Chemical Shift Artifacts

The chemical shift artifact is commonly noticed in the spine at the vertebral body end plates, in the abdomen, and in the orbits where fat and other tissues form borders. In the frequency direction, the MRI scanner uses the frequency of the signal to indicate spatial position. Since water in organs and muscle resonate at a different frequency than fat, the MRI scanner mistakes the frequency difference as a spatial (positional) difference. As a result, fat containing structures are shifted in the frequency direction from their true positions. In the spine, this causes one end plate to appear thicker than the opposite one; in the abdomen and orbits, this causes a black border at one fat-water interface, and a bright border at the opposite border. This artifact is shown in the following axial image of a kidney where the bright border along the top of the kidney and the dark border along the bottom of the kidney represent the artifact. This artifact is greater at higher field strengths and lesser at higher gradient strengths. Practically about the only way to eliminate this artifact is to use a fat suppression technique.