Simultaneous highly selective MR water and fat imaging using a simple new type of spectral-spatial excitation.

In a recent contribution [MRM 38:269-274 (1997)], it was reported that an excitation by a series of sinc-shaped slice-selective RF pulses with binomial amplitude ratios and complete spin refocusing between consecutive pulses leads to water- or fat-selective images of high quality. A method for simultaneous water and fat imaging in multislice operation is presented based on the principle of alternated line scanning and linear superposition of several excitations. For example, a 1 - 3 - 3 - 1 pulse train with suitable interpulse delays results in a water-selective excitation, whereas a 1 - 3 - 3 - 1 train leads to a selective excitation of fat (transmitter frequency corresponds with the Larmor frequency of water protons). Phase cycling of the excitation (1 - 3 - 3 - 1 for the even line numbers in k-space, but 1 - 3 - 3 - 1 for the odd line numbers) causes a shift of n/2 lines in phase-encode direction for the fat signals in an n x m matrix. The principle of linear superposition explains why an excitation of 2 - 0 - 6 - 0 for the even lines and 0 - 6 - 0 - 2 for the odd lines results in a final image with unshifted water signals and shifted fat signals. Both water and fat portions are simultaneously exhibited and separated without any signal loss. Examples recorded by a gradient-echo sequence demonstrate the potential of the new technique that allows a reduction of up to 50% of measuring time compared with former frequency-selective imaging methods.