Chemical Shift Artefact
Chemical shift misregistration artefact manifests as bright or dark outlines predominantly at fat / water (and silicone) interfaces. They are anatomically incorrect and can alter or even shield true anatomy. Once recognised, chemical shift artefacts can generally be ignored in image interpretation. It does however, present problems in the detection of subtle abnormalities of the optic nerve or small disc herniations in the thoracic spine, for example.
In the frequency-encode direction, the MRI scanner uses the frequency of the signal to indicate spatial position. Since water in organs and muscle resonate at a slightly different frequency than that of fat, the MRI scanner mistakes the frequency difference as a spatial (positional) difference. This frequency difference results from the different electron environments of the protons of water and of fat. The difference in chemical shift is approximately 3.5 parts-per-million (ppm) which at 1.5 Tesla corresponds to a frequency difference between that of fat and water of approximately 220 Hz.
As a result, fat containing structures are shifted in the frequency direction from their true positions. This type of chemical shift is sometimes referred to as chemical shift of the first kind.
[Note: in echo planar imaging (EPI), this artefact is pronounced and seen in the phase encoding direction (as opposed to the frequency encoding direction).]
To understand how chemical shift artefact depends on the receiver bandwidth, see the Chemical Shift animated tutorial.