title: EPI distortion correction from a simultaneously acquired distortion map using TRAIL.
abstract: OBJECTIVE
  To develop a method for shot-by-shot distortion correction of single-shot echo-planar imaging (EPI) that is capable of correcting each image individually using a distortion measurement performed during acquisition of the image itself.
  
  
  METHODS
  The recently-introduced method known as two reduced acquisitions interleaved (TRAIL) was extended to measure the distribution of the main magnetic field B0 with each shot. This corresponded to a map of distortion, and allowed distortion to be corrected in the acquired images.
  
  
  RESULTS
  Distortion-corrected images were demonstrated in the human brain. The distortion field could be directly visualized using the "stripe" distribution imposed by the TRAIL pulse sequence. This confirmed the success of the correction. Over a time-course measurement of 10 images, variance was reduced by using shot-by-shot distortion correction compared to correction with a constant field map.
  
  
  CONCLUSIONS
  Shot-by-shot distortion correction may be performed for EPI images acquired using an extension of the TRAIL technique, ensuring that the correction reflects the actual distortion pattern and not merely a previously measured, but possibly no longer valid, distortion field. This avoids errors due to changes in the distortion field or misregistration of a previously measured distortion map resulting from subject motion.
authors.affiliation: UCL Hospitals NHS Trust, Department of Medical Physics and Bioengineering, London, United Kingdom. a.priest@uke.uni-hamburg.de
authors.forename: Andrew N
authors.initials: AN
authors.lastname: Priest
authors.forename: Enrico
authors.initials: E
authors.lastname: De Vita
authors.forename: David L
authors.initials: DL
authors.lastname: Thomas
authors.forename: Roger J
authors.initials: RJ
authors.lastname: Ordidge
mesh_terms: D001931:Brain Mapping / Q000379:methods*; D017352:Echo-Planar Imaging / Q000379:methods*; D006801:Humans; D007091:Image Processing, Computer-Assisted