Distortion Correction and Field Maps

Distortion? What distortion?
The various tissues and air spaces in the head cause inhomogeneities in the B0 magnetic field. The inhomogeneities cause distortions in functional images.

A field map scan measures the inhomogeneities in B0. There are several tools that apply the information from a field map to correct EPI distortion.

Corrections using FSL: Prelude and Fugue
http://www.fmrib.ox.ac.uk/fsl/fugue/index.html