Phase-encode order and its effect on contrast and artifact in single-shot RARE sequences

Substantial manipulation of tissue contrast can be achieved by varying the order in which phase-encode values are applied to individual echoes within a 128-echo single-shot rapid acquisition relaxation enhanced (RARE) sequence. Appropriate ordering can then permit imaging of short T2 species like muscle and white matter with single-shot RARE. For sequential phase encoding with an arbitrary initial phase-encode value, the timing of the zero phase (ZP) encoded echo is found to be analogous to the echo time (TE) of standard spin-echo sequences. This is demonstrated qualitatively with human brain images and is verified quantitatively with NiCl2 phantoms by correlating the time constant for signal decay with ZP echo time, with transverse relaxation times T2, as obtained with a 128-echo Carr-Purcell-Meiboom-Gill (CPMG) imaging sequence. Banding artifacts accompanying the discontinuous traverse through K space are experimentally demonstrated in a rectangular phantom and expressions are developed for determining the dependence of this artifact on the phase-encode gradient increments and durations, the ZP echo number, echo spacing, and T2. Simulations based on the expressions are shown to be useful for characterizing the observed "banding" artifacts perpendicular to the phase-encode direction and for predicting the extent of tissue-tissue overlap to be expected with the use of this ultrafast rf echo planar imaging method.