The process of locating a MR signal by altering the phase of spins in one dimension with a pulsed magnetic field gradient along that dimension prior to the acquisition of the signal.
If a gradient field is briefly switched on and then off again at the beginning of the pulse sequence right after the radio frequency pulse, the magnetization of the external voxels will either precess faster or slower relative to those of the central voxels.
During readout of the signal, the phase of the xy-magnetization vector in different columns will thus systematically differ. When the x- or y- component of the signal is plotted as a function of the phase encoding step number n and thus of time n TR, it varies sinusoidally, fast at the left and right edges and slow at the center of the image. Voxels at the image edges along the phase encoding direction are thus characterized by a higher 'frequency' of rotation of their magnetization vectors than those towards the center.
As each signal component has experienced a different phase encoding gradient pulse, its exact spatial reconstruction can be specifically and precisely located by the Fourier transformation analysis. Spatial resolution is directly related to the number of phase encoding levels (gradients) used. The phase encoding direction can be chosen, e.g. whenever oblique MR images are acquired or when exchanging frequency and phase encoding directions to control wrap around artifacts.

The temporal order in which the phase encoding gradient pulses are applied. The order can be sequential, centric, reverse centric, random, etc.

A pulse sequence is a preselected set of defined RF and gradient pulses, usually repeated many times during a scan, wherein the time interval between pulses and the amplitude and shape of the gradient waveforms will control NMR signal reception and affect the characteristics of the MR images. Pulse sequences are computer programs that control all hardware aspects of the MRI measurement process.
Usual to describe pulse sequences, is to list the repetition time (TR), the echo time (TE), if using inversion recovery, the inversion time (TI) with all times given in milliseconds, and in case of a gradient echo sequence, the flip angle. For example, 3000/30/1000 would indicate an inversion recovery pulse sequence with TR of 3000 msec., TE of 30 msec., and TI of 1000 msec.
Specific pulse sequence weightings are dependent on the field strength, the manufacturer and the pathology.

See also Interpulse Times.

Briefly applied magnetic field gradients.

See also Bipolar Gradient Pulse and Gradient Magnetic Field.

The use of varying phase or timing of the RF pulses to prevent setting up a condition of steady state free precession, e.g. in rapid excitation MR imaging.

See also Spoiler Gradient Pulse and Rapid Excitation Magnetic Resonance Imaging.