Respiration impacts phase difference-based field maps in echo planar imaging.

PURPOSE: To investigate the influence of respiration on field maps for geometric distortion correction derived from two rapidly acquired consecutive echo planar images.
METHODS: Displacement maps of the brains of seven healthy volunteers were acquired under breath hold and free breathing for a 64 × 64 pixel image matrix using phase labeling for additional coordinate encoding (PLACE). The maps were transformed into undistorted gradient echo space and analyzed with regard to standard deviation and absolute deviation from an accurate reference field map derived from a multiecho reference scan.
RESULTS: Standard deviations between PLACE field maps and absolute difference from the reference field map are a factor of about 3 higher under free breathing than under breath hold. The mean deviation decreases from 3 pixels in the slice closest to the lung to 1 pixel in the most superior slice under free breathing and from 1 to <0.5 pixels under breath hold.
CONCLUSION: Maps obtained under free breathing can significantly impact the field map and thus corrupt the geometric distortion correction. The effect can be greatly reduced by acquiring the field map data under breath hold. Data acquired under free breathing can be improved with retrospective phase correction or by averaging several field maps.