Dynamic field mapping and distortion correction for fMRI

Abstract

Echo planar images (EPI) suffer from geometric distortions caused by static-field inhomogeneity. Correction techniques have been suggested based on field maps obtained before or after the EPI acquisition. However, when a relatively long time series of images is required, as in fMRI studies, the inhomogeneity varies from image to image because of gross motion and physiological activity such as respiration and cardiac motion. It is not ideal to approximate the varying maps of field inhomogeneity by means of one map. To overcome this limitation, multiple field maps are desirable for correcting the distortions that are dynamically changing. Some groups have explored the possibility of acquiring multiple field maps, but either the increased scan time is not affordable for most fMRI studies or the field map acquisition is embedded in EPI pulse sequence, which produces a map of insufficient resolution to support a complete distortion correction. In this paper, we propose a dynamic field mapping technique that uses a single reference image and a single corresponding acquired field map and the phase information extracted from the complex image data of each EPI image in the time series. From this information, a separate field map is then derived individually for each EPI image. The derived field maps are then used for distortion correction. This approach, which is particularly suitable for fMRI studies, can correct for image distortion that varies dynamically without sacrificing temporal resolution. We validate this technique using simulated data, and the experimental results show improved performance in comparison to correction using a single field map.