Two PFG of equal strength and polarity before and after a 180° pulse applied on a single-spin nucleus provide the opportunity to select terms that have a transverse spin operator both before and after this pulse. In this way, pulse imperfections (transfer from I_z to I_x, or viceversa) are removed and transverse magnetization that do not experience the 180° pulse is effectively dephased. Examples should be:

• A selective 180 pulse for selective excitation
• A WATERGATE solvent suppression scheme
• A BIRD element for selective inversion
• In heteronuclear systems this is also possible, as widely used in regular INEPT pulse trains included into multidimensional HSQC experiments:

Easy implementation on AVANCE spectrometers equipped with gradient technology.

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

... d13 p16:gp1 d16 (refocusing element to be defined accordingly) d13 p16:gp1 d16 ...

in which:

• The refocusing element can be a single 180 pulse, simultaneous 180 pulses in heteronuclear systems, a selective 180 pulse, a BIRD element, a WATERGATE scheme ......
• p16 is the duration of the gradient (in microseconds)
• d16 is the recovery time (in seconds) needed after the gradient
• gp1 stands for the gradient strength defined in the gpz1 command. Otherwhise, gpnam1 defines the shape of the gradient.