Excitation of narrow frequency bands with reduced relaxation-related signal losses: methodology and preliminary applications

Selective excitation of a narrow frequency band is usually obtained by a long-duration, symmetrical-shaped RF pulse or by a series of short pulses with a symmetrical envelope. In species with fast transverse relaxation, both approaches lead to marked signal losses. Asymmetrical excitations applying truncated shaped pulses or half-Gaussian envelopes for trains of equidistant hard pulses were reported to provide higher signal intensity, but the frequency response is clearly inferior to the corresponding symmetrical excitations. Methods allowing asymmetrical excitation, but excellent frequency response are described in the present work. An additional 90° pulse is applied after a train of equidistant hard pulses with a half-Gaussian envelope. Suitable timing in the entire sequence of pulses and a phase cycle with length 2 only for the additional 90° pulse combined with an even number of scans allow the removal of undesired transverse magnetization outside narrow frequency bands. Thus, a periodical excitation with a very small bandwidth is obtained. In imaging sequences with standard 2D Fourier reconstruction the new excitation strategies can be included to generate a normal image representing morphology beside a band pattern with chemical information, if an odd number of scans is used. The separation of both parts in the final image is based on the principle of alternated line scanning. Macroscopic and microscopic field inhomogeneities in tissue are assessable in a single experiment. Preliminary applications on specimens with limited homogeneity of the magnetic field and on human tissue are demonstrated.