Echo-antiecho approach

Quadrature detection As described above, some care is required when employing gradients in phase-sensitive experiments. This either requires that gradients are not placed within time domain evolution periods or that the echo-antiecho approach be used. 

Figure 6.10. A gradient-selected, phase-sensitive HSQC sequence using the echo-antiecho approach. The N- and P-type pathways are selected by the last gradient.

A similar logic to that above applies to gradient selection in the HSQC experiment, for which a variety of different approaches are also possible [7]. A suitable sequence employing the echo-antiecho approach is illustrated in Fig. 6.10, and requires only two gradients in proportion to the magnetogyric ratios of the X and H spins since each acts on single-quantum X and H magnetisation. Thus, for a correlation experiment, ratios of 4 1 and... 

Finally, we note an alternative approach to quadrature detection known as echo-antiecho selection [11], which is applicable only to pulsed field gradient selected 2D methods and which now finds widespread use. As this involves a quite different procedure, it will not be considered further here but will be introduced in Section 5.5.2 after field gradients have been described. 

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