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• W2074648063 abstract "Selective excitation of a single peak or a small group of peaks in a complicated spectrum presents a series of challenging problems. Excellent localization of the excitation on and around resonance, as several groups have shown previously (I, 2), can be achieved by pulse tailoring. In addition, the selectivity of any pulse can of course be improved by making it longer. However, long pulses are well known to normally excite spectra with severe phase rolls. In the simple example of a long rectangular pulse of duration 7, phase roll originates from the delay of 7/2 between the pulse center (since the Fourier transform of the pulse is only purely real if this point is chosen as t = 0) and the end of the pulse where detection is possible. Phase rolls are usually not serious in routine spectroscopy of liquids, which uses short intense 7r/2 or P pulses making Aw7/2 small for the entire spectrum. On the other hand, it is not unreasonable to apply a 50 ms long pulse in order to achieve selectivity of better than 100 Hz, and the resulting 25 ms “acquisition delay” from the center of the pulse will induce a large phase roll. This problem is dramatically amplified in sequences which also include strong pulses or in solvent suppression experiments, and often makes only magnitude spectra usable. Since the magnitude spectrum introduces line broadening, distortion of peaks, and ultimately a decrease in resolution, it is to be avoided if possible. In this communication we present what we believe are the first experimental results for elimination of phase roll from weak pulses. We compare and contrast two different approaches. In the first approach the phase roll was minimized by contructing amplitude-modulated pulses (Fig. 1A) with a total flip angle of 3a/2. A numerical simulation of “self-refocused 7r/2 pulse” was published recently by Ngo and Morris (3); we believe that shape is in fact a 3a/2 pulse. One can think of this family of pulses as analogs of a spin-echo sequence but with no delays either between or after the pulses. The theoretical excitation profile is shown in Fig. 2A. We find this shape quite difficult to use because of its extreme sensitivity to RF inhomogeneity. In the second approach, which produces much more usable shapes, we optimized a phaseand amplitudemodulated pulse (Fig. 1B) in such a way as to produce a phase predistortion capable of compensating for the effects of the resonance offset. The theoretical excitation profile for this pulse is presented in Fig. 2B. In both cases detection of the signal was assumed to begin directly after the application of the pulse and no phase correction was assumed. Both pulse shapes were obtained using a minimization program based on Powell’s" @default.
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• W2074648063 date "1988-03-01" @default.
• W2074648063 modified "2023-10-10" @default.
• W2074648063 title "Selective excitation without phase distortion using self-refocused amplitude- and amplitude/phase-modulated pulses" @default.
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• W2074648063 doi "https://doi.org/10.1016/0022-2364(88)90044-3" @default.
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