Improved three-dimensional GRASE imaging with the SORT phase-encoding strategy

The phase-encoding strategy plays a critical role in determining the qualit y of gradient- and spin-echo (GRASE) images. Phase-encoding methods develop ed for two-dimensional GRASE imaging strive to achieve a balance between ar tifacts from T2-dependent signal amplitude modulations and off-resonance-de pendent signal phase shifts, although no current method provides smooth and nonperiodic evolutions for both of these signal changes. In three-dimensio nal GRASE imaging, the use of two phase-encoding directions presents the op portunity for improved phase-encoding strategies. In this report a phase-en coding strategy for three-dimensional GRASE, termed SORT, is described; thi s strategy separates off-resonance and T2 effects, mapping one along each o f the two phase-encoding directions. Thus, off-resonance-induced artifacts can be minimized while eliminating T2-dependent periodic signal modulations and allowing complete flexibility in the selection of echo time. The perfo rmance of the SORT phase-encoding method for T2-weighted GRASE imaging was compared with that of existing methods based on calculated point spread fun ctions and simulated images. The predicted performance of SORT phase encodi ng was verified experimentally using T2-weighted three-dimensional GRASE im aging of the brain. Generally artifact-free images were obtained even in th e presence of fat, susceptibility interfaces, and a wide range of T2 values . J. Magn, Reson, Imaging 1999;9:604-612. (C) 1999 Wiley-Liss, Inc.