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

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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. Magn Reson Med 47:337–343, 2002.