title: Cardiac imaging at 7 Tesla: Single- and two-spoke radiofrequency pulse design with 16-channel parallel excitation.
abstract: OBJECTIVE
  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.
  
  
  CONCLUSIONS
  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.
authors.affiliation: University of Minnesota, Center for Magnetic Resonance Research, Minneapolis, Minnesota, USA.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Schmitter
authors.forename: Lance
authors.initials: L
authors.lastname: DelaBarre
authors.forename: Xiaoping
authors.initials: X
authors.lastname: Wu
authors.forename: Andreas
authors.initials: A
authors.lastname: Greiser
authors.forename: Dingxin
authors.initials: D
authors.lastname: Wang
authors.forename: Edward J
authors.initials: EJ
authors.lastname: Auerbach
authors.forename: J Thomas
authors.initials: JT
authors.lastname: Vaughan
authors.forename: Kâmil
authors.initials: K
authors.lastname: Uğurbil
authors.forename: Pierre-François
authors.initials: PF
authors.lastname: Van de Moortele
mesh_terms: D000465:Algorithms; D062485:Breath Holding; D055424:Cardiac-Gated Imaging Techniques / Q000379:methods*; D004867:Equipment Design; D019544:Equipment Failure Analysis; D005240:Feasibility Studies; D006321:Heart / Q000033:anatomy & histology*; D006801:Humans; D007089:Image Enhancement / Q000295:instrumentation / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000295:instrumentation / Q000379:methods*; D019028:Magnetic Resonance Imaging, Cine / Q000379:methods*; D011846:Radio Waves; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted / Q000295:instrumentation
keywords: 7T; RF pulse design; cardiac CINE imaging; pTX; parallel transmission; spoke excitation
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