A NOVEL METHOD FOR FAT SUPPRESSION IN RARE SEQUENCES

Rapid acquisition relaxation‐enhanced (RARE) sequences (Hennig et al., Magn. Reson. Med. 3, 823 (1986)) utilize one or several Cam‐Purcell–Meiboom–Gill (CPMG) echo trains to sample a number of k‐space lines each repetition time TR. The technique can rapidly generate multislice T2‐weighted images which, as a rule, are strikingly similar in contrast to conventional T2‐weighted spin‐echo (SE) images. An exception to this rule is the appearance of very bright signal from fat in T2‐weighted RARE images as compared to conventional T2‐weighted SE images. To reduce this fat signal, we introduce a time delay, τc, between the 90ox and first 180oy pulse of each echo train such that a phase angle of π/2 develops between fat and the reference (water) line at echo maxima. The technique leads to single‐acquisition fat suppression without the use of frequency‐selective saturation pulses and concomitant loss of slices per TR. A Bloch equation analysis is used to identify two major mechanisms contributing to suppression of off‐resonance spins such that Wτc = π/2. Namely, the CPMG sequence becomes a CP sequence with no self‐correction properties for imperfect 180° pulses leading to enhanced signal decay, and the raw k‐space data matrix become segemented into blocks alternately multiplied by ±i, leading to signal dispersion following Fourier transformation. Copyright © 1992 Wiley‐Liss, Inc., A Wiley Company