PHASE ROBUST PULSE PAIRS FOR CHEMICAL SHIFT SELECTIVE SPIN ECHOES

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Introduction: An important side effect of applying a frequency selective pulse is the introduction of a phase gradient across the spectrum. Flip angle deviations on the order of 5-10% significantly change the phase profile across the passband of the pulse. Methods: The frequency response of a spin-echo is calculated from the Cayley-Klein parameters for the two pulses in a 90-180 spin-echo pair. To design the pulses we first establish a goal profile which defines the ideal profile for a pair of amplitude-modulated pulses subject to B1 inhomogeneity. Least squares optimization by simulated annealing is then used to find the pulse pair that minimizes the deviation between the actual profile and the goal profile, for +/-20% variation in B1. Results: The pulses were implemented as spatial-spectral pulses for use in a fast spectroscopic imaging experiment. Measurements of phase variations over the volume of interest confirmed the stability of the phase profile. Conclusions: A joint design of 90-180 spin-echo pairs offers frequency profiles that are immune to up to 20% variation in RF homogeneity.