title: Phase and amplitude correction for multi-echo water-fat separation with bipolar acquisitions.
abstract: OBJECTIVE
  To address phase and amplitude errors for multi-point water-fat separation with "bipolar" acquisitions, which efficiently collect all echoes with alternating read-out gradient polarities in one repetition.
  
  
  METHODS
  With the bipolar acquisitions, eddy currents and other system nonidealities can induce inconsistent phase errors between echoes, disrupting water-fat separation. Previous studies have addressed phase correction in the read-out direction. However, the bipolar acquisitions may be subject to spatially high order phase errors as well as an amplitude modulation in the read-out direction. A method to correct for the 2D phase and amplitude errors is introduced. Low resolution reference data with reversed gradient polarities are collected. From the pair of low-resolution data collected with opposite gradient polarities, the two-dimensional phase errors are estimated and corrected. The pair of data are then combined for water-fat separation.
  
  
  RESULTS
  We demonstrate that the proposed method can effectively remove the high order errors with phantom and in vivo experiments, including obliquely oriented scans.
  
  
  CONCLUSIONS
  For bipolar multi-echo acquisitions, uniform water-fat separation can be achieved by removing high order phase errors with the proposed method.
authors.affiliation: Applied Science Laboratory, GE Healthcare, Menlo Park, California, USA. huanzhou.yu@ge.com
authors.forename: Huanzhou
authors.initials: H
authors.lastname: Yu
authors.forename: Ann
authors.initials: A
authors.lastname: Shimakawa
authors.forename: Charles A
authors.initials: CA
authors.lastname: McKenzie
authors.forename: Wenmiao
authors.initials: W
authors.lastname: Lu
authors.forename: Scott B
authors.initials: SB
authors.lastname: Reeder
authors.forename: R Scott
authors.initials: RS
authors.lastname: Hinks
authors.forename: Jean H
authors.initials: JH
authors.lastname: Brittain
mesh_terms: D000273:Adipose Tissue / Q000033:anatomy & histology*; D000465:Algorithms; D016477:Artifacts; D001834:Body Water / Q000166:cytology*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D021621:Imaging, Three-Dimensional / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000295:instrumentation / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity; D013382:Subtraction Technique