Improved hard-pulse sequences for frequency-selective presaturation in magnetic resonance

Abstract

We have been investigating several fat-suppressing sequences compatible with mu ltislice “instant” (I, 2) (or echo planar) imaging. Wh ile lipid suppression may be used in conventional imaging both to reduce chemical-shift artifact and to improve visualization of body tissues embedded in fat, lipid suppression is crucial in instant imaging because of the extreme shift artifact. In one promising selective saturation technique (3-5) the lipid protons are excited selectively by a binomial series of hard pulses (6). A magnetic field gradient then dephases the resulting transverse magnetization prior to slice selection. The binomial sequences (and their variants, e.g., ( 7))) invented for solvent suppression, leave little transverse excitation in a broad frequency range near the species that is to be suppressed. However, in order to suppress fat by selective saturation, the remaining longitudinal magnetization determines the amount of suppression. Therefore, for MR imaging, a better strategy would be to make a sequence that produces or leaves m inimum longitudinal magnetization in a broad frequency range around the species to be suppressed. In this Note, we derive simple hard-pulse sequences using this strategy in order to reduce dramatically the sensitivity of selective saturation suppression to local BO inhomogeneity. We shall derive these new hard-pulse sequences in a manner inspired by Hore’s original paper, al though differing a bit in approach. We will examine how requirements on the excitation spectrum constrain the amp litude of the pulses that make up a sequence. Consider an excitation, A(t), composed of IZ + 1 hard pulses along one of the transverse axes (say 2;)) equally separated in time by half of the fat-water precession period, 7hv,