If the 180° pulse is used to invert z-magnetization (from z to -z) the greatest dephasing of undesired coherences is obtained when the second gradient is applied in the opposite sense to the first (fig. 1e). In this way, any residual transverse magnetization created by the imperfect 180° pulse is eliminated.

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:gp2 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 and gp2 stand for the gradient strength defined in the gpz1 and gpz2 commands, where gpz1=-gpz2. Otherwhise, gpnam1 and gpnam2 define the shape of the gradients.