phase encoding 

IMAGING

An MRI term for the process of locating an MR signal by altering the phase of spins in one dimension with a pulsed magnetic field gradient along that dimension before acquiring the signal.

Because each signal component experiences a different phase encoding gradient pulse, its exact spatial reconstruction can be specifically and precisely located by Fourier transformation analysis. Spatial resolution is directly correlated to the number of phase encoding levels (gradients) used. 

Reference http://fonar.com/glossary.htm