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

The presence of metal objects in the scan field can lead to severe streaking artifacts. They occur because the density of the metal is beyond the normal range that can be handled by the computer, resulting in incomplete attenuation profiles. Additional artifacts due to beam hardening, partial volume, and aliasing are likely to compound the problem when scanning very dense objects.
Patients are normally asked to take off removable metal objects such as jewelry before scanning commences. For non-removable items, such as dental fillings, prosthetic devices, and surgical clips, it is sometimes possible to use gantry angulation to exclude the metal inserts from scans of nearby anatomy. When it is impossible to scan the required anatomy without including metal objects, increasing technique, especially kilovoltage, may help penetrate some objects, and using thin sections will reduce the contribution due to partial volume artifact.
Streaking caused by over-ranging can be greatly reduced by means of special software corrections. Manufacturers use a variety of interpolation techniques to substitute the over-range values in attenuation profiles.
The usefulness of metal artifact reduction software is sometimes limited because, although streaking distant from the metal implants is removed, there still remains a loss of detail around the metal-tissue interface, which is often the main area of diagnostic interest. Beam hardening correction software should also be used when scanning metal objects to minimize the additional artifacts due to beam hardening.
Article from: http://radiographics.rsna.org/content/24/6/1679.full

Motion Artefacts

Multislice Artefacts

Partial Volume Artefacts

Ring Artefacts

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