Nuclear magnetic resonance imaging

A method of investigating a body by nuclear magnetic resonance comprises preferentially exciting resonance in a slice of the body; applying a magnetic field to the body having first and second gradients in different directions in the slice, both gradients being periodically reversed at regular intervals; detecting the free induction decay signals occurring when the gradients are applied; and processing the detected signals to retrieve chemical shift information for each of a two dimensional array of piscels in the slice. The method is an adaptation of the echo-planar technique. Preferably the period of reversal of one gradient is a multiple of that of the other. The method may be repeated with the polarity of one gradient (amf) reversed relative to that in the first cycle. (author)