Ring artefacts in cranial CT: BMJ case report
A ring or arc artefact in a CT is a hardware related artefact that occurs due to a defective or miscalibrated detector. It is more common with third-generation CT scanners with solid-state detectors.

It appears as complete circles (sequential scans), annular rings (multiple circles) or partial rings (helical/multislice CT) at the same spot near the isocentre in multiple sections.1 2 Recognising this artefact is important as it may be subtle and misinterpreted as a pathological entity by the novice clinician.

This artefact is usually more clearly visualised in the soft-tissue window of CT scans than in the bone window. Presented here is a case where an arc artefact was noted in the CT made for the evaluation of facial fractures. The artefact posed no diagnostic challenge as it exhibited the characteristic features for easy identification and was away from the region of interest.

Source: http://casereports.bmj.com/content/2018/bcr-2018-226097.full
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