Coherence Transfer Pathways

In this chapter, the authors discuss coherence transfer pathways (CTPs), which represent a more abstract concept for describing pulse sequences. CTP selection can be applied in many cases in which the use of the product operator formulation is too difficult or impossible. The authors also discuss the main methods used in coherence pathway selection, phase cycling and pulsed gradients, and provide examples of how these are implemented in important classes of experiments. Cogwheel phase cycling is a type of concerted phase incrementation, but it had not been exploited until it was explicitly formulated by Levitt et al. Both phase cycling and pulsed gradients can be combined together in a single pulse sequence for gradient selection. For heteronuclear spin systems, one needs to keep track of the individual coherence orders for each nucleus. This approach is more straightforward for phase cycling than it is for pulsed-field gradients.