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.
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