Elimination of Nyquist ghosting caused by read-out to phase-encode gradient cross-terms in EPI.

Abstract

Echo-planar imaging (EPI) commonly suffers from ghosting artifacts caused by zero- or first-order phase differences between the odd and even echoes that constitute an EPI dataset. Small-bore imaging systems with shielded gradients may suffer significantly from cross-term eddy currents due to the high degree of manufacturing precision required in such systems compared to larger whole-body coils. A Nyquist ghost caused by cross-term eddy current contributions from the read-out to the phase-encode axis was identified in a small-bore system and characterized using a modified EPI experiment and a computer simulation. The artifact was corrected for using both a postprocessing approach and compensation blips along the phase-encode axis. Correction using compensation blips proved to be a more effective strategy to reduce this artifact than the postprocessing method used.