title: Minimizing the effects of magnetization transfer asymmetry on inhomogeneous magnetization transfer (ihMT) at ultra-high magnetic field (11.75 T).
abstract: OBJECTIVE
  The recently reported inhomogeneous magnetization transfer technique (ihMT) has been proposed for specific imaging of inhomogeneously broadened lines, and has shown great promise for characterizing myelinated tissues. The ihMT contrast is obtained by subtracting magnetization transfer images obtained with simultaneous saturation at positive and negative frequency offsets (dual frequency saturation experiment, MT (+/-)) from those obtained with single frequency saturation (MT (+)) at the same total power. Hence, ihMT may be biased by MT-asymmetry, especially at ultra-high magnetic field. Use of the average of single positive and negative frequency offset saturation MT images, i.e., (MT (+)+MT (-)) has been proposed to correct the ihMT signal from MT-asymmetry signal.
  
  
  METHODS
  The efficiency of this correction method was experimentally assessed in this study, performed at 11.75 T on mice. Quantitative corrected ihMT and MT-asymmetry ratios (ihMTR and MTRasym) were measured in mouse brain structures for several MT-asymmetry magnitudes and different saturation parameter sets.
  
  
  RESULTS
  Our results indicated a "safe" range of magnitudes (/MTRasym/<4 %) for which MT-asymmetry signal did not bias the corrected ihMT signal. Moreover, experimental evidence of the different natures of both MT-asymmetry and inhomogeneous MT contrasts were provided. In particular, non-zero ihMT ratios were obtained at zero MTRasym values.
  
  
  CONCLUSIONS
  MTRasym is not a confounding factor for ihMT quantification, even at ultra-high field, as long as MTRasym is restricted to ±4 %.
authors.affiliation: Centre de Résonance Magnétique Biologique et Médicale, CRMBM-CEMEREM, UMR 7339, CNRS, Faculté de Médecine, Aix-Marseille Université (AMU), 27 Boulevard Jean Moulin, 13005, Marseille, France.
authors.forename: Valentin H
authors.initials: VH
authors.lastname: Prevost
authors.affiliation: Centre de Résonance Magnétique Biologique et Médicale, CRMBM-CEMEREM, UMR 7339, CNRS, Faculté de Médecine, Aix-Marseille Université (AMU), 27 Boulevard Jean Moulin, 13005, Marseille, France.
authors.forename: Olivier M
authors.initials: OM
authors.lastname: Girard
authors.affiliation: Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA.
authors.forename: Gopal
authors.initials: G
authors.lastname: Varma
authors.affiliation: Department of Radiology, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, MA, USA.
authors.forename: David C
authors.initials: DC
authors.lastname: Alsop
authors.affiliation: Centre de Résonance Magnétique Biologique et Médicale, CRMBM-CEMEREM, UMR 7339, CNRS, Faculté de Médecine, Aix-Marseille Université (AMU), 27 Boulevard Jean Moulin, 13005, Marseille, France. guillaume.duhamel@univ-amu.fr.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Duhamel
mesh_terms: D000465:Algorithms; D000818:Animals; D001921:Brain / Q000000981:diagnostic imaging; D005260:Female; D007089:Image Enhancement / Q000379:methods; D007091:Image Processing, Computer-Assisted / Q000379:methods; D060526:Magnetic Fields; D008279:Magnetic Resonance Imaging; D008280:Magnetics; D051379:Mice; D008810:Mice, Inbred C57BL; D009186:Myelin Sheath / Q000737:chemistry*
keywords: Dipolar order; Dipolar relaxation time; Inhomogeneous magnetization transfer; MT-asymmetry; Myelin; ihMT