What are pulsed field gradients?
Imagine if you could really mess up the Z1 resolution for a few milliseconds then restore it to its proper value during the course of the pulse sequence. This is an oversimplification, since pulsed field gradients do not use the normal shim circuits. A special PFG probe, and a PFG amplifier are necessary. By applying a gradient to the magnetic field, the top of the sample experiences a slightly different magnetic field to the bottom of the sample. Since magnetisation precesses at different rates in different fields, it is possible after a 90 degree pulse and a PFG of a few milliseconds to have the magnetisation vectors along the length of the tube pointing in all directions instead of nicely aligned along one axis of the rotating frame. Obviously if the magnetisation vectors are pointing in all directions, there is no net signal. The vectors are said to be dephased. If you now apply a PFG of opposite sign for the same time, you will rephase the magnetisation, and get your signal back.
