Coherence artifacts suppression

Figure 1. Gradient-enhanced heteronuclear single quantum coherence pulse sequence with coherence transfer selection and artifact suppression gradients. All pulses are of phase x unless otherwise indicated.

Pulsed field gradients are added to existing NMR pulse sequences in order to suppress artifacts and/or to select certain coherence transfer pathways. The application of gradients for these two purposes maintain different requirements and benefits. 

Phase cycling is a fundamental procedure in most NMR experiments and is used not only for removing instrument artifacts, but also for selecting or suppressing signals, specially for achievement of specific coherence transfer pathways [5,13]. In NMR experiments, one must be aware of the importance of phase cycling, which sometimes is more difficult to understand than the basic aspects of the pulse sequences. 

Before the development of pulsed field gradient sequences (see below), most NMR pulse sequences included a phase cycle in which the phases of at least one pulse and the receiver were varied systematically. This was needed for one or more of several reasons, e.g. suppression of unwanted signals, suppression of artifacts due to hardware imperfections and/or incomplete return to equilibrium between scans and coherence pathway selection in multidimensional NMR. One example of each the first two kinds of phase cycle is briefly discussed below. 

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