title: Inter-echo variance as a weighting factor for multi-channel combination in multi-echo acquisition for local frequency shift mapping.
abstract: OBJECTIVE
  To develop and evaluate a local frequency shift (LFS) mapping method specifically designed for multi-echo acquisitions and multi-channel receive coils.
  
  
  METHODS
  The proposed method uses the pixel-by-pixel inter-echo variance (IEV) as a weighting factor during channel-combination. Five healthy volunteers were scanned at 7 T. The IEV-weighted method was quantitatively compared to established (adaptive and Hermitian product) channel-combination methods with respect to IEV of LFS over the entire brain.
  
  
  RESULTS
  In all experiments, the IEV-weighted method generated LFS maps free of artifacts caused by unwrapping errors. Based on measurements of the inter-echo frequency variance throughout the whole brain, the IEV-weighted method produced the lowest variation and the best contrast at the edge of the brain.
  
  
  CONCLUSIONS
  The primary finding of the present study is that accurate LFS maps are achievable if the data from each channel is processed independently prior to combination followed by a weighted combination using IEV as the weighting term. The software is freely available to the scientific community.
authors.affiliation: Imaging Research Laboratories, Robarts Research Institute, University of Western Ontario, London, Canada.
authors.forename: Junmin
authors.initials: J
authors.lastname: Liu
authors.forename: David A
authors.initials: DA
authors.lastname: Rudko
authors.forename: Joseph S
authors.initials: JS
authors.lastname: Gati
authors.forename: Ravi S
authors.initials: RS
authors.lastname: Menon
authors.forename: Maria
authors.initials: M
authors.lastname: Drangova
mesh_terms: D000328:Adult; D000465:Algorithms; D001921:Brain / Q000033:anatomy & histology*; D017352:Echo-Planar Imaging / Q000379:methods*; D005260:Female; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008297:Male; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted
keywords: channel-combination; inter-echo variance; local frequency shift mapping; multi-echo; phase unwrapping error