Air bubble artifact (CT)
The air bubble artifact on CT is due to the presence of air in the x-ray tube oil, and has only been described on examinations of the brain. Subtle low density is observed in the brain parenchyma. It was first described in 2016.
The x-ray tube in a CT scanner is prevented from overheating by a system which uses oil as its coolant. The abnormal bubbles of air in the system subtly modify the transmission of the primary x-ray beam, decreasing its attenuation by up to 3 HU. The number and precise location of the bubbles may vary over time - due to their movement in the coolant - so that fluctuating attenuation of the x-rays occurs as the tube rotates. Therefore the position and severity of the artefact also varies. As the effect on the attenuation of the x-ray beam is very mild this artifact has only been seen when narrow window widths are used, which for practical purposes is generally CT brain only, primarily on "stroke" window settings.
History and etymology
This artifact was first described in 2016 by two separate groups (one in Scotland, the other in the US) in two different publications, each seemingly unaware of the other 1,2.