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

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 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.

Water–fat separation with bipolar multiecho sequences

“…With some algebraic manipulation, the noise amplification factor η can be computed explicitly by Given the acquisition times t 1,2,3 at each k-space location, we can compute the noise amplification factor η of the corresponding matrix A. Figure 3a, b show the variation of the noise amplification factor η in two imaging cases at 1.5 T. The relevant imaging parameters that determine the acquisition times are: TE 1…”

“…Compared to fat suppression methods based on spectrallyselective excitation (1) and STIR (short-TI inversion recovery) (2), multipoint water-fat separation methods based on chemical-shift-induced phase differences (3)(4)(5)(6)(7)(8) are often less susceptible to magnetic field inhomogeneities. To correct field inhomogeneities before water-fat separation, the latter group of methods incur long scan times to acquire multiple images for field map estimation.…”

Design of improved spectral-spatial pulses for routine clinical use

“…[6], is f p and f n , respectively. When f p Ϫf n the tip angle of each odd subpulse is different from the tip angle of an adjacent even subpulse.…”

3D interleaved water and fat image acquisition with chemical-shift correction

“…However, this approach does not allow acquisition of both water and fat images in a single scan. In an effort to improve the ratio between information provided and imaging time used, simultaneous water and fat imaging techniques were developed to acquire water and fat data in alternate phase encoding line (18,20).…”