title: B1+ inhomogeneity mitigation in CEST using parallel transmission.
abstract: OBJECTIVE
  In order to benefit from the increased spectral bandwidth at ultrahigh field (UHF), the use of parallel transmission (pTx) to mitigate flip-angle inhomogeneity in chemical exchange saturation transfer (CEST) imaging is investigated.
  
  
  METHODS
  A pTx basis pulse is homogenised by magnitude least-squares (MLS) optimization and expanded to form a frequency-selective saturation pulse for CEST. The pTx saturation pulse was simulated with a three-pool Bloch-McConnell equation to evaluate the impact of pTx on CEST contrast. In vivo CEST imaging performance (7 T) of the pTx saturation pulse and the standard Gaussian saturation in circularly polarized mode were compared. Two-spokes pTx homogeneous excitation was used in all in vivo experiments to ensure fair comparison of the two saturation pulses. Magnetization transfer ratio and inverse Z-spectrum analyses were used as metrics in evaluating the data from 3 healthy volunteers.
  
  
  RESULTS
  Bloch-McConnell simulations showed that side bands of the pTx saturation pulse at ±20 ppm did not affect any CEST contrast. Improved homogeneity in contrasts and relaxation-compensated CEST metrics were observed in our in vivo data when the pTx saturation pulse was used.
  
  
  CONCLUSIONS
  A pTx-based pulsed CEST presaturation scheme is proposed and validated by simulations and 7T in vivo imaging. Magn Reson Med 78:2216-2225, 2017. © 2017 International Society for Magnetic Resonance in Medicine.
authors.affiliation: Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands.
authors.forename: Desmond H Y
authors.initials: DHY
authors.lastname: Tse
authors.affiliation: Institute of Neuroscience and Medicine-4, Forschungszentrum Juelich GmbH, Wilhelm-Johnen-Strasse, Juelich, Germany.
authors.forename: Nuno Andre
authors.initials: NA
authors.lastname: da Silva
authors.affiliation: Faculty of Psychology and Neuroscience, Maastricht University, Maastricht, The Netherlands.
authors.forename: Benedikt A
authors.initials: BA
authors.lastname: Poser
authors.affiliation: Institute of Neuroscience and Medicine-4, Forschungszentrum Juelich GmbH, Wilhelm-Johnen-Strasse, Juelich, Germany.|Department of Neurology, Faculty of Medicine, RWTH Aachen University, JARA, Aachen, Germany.|Department of Electrical and Computer Systems Engineering, and Monash Biomedical Imaging, School of Psychological Sciences, Monash University, Melbourne, Victoria, Australia.
authors.forename: N Jon
authors.initials: NJ
authors.lastname: Shah
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000000981:diagnostic imaging*; D003198:Computer Simulation; D064368:Healthy Volunteers; D006801:Humans; D007089:Image Enhancement; D007090:Image Interpretation, Computer-Assisted; D016018:Least-Squares Analysis; D008279:Magnetic Resonance Imaging; D008297:Male; D016011:Normal Distribution; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D013053:Spectrophotometry
keywords: RF inhomogeneity; RF pulse design; chemical exchange saturation transfer; parallel transmission; pulsed CEST; ultra-high-field MRI