Revision 9 for 'Air bubble artifact'

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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.

Cause

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/gas 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.

Formation of air bubbles

Gas/air bubbles can arise within the coolant oil via several different mechanisms 3:

  • during CT service/repair, e.g. oil changes/top-ups
  • loss of integrity of the tube envelope/heat exchanger: it is designed to be a self-contained unit with no communication with the external environment
  • spontaneous formation of gas within the oil when in situ, due to vaporization in the system, increasingly common as the tube ages
Removing the artifact

Resolving the artifact requires an engineer to replace the oil and treat any underlying defect in the system e.g. a leak in the tube housing.

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 case-series, each seemingly unaware of the other 1,2.

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