Ring artefacts are a CT phenomenon that occur due to miscalibration or failure of one or more detector elements in a CT scanner. They occur close to the isocentre of the scan and are usually visible on multiple slices at the same location. They are a common problem in cranial CT.
The remedy is usually simple - recalibrate the scanner and let the referrer know that the worrying-looking ring shadows are artefactual.
Some people like to make something out of nothing and the former German physician (never a radiologist) Ryke Geerd Hamer created an entire mythology about them. He shows ring artifacts in his book “Testament einer neuen Medicin” and calls them “Hamer-foci”. The logic is a little difficult to follow, but apparently the appearance of these “Hamer-foci” in the brain lead to the instantaneous development of cancer somewhere else in the body.
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