title: Phase-corrected bipolar gradients in multi-echo gradient-echo sequences for quantitative susceptibility mapping.
abstract: OBJECTIVE
  Large echo spacing of unipolar readout gradients in current multi-echo gradient-echo (GRE) sequences for mapping fields in quantitative susceptibility mapping (QSM) can be reduced using bipolar readout gradients thereby improving acquisition efficiency.
  
  
  METHODS
  Phase discrepancies between odd and even echoes in the bipolar readout gradients caused by non-ideal gradient behaviors were measured, modeled as polynomials in space and corrected for accordingly in field mapping. The bipolar approach for multi-echo GRE field mapping was compared with the unipolar approach for QSM.
  
  
  RESULTS
  The odd-even-echo phase discrepancies were approximately constant along the phase encoding direction and linear along the readout and slice-selection directions. A simple linear phase correction in all three spatial directions was shown to enable accurate QSM of the human brain using a bipolar multi-echo GRE sequence. Bipolar multi-echo acquisition provides QSM in good quantitative agreement with unipolar acquisition while also reducing noise.
  
  
  CONCLUSIONS
  With a linear phase correction between odd-even echoes, bipolar readout gradients can be used in multi-echo GRE sequences for QSM.
authors.affiliation: Shanghai Key Laboratory of Magnetic Resonance and Department of Physics, East China Normal University, 3663 North Zhongshan Road, Shanghai, 200062, China, jqli@phy.ecnu.edu.cn.
authors.forename: Jianqi
authors.initials: J
authors.lastname: Li
authors.forename: Shixin
authors.initials: S
authors.lastname: Chang
authors.forename: Tian
authors.initials: T
authors.lastname: Liu
authors.forename: Hongwei
authors.initials: H
authors.lastname: Jiang
authors.forename: Fang
authors.initials: F
authors.lastname: Dong
authors.forename: Mengchao
authors.initials: M
authors.lastname: Pei
authors.forename: Qianfeng
authors.initials: Q
authors.lastname: Wang
authors.forename: Yi
authors.initials: Y
authors.lastname: Wang
mesh_terms: D001931:Brain Mapping / Q000379:methods*; D006801:Humans; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D008279:Magnetic Resonance Imaging / Q000379:methods*