From the case:
Ring artifact
Axial non-contrast
Two images from a non-contrast CT of the abdomen demonstrate concentric rings centered on the caudate lobe of the liver. These do not conform to normal anatomy and represent ring artefact.
Case Discussion
Ring artifact is usually obvious and rarely is mistaken for pathology, although it can obscure pathology in affected parts of the scan. It is most often due to a faulty x-ray detector.