Simultaneous correction of ghost and geometric distortion artifacts in EPIusing a multiecho reference scan

A computationally efficient technique is described for the simultaneous rem oval of ghosting and geometrical distortion artifacts iri echo-planar imagi ng (EPI) utilizing a multiecho, gradient-echo reference scan. Nyquist ghosts occur in EPI reconstructions because odd and even Lines of I c-space are acquired with opposite polarity, and experimental imperfections such as gradient eddy currents, imperfect pulse sequence timing, B-0 field inhomogeneity, susceptibility, and chemical shift result in the even and o dd lines of k-space being offset by different amounts relative to the true center of the acquisition window Geometrical distortion occurs due to the l imited bandwidth of the EPI images in the phase-encode direction. This dist ortion can be problematic when attempting to overlay an activation map from a functional magnetic resonance imaging experiment generated from EPI data on a high-resolution anatomical image. The method described here corrects for geometrical distortion related to Be inhomogeneity, gradient eddy currents, radio frequency pulse frequency off set, and chemical shift effect. The algorithm for removing ghost artifacts utilizes phase information in two dimensions and is, thus, more robust than conventional one-dimensional methods. An additional reference scan is requ ired which takes approximately 2 min for a matrix size of 64 X 64 and a rep etition time of 2 s. Results from a water phantom and a human brain at 3 T demonstrate the effectiveness of the method for removing ghosts and geometr ic distortion artifacts.