Correction of geometric distortion in fMRI data.

The early functional MRI research programme at the National Institutes of Health, described by Robert Turner in an accompanying article in this volume, was the first to combine echo planar imaging (EPI) and high field in the pursuit of fMRI. As such, it soon became apparent that one of the obstacles to interpreting fMRI data using EPI was the presence of geometric distortions caused by static field inhomogeneities. This meant that EPI data did not properly align spatially with conventionally acquired MRI scans that showed structural information. This article describes some of the approaches that have been adopted to ensure that spatial warping caused by field inhomogeneities can be corrected so that functional and structural information can be co-aligned