Inter-echo variance as a weighting factor for multi-channel combination in multi-echo acquisition for local frequency shift mapping

Magn Reson Med. 2015 Apr;73(4):1654-61. doi: 10.1002/mrm.25247. Epub 2014 Apr 18.

Abstract

Purpose: 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.

Conclusion: 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.

Keywords: channel-combination; inter-echo variance; local frequency shift mapping; multi-echo; phase unwrapping error.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Adult
  • Algorithms*
  • Brain / anatomy & histology*
  • Echo-Planar Imaging / methods*
  • Female
  • Humans
  • Image Enhancement / methods*
  • Image Interpretation, Computer-Assisted / methods*
  • Male
  • Reproducibility of Results
  • Sensitivity and Specificity
  • Signal Processing, Computer-Assisted