OFF-RESONANCE IMAGE ARTIFACTS IN INTERLEAVED EPI AND GRASE PULSE SEQUENCES

Echo-time shifting (ETS) is used in GRASE and interleaved-EPI sequence s to improve the phase evolutions for off-resonance signal sources. Ho wever, even with ETS the phase evolutions still exhibit discontinuitie s. In this work, we extend previous studies of ETS by quantitatively e valuating the magnitude and form of the image artifacts that result fr om these phase discontinuities. The functional form of the phase evolu tion is used to derive the general conditions under which artifacts ar e expected. The artifacts for two sequence structures are then evaluat ed as a function of off-resonance frequency and data sampling period b y calculating point spread functions and simulated images. It was foun d that even when ETS is used to improve the phase evolutions, periodic phase discontinuities may degrade image quality by producing ghosting artifacts of edges. These artifacts are similar to those that commonl y occur with periodic motion. From our results recommendations are der ived for limiting the ghosting artifacts.