Excitation Sculpting in High-Resolution Nuclear Magnetic Resonance Spectroscopy: Application to Selective NOE Experiments

摘要

Selective pulses are key elements in high-resolution NMR experiments, so great effort has been put into designing pulse shapes with desirable properties. In this communication we describe a selective excitation technique which, judged by the usual criteria, outdistances existing methods. Our method gives constant phase and amplitude excitation over an easily adjustable bandwidth, can achieve given selectivity in a shorter time than existing methods, has no out-of-band sidelobes, and in exciting a multiplet, refocuses the evolution of scalar coupling. The method is tolerant of radio-frequency (rf) field inhomogeneity, and altering the net flip angle is easy. The use of pulsed field gradients (PFGs) results in these crucial properties being achieved in a single scan, without difference spectroscopy or phase cycling: magnetization from outside the desired bandwidth is destroyed, thus simplifying the subsequent manipulation of the excited magnetization. While PFGs have been used to tailor spectral response using single spin echoes, for example with the WATERGATE sequence, and while selective 180° pulses have been used for selective excitation in conjunction with difference spectroscopy, the approach described here is more general.