title: The effects of geometric distortion correction on motion realignment in fMRI.
abstract: OBJECTIVE
  Subject motion is well recognized as a significant impediment to resolution and sensitivity in functional magnetic resonance imaging (fMRI). A parallel confounder to fMRI data quality is geometric image distortion, particularly at high field strengths, due to susceptibility-induced magnetic field inhomogeneity. Consequently, many high-field echo-planar imaging methods incorporate a post-processing distortion correction by acquiring a field map of the sample prior to the fMRI measurement. However, field mapping methods impose a spatial mask on the data, since field information is only obtainable from regions with adequate signal-to-noise ratio (SNR). This masking, when applied to subsequent images in the fMRI time series, can clip the effects of motion, resulting in inaccurate estimation and correction of motion-based changes in the images.
  
  
  METHODS
  The effects of geometric distortion correction on automated realignment (motion correction) of fMRI data are investigated from data acquired at 4 T. The results of image realignment with and without prior application of distortion correction are compared, using the estimated motion parameters and overall image realignment as metrics.
  
  
  RESULTS
  The application of field-map-based distortion correction prior to image realignment reduces the amount of motion detected by a standard motion correction algorithm. Moreover, motion correction applied before distortion correction is shown to result in superior realignment of motion-correction images.
  
  
  CONCLUSIONS
  It is preferable to perform motion realignment prior to correcting for geometric distortion.
authors.affiliation: Department of Radiology, University of Pennsylvania, B1 Stellar Chance Labs, Philadelphia, PA 19104, USA. mark@mail.mmrrcc.upenn.edu
authors.forename: Mark A
authors.initials: MA
authors.lastname: Elliott
authors.forename: Eugene E
authors.initials: EE
authors.lastname: Gualtieri
authors.forename: Justin
authors.initials: J
authors.lastname: Hulvershorn
authors.forename: J Daniel
authors.initials: JD
authors.lastname: Ragland
authors.forename: Ruben
authors.initials: R
authors.lastname: Gur
mesh_terms: D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods; D009038:Motion; D010469:Perceptual Distortion