title: Large dynamic range relative B1+ mapping.
abstract: Parallel transmission (PTx) requires knowledge of the B1+ produced by each element. However, B1+ mapping can be challenging when transmit fields exhibit large dynamic range. This study presents a method to produce high quality relative B1+ maps when this is the case.
  
  
  
  The proposed technique involves the acquisition of spoiled gradient echo (SPGR) images at multiple radiofrequency drive levels for each transmitter. The images are combined using knowledge of the SPGR signal equation using maximum likelihood estimation, yielding an image for each channel whose signal is proportional to the B1+ field strength. Relative B1+ maps are then obtained by taking image ratios. The method was tested using numerical simulations, phantom imaging, and through in vivo experiments.
  
  
  
  The numerical simulations demonstrated that the proposed method can reconstruct relative transmit sensitivities over a wide range of B1+ amplitudes and at several SNR levels. The method was validated at 3 Tesla (T) by comparing it with an alternative B1+ mapping method, and demonstrated in vivo at 7T.
  
  
  
  Relative B1+ mapping in the presence of large dynamic range has been demonstrated through numerical simulations, phantom imaging at 3T and experimentally at 7T. The method will enable PTx to be applied in challenging imaging scenarios at ultrahigh field. Magn Reson Med 76:490-499, 2016. © 2015 The Authors. Magnetic Resonance in Medicine published by Wiley Periodicals, Inc. on behalf of International Society for Magnetic Resonance in Medicine. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
authors: King's College London, Division of Imaging Sciences and Biomedical Engineering, The Rayne Institute, St Thomas' Hospital, London, United Kingdom.
authors: Francesco
authors: F
authors: Padormo
authors: University of Oxford Centre for Clinical Magnetic Resonance Research, Division of Cardiovascular Medicine, Radcliffe Department of Medicine, Oxford, United Kingdom.
authors: Aaron T
authors: AT
authors: Hess
authors: King's College London, Division of Imaging Sciences and Biomedical Engineering, The Rayne Institute, St Thomas' Hospital, London, United Kingdom.
authors: Paul
authors: P
authors: Aljabar
authors: King's College London, Division of Imaging Sciences and Biomedical Engineering, The Rayne Institute, St Thomas' Hospital, London, United Kingdom.
authors: Shaihan J
authors: SJ
authors: Malik
authors: Centre for Functional Magnetic Resonance Imaging of the Brain, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom.
authors: Peter
authors: P
authors: Jezzard
authors: University of Oxford Centre for Clinical Magnetic Resonance Research, Division of Cardiovascular Medicine, Radcliffe Department of Medicine, Oxford, United Kingdom.
authors: Matthew D
authors: MD
authors: Robson
authors: King's College London, Division of Imaging Sciences and Biomedical Engineering, The Rayne Institute, St Thomas' Hospital, London, United Kingdom.|King's College London, Centre for the Developing Brain, Division of Imaging Sciences and Biomedical Engineering, The Rayne Institute, St Thomas' Hospital, London, United Kingdom.
authors: Joseph V
authors: JV
authors: Hajnal
authors: Centre for Functional Magnetic Resonance Imaging of the Brain, Nuffield Department of Clinical Neurosciences, University of Oxford, Oxford, United Kingdom.
authors: Peter J
authors: PJ
authors: Koopmans
mesh_terms: D000465:Algorithms; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008099:Liver / Q000033:anatomy & histology*; D008279:Magnetic Resonance Imaging / Q000379:methods*; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity
keywords: B1 mapping; parallel transmission; ultrahigh field MRI