title: Phase-labeled reference EPI for frequency-segmented inhomogeneity corrections (PREFICS).
abstract: OBJECTIVE
  To develop a method to correct for geometric distortions in non-Cartesian echo planar imaging using data acquired in a fast prescan.
  
  
  METHODS
  Echo planar images were obtained using a non-Cartesian density weighted k-space trajectory. In contrast to linear Cartesian acquisition, distortions arising in non-Cartesian acquisitions can only be corrected using conjugate phase-based methods. Up to now these methods require a time consuming field map acquisition in undistorted coordinates. In this work, a phase-labeled reference EPI scan was performed in a very short time, yielding a displacement map in distorted coordinates. Subsequently, this map was applied to itself and thus transformed into undistorted coordinates. The echo planar images were then corrected with a frequency-segmented conjugate phase method. The results were compared with corrections based on a multi-echo reference field map acquired in undistorted coordinates.
  
  
  RESULTS
  Uncorrected density weighted EPI exhibited geometric distortions and severe ringing artifacts. These distortions could be successfully corrected using the novel method. The quality was comparable to corrections based on the multi-echo reference field map. The novel method shortens the field map acquisition duration by a factor of 64.
  
  
  CONCLUSIONS
  The method presented allows correcting non-Cartesian EPI from a reference scan consisting of only two phase-labeled echo planar images.
authors.affiliation: University of Würzburg, University Clinic, Institute of Radiology, Würzburg, Germany.
authors.forename: Mario
authors.initials: M
authors.lastname: Zeller
authors.forename: Alexander
authors.initials: A
authors.lastname: Müller
authors.forename: Dietbert
authors.initials: D
authors.lastname: Hahn
authors.forename: Herbert
authors.initials: H
authors.lastname: Köstler
mesh_terms: D000465:Algorithms; D016477:Artifacts; D017352:Echo-Planar Imaging / Q000295:instrumentation / Q000379:methods*; D006801:Humans; D007089:Image Enhancement / Q000379:methods*; D007090:Image Interpretation, Computer-Assisted / Q000379:methods*; D019047:Phantoms, Imaging; D015203:Reproducibility of Results; D012680:Sensitivity and Specificity