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

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Abstract
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 × 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.