Identification of a unique cause of ring artifact seen in computed tomography trans-axial images

Artifacts present in computed tomography (CT) image often degrade the image quality and ultimately, the diagnostic outcome. Ring artifact in trans-axial image is caused by either miscalibrated or defective detector element of detector row, which is often categorized as scanner based artifact. A ring artifact detected on trans-axial CT image of positron emission tomography/computed tomography (PET/CT), was caused by contamination of CT tube aperture by droplet of injectable contrast medium. This artifact was corrected by removal of contrast droplet from CT tube aperture. The ring artifact is a very common artifact, commonly cited in the literature. Our case puts forward an uncommon cause of this artifact and its method of correction, which also, has no mention in the existing literature.