Aim\ To research how the non-uniformity in the response of detector elements causes ring artifacts and their different appearance in parallel-beam 2D CT and in fan-beam CT. Methods\ Based on the principle of image reconstruction and the physical meaning of back-projection
the simulation is used. Results\ The ring artifacts are closely related to the angle range of x-ray source
as well as the position of the odd elements with abnormal response. Conclusion\ Artifacts in parallel-beam CT are actually semi-annualar artifacts while those in fan-beam CT are a series of whole-rings.