Aliasing or "Wrap-around "

Aliasing or wrap-around is a common artifact that occurs when the field of view (FOV) is smaller than the body part being imaged. The part of the body that lies beyond the edge of the FOV is projected on to the other side of the image. This can be corrected, if necessary, by oversampling the data. In the frequency direction, this is accomplished by sampling the signal twice as fast. In the phase direction, the number of phase-encoding steps must be increased with a longer study as a result.
The following axial images of the brain demonstrate this artifact. the first image shows wrap-around of the back of the head on to the front of the head, where the phase-encoded direction is anterior-posterior.The second image has the phase and frequency directions reversed resulting in absence of the aliasing artifact. Oversampling was used in the frequency direction to eliminate the aliasing.