One of three linear magnetization waveforms superimposed on the main magnetic field at specific times within a pulse sequence to select the imaging region or provide necessary spatial localization information. A magnetic gradient is defined as the amount and direction of the linear rate of change of the magnetic field in space.

(FFE) An echo signal generated from a FID by means of a bipolar switched magnetic gradient. The preparation module of the pulse sequence consists of an excitation pulse. The magnetization tilts by a flip angle between 0° and 90°.

See Gradient Echo Sequence and Refocused Gradient Echo Sequence.

(Gf) A frequency encoding gradient produces a magnetic gradient field present during MR signal acquisition to encode signals into different frequencies, depending on their position toward the gradient.
Also called readout gradient.

(GE) An echo signal generated from a free induction decay by means of a bipolar switched magnetic gradient. The echo is produced by reversing the direction of a magnetic field gradient or by applying balanced pulses of magnetic field gradient before and after a refocusing RF pulse so as to cancel out the position dependent phase shifts that have accumulated due to the gradient.
In the latter case, the gradient echo is generally adjusted to be coincident with the RF spin echo. When the RF and gradient echoes are not coincident, the time of the gradient echo is denoted echo time (TE) and the difference in time between the echoes is denoted time difference (TD).
Gradient echo does not refocus the effects of main field inhomogeneity and therefore is generally used with a short echo time. Disadvantages of gradient echo imaging are compromised anatomic details and artifacts in regions with varying susceptibility e.g. between the air-containing sinuses and brain and especially between haemorrhages and normal tissue.

See also Susceptibility Artifact.

The process of returning out of phase magnetic moments back into phase coherence. Caused either by rapidly reversing a magnetic gradient (Field Echo) or by applying a 180° RF pulse (Spin Echo). In the spin echo pulse sequence this action effectively cancels out the spurious T2* information from the signal.

See also Spin Echo Sequence and Gradient Echo Sequence.