Ring artefacts in X-ray computed tomography

The present authors reply to a letter by Coulden and Dixon (1987) and contend that low photon flux alone cannot cause central ring artefacts, and that they are necessarily caused by detector miscalibration or malfunction. The effects of some types of detector error will be more visible at low photon fluxes and hence may only be obvious when scanning obese patients. However, if the machine is functioning correctly such artefacts will not be produced, however large the patient. (author)