The patient's movements are the most common cause of image artifacts.
There are 2 types of artifacts due to motion:
Motion artifacts mainly propagate in the phase-encode direction. This is due to movement of the spins between 2 excitations or between phase-encoding and signal reading: in the first case, the spins will not be recorded at the same position between excitations, in the second case, their phase-encoding will not be correct. As a result, the phase-encoding of these voxels is corrupted and this will be responsible for artifacts in the phase-encode direction.
On the other hand, signal sampling and spatial-encoding in the frequency-encode direction are done so fast that physiological motion will only produce a small amount of spatial blurring in that direction.
When the movements are periodical (cardiac beats, arterial or CSF pulsations, respiration), they can produce ghost images, propagated in the phase-encode direction, even outside the anatomic limits. The intensity of these ghost images becomes more extreme with the intensity of the moving structure and with the amplitude of movement. These ghost images can show up as an increase or decrease of the true image signal.
The spacing between ghost images varies with the direction of the movement, its amplitude and its periodicity relative to the phase-sampling interval (TR).