title: GABA editing with macromolecule suppression using an improved MEGA-SPECIAL sequence.
abstract: OBJECTIVE
  The most common γ-aminobutyric-acid (GABA) editing approach, MEGA-PRESS, uses J-editing to measure GABA distinct from larger overlapping metabolites, but suffers contamination from coedited macromolecules (MMs) comprising 40 to 60% of the observed signal. MEGA-SPECIAL is an alternative method with better MM suppression, but is not widely used primarily because of its relatively poor spatial localization. Our goal was to develop an improved MM-suppressed GABA editing sequence at 3 Tesla.
  
  
  METHODS
  We modified a single-voxel MEGA-SPECIAL sequence with an oscillating readout gradient for improved spatial localization, and used very selective 30-ms editing pulses for improved suppression of coedited MMs.
  
  
  RESULTS
  Simulation and in vivo experiments confirmed excellent MM suppression, insensitive to the range of B0frequency drifts typically encountered in vivo. Both intersubject and intrasubject studies showed that MMs, when suppressed by the improved MEGA-SPECIAL method, contributed approximately 40% to the corresponding MEGA-PRESS measurements. From the intersubject study, the coefficient of variation for GABA+/Cre (MEGA-PRESS) was 11.2% versus 7% for GABA/Cre (improved MEGA-SPECIAL), demonstrating significantly reduced variance (P = 0.005), likely coming from coedited MMs.
  
  
  CONCLUSIONS
  This improved MEGA-SPECIAL sequence provides unbiased GABA measurements with reduced variance as compared with conventional MEGA-PRESS. This approach is also relatively insensitive to the range of B0drifts typically observed in in vivo human studies. Magn Reson Med 79:41-47, 2018. © 2017 International Society for Magnetic Resonance in Medicine.
authors.affiliation: Department of Radiology, Stanford University, Stanford, California, USA.
authors.forename: Meng
authors.initials: M
authors.lastname: Gu
authors.affiliation: GE Healthcare, Menlo Park, California, USA.
authors.forename: Ralph
authors.initials: R
authors.lastname: Hurd
authors.affiliation: GE Healthcare, Berlin, Germany.
authors.forename: Ralph
authors.initials: R
authors.lastname: Noeske
authors.affiliation: Center for Cognitive and Neurobiological Imaging, Stanford University, Stanford, California, USA.
authors.forename: Laima
authors.initials: L
authors.lastname: Baltusis
authors.affiliation: Department of Psychiatry, University of California, San Francisco, California, USA.
authors.forename: Roeland
authors.initials: R
authors.lastname: Hancock
authors.affiliation: Neurosciences Program and Psychology, Stanford University, Stanford, California, USA.
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Sacchet
authors.affiliation: Neurosciences Program and Psychology, Stanford University, Stanford, California, USA.
authors.forename: Ian H
authors.initials: IH
authors.lastname: Gotlib
authors.affiliation: Department of Radiology, Stanford University, Stanford, California, USA.
authors.forename: Frederick T
authors.initials: FT
authors.lastname: Chin
authors.affiliation: Department of Radiology, Stanford University, Stanford, California, USA.
authors.forename: Daniel M
authors.initials: DM
authors.lastname: Spielman
mesh_terms: D000465:Algorithms; D001921:Brain / Q000000981:diagnostic imaging; D001931:Brain Mapping / Q000379:methods; D003198:Computer Simulation; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods*; D046911:Macromolecular Substances; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D016011:Normal Distribution; D009991:Oscillometry; D019047:Phantoms, Imaging; D011846:Radio Waves; D015203:Reproducibility of Results; D005680:gamma-Aminobutyric Acid / Q000737:chemistry*
chemical_list: D046911:Macromolecular Substances; D005680:gamma-Aminobutyric Acid
keywords: GABA; MEGA-PRESS; MEGA-SPECIAL; MRS; macromolecule suppression