Phase-Modulated binomial RF pulses for fast spectrally-selective musculoskeletal imaging

Two fat-suppressed three-dimensional gradient echo pulse sequences, FLASH and DESS (Double Echo in Steady-State), that have significantly reduced scan time compared with conventional chemical shift fat-suppression sequences are presented. This fat-suppression technique is based on selectively exciting water spins using a time-optimized binomial RF pulse pair at the water resonance frequency with a null in the excitation profile at the fat frequency. To minimize the total pulse length, the delay between the binomial components of the RF pulse is decreased from a standard jump-return implementation. To maintain the proper null frequency, the phase of the second RF pulse is shifted such that it returns the fat resonance back to its initial z position while further tipping the water spins. Nonselective phase-modulated RF pulse pairs can be implemented in 1.20 ms at 1.0 T, significantly reducing the minimum TR.