Magnetic Resonance Imaging, Cine - methods; pTX; Radio Waves; respiration; RF pulse design; Signal Processing, Computer-Assisted; spoke pulses
• ISSN: 0740-3194; 1522-2594
• DOI: 10.1002/mrm.25512

Purpose Two‐spoke parallel transmission (pTX) radiofrequency (RF) pulses have been demonstrated in cardiac MRI at 7T. However, current pulse designs rely on a single set of B1+/B0 maps that may not be valid for subsequent scans acquired at another phase of the respiration cycle because of organ displacement. Such mismatches may yield severe excitation profile degradation. Methods B1+/B0 maps were obtained, using 16 transmit channels at 7T, at three breath‐hold positions: exhale, half‐inhale, and inhale. Standard and robust RF pulses were designed using maps obtained at exhale only, and at multiple respiratory positions, respectively. Excitation patterns were analyzed for all positions using Bloch simulations. Flip‐angle homogeneity was compared in vivo in cardiac CINE acquisitions. Results Standard one‐ and two‐spoke pTX RF pulses are sensitive to breath‐hold position, primarily due to B1+ alterations, with high dependency on excitation trajectory for two spokes. In vivo excitation inhomogeneity varied from nRMSE = 8.2% (exhale) up to 32.5% (inhale) with the standard design; much more stable results were obtained with the robust design with nRMSE = 9.1% (exhale) and 10.6% (inhale). Conclusion A new pTX RF pulse design robust against respiration induced variations of B1+/B0 maps is demonstrated and is expected to have a positive impact on cardiac MRI in breath‐hold, free‐breathing, and real‐time acquisitions. Magn Reson Med 74:1291–1305, 2015. © 2014 Wiley Periodicals, Inc.