The effects of geometric distortion correction on motion realignment in fMRI.
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.
Elliott
Mark A
MA
Department of Radiology, University of Pennsylvania, B1 Stellar Chance Labs, Philadelphia, PA 19104, USA. mark@mail.mmrrcc.upenn.edu
Gualtieri
Eugene E
EE
Hulvershorn
Justin
J
Ragland
J Daniel
JD
Gur
Ruben
R
D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008279:Magnetic Resonance Imaging / Q000379:methods; D009038:Motion; D010469:Perceptual Distortion