Chemical Shift Artifact

Chemical shift artifact

This diagram illustrates chemical shift artifact around an intracranial lipoma.

Chemical shift artifact, is a common finding on some MRI sequences, and used in MRS, and results from the effect of the electron cloud to greater or lesser degree shielding the nucleus from the external static magnetic field (Bo). The Lamor frequency which determines the frequency at which a particular nucleus resonates is established at the nucleus, and therefore different tissues will have slightly different Lamor frequencies depending on their chemical composition.

In MRS the shift in Lamor frequency allows separation of different chemical peaks. The actual amount of chemical shift as an absolute value is difficult to measure, so instead it is represented relative to a reference, and expressed in parts per million (ppm).

In MRI, both spin echo sequences (SE) and gradient echo sequences (GE) may demonstrate chemical shift artefact, but it will appear differently depending on the sequence.

In SE sequences the mismapping will occur in the frequency encoding direction, and show up as a bright band on one side and a dark band on the other side of a fat-soft tissue interface (see diagram here)

In GE sequences the mismapping will occur in all directions, and result in an ‘india ink’ outline to the chemical artifact region.

These effects can be used to confirm, as in this example, the presence of fat in a lesion.

For original images of this case please visit here.

1. MF Reiser et al “Magnetic Resonance Tomography” Springer 2007

Credit: Dr Frank Gaillard