title: Hemi-spectrum substitution after water signal fitting (HESWAF): an improvement of the modulus post-processing of MR spectra.
abstract: OBJECTIVE
  In a previous study, we have shown that modulus post-processing is a simple and efficient tool to both phase correct and frequency align magnetic resonance (MR) spectra automatically. Furthermore, this technique also eliminates sidebands and phase distortions. The advantages of the modulus technique have been illustrated in several applications to brain proton MR spectroscopy. Two possible drawbacks have also been pointed out. The first one is the theoretical decrease in signal-to-noise ratio (SNR) by a factor up to √2 when comparing the spectrum obtained after modulus versus conventional post-processing. The second pitfall results from the symmetrization of the spectrum induced by modulus post-processing, since any resonance or artifact located at the left of the water resonance is duplicated at the right of the water resonance, thus contaminating the region of the spectrum containing the resonances of interest. Herein, we propose a strategy in order to eliminate these two limitations.
  
  
  METHODS
  Concerning the SNR issue, two complementary approaches are presented here. The first is based on the application of modulus post-processing before spatial apodization, and the second consists in substituting the left half of the spectrum by the fit of the water resonance before applying modulus post-processing. The symmetrization induced by modulus post-processing then combines the right half of the original spectrum containing the resonances of interest with the left half of the water fit, free of noise and artifacts. Consequently, the SNR is improved when compared to modulus post-processing alone. As a bonus, any artifact or resonance present in the left half of the original spectrum is removed. This solves the second limitation.
  
  
  RESULTS
  After validation of the technique on simulations, we demonstrated that this improvement of the modulus technique is significantly advantageous for both in vitro and in vivo applications.
  
  
  CONCLUSIONS
  By improving the SNR of the spectra and eliminating eventual contaminations, the new strategies proposed here confer an additional competitive advantage to the modulus post-processing technique.
authors.affiliation: CRMBM UMR 7339, Aix-Marseille Université, CNRS, 13385, Marseille, France, yann.le-fur@univ-amu.fr.
authors.forename: Yann
authors.initials: Y
authors.lastname: Le Fur
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: Cozzone
mesh_terms: D000465:Algorithms; D016477:Artifacts; D001923:Brain Chemistry; D006801:Humans; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D014867:Water / Q000737:chemistry*
chemical_list: D014867:Water