An artefact is ...

Beam Hardening Artefacts

Beam hardening artefacts appear as streaks and shadows adjacent to areas of high density such as the petrous bones, shoulders, and hips. The artefact occurs because the high density anatomy absorbs the lower energy photons while the higher energy photons pass through to the detectors which results in the beam becoming 'harder'.

Metal Artefacts

Motion Artefacts

Multislice Artefacts

Partial Volume Artefacts

Ring Artefacts

Insufficient exposure results in concentric circles in the center of the field of view.