During the application of a PFG, chemical shift and J-coupling evolutions of the nuclei involved in the selected coherence take place in the usual way. In order to minimise the resulting phase errors owing to these evolutions, PFG are usually included into spin echoes. On the other hand, a PFG can be placed into constant-time defocusing-refocusing periods without further optimizations.
 

Easy implementation on AVANCE spectrometers equipped with gradient technology.

The standard way to implement a gradient-based echo in a pulse program is:

"d20=p16+d16+d13" ... d20 p2 ph1 d13 p16:gp1 d16 ...

in which:

• The d20 delay is automatically calculated before the pulse program is executed as defined in the first line usually found before the ze command.
• p2 is the high-power 180 degree pulse
• p16 is the duration of the gradient (in microseconds)
• d16 is the recovery time (in seconds) needed after the gradient
• gp1 is the gradient strength defined in the gpz1 command. Otherwhise, gpnam1 defines the shape of the gradient.

In the case of PFGs are applied after and before the 180 pulse:

... d13 p16:gp1 d16 p2 ph1 d13 p16:gp2 d16 ...

in which gpz1=-gpz2.