High spatial resolution EPI using an odd number of interleaves

Ghost artifacts in echoplanar imaging (EPI) arise from phase errors caused by differences in eddy currents and gradient ramping during left-to-right t raversal of k(x) (forward echo) versus right-to-left traversal of k(x) (rev erse echo). Reference scans do not always reduce the artifact and may make image quality worse. To eliminate the need for reference scans, a ghost art ifact reduction technique based on image phase correction was developed, in which phase errors are directly estimated from images reconstructed separa tely using only the forward or only the reverse echos, In practice, this te chnique is applicable only to single-shot EPI that produces only one ghost (shifted 1/2 the field of view from the parent image), because the techniqu e requires that the ghosts do not completely overlap the parent image. For higher spatial resolution, typically an even number of separate k-space tra versals (interleaves) are combined to produce one large data set. In this p aper, we show that data obtained from an even number of interleaves cannot be combined to produce only one ghost, and image phase correction cannot be applied. We then show that data obtained from an odd number of interleaves can be combined to produce only one ghost, and image phase correction can be applied to reduce ghost intensity significantly. This "odd-number interl eaf EPI" provides spatial and temporal resolution tradeoffs that are comple mentary to, or can replace, those of even-number interleaf EPI. Odd-number interleaf EPI may be particularly useful for NIR systems in which reference scans have been unreliable. (C) 1999 Wiley-Liss, Inc.