Air bubble artifact
Updates to Article Attributes
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 vaporisation in the system, increasingly common as the tube ages
Detecting the artifact
It can be difficult for radiologists/radiographers to realise that this artifact is present, as true subtle low density on CT brain is often seen in early ischaemia and other pathologies. Radiologists or clinical referrers may become suspicious when consistent subtle findings on CT are not visible on repeat CT or MRI.
If there is suspicion that the artifact might be present then the vendor needs to be contacted for testing of the scanner by qualified personnel. The service engineer is able to perform an 'air detection test' to determine how many gas bubbles are in the coolant.
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 publicationscase-series, each seemingly unaware of the other 1,2.
-<p>The <strong>air bubble artifact</strong> on <a title="CT" href="/articles/computed-tomography">CT</a> 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.</p><h4>Cause</h4><p>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 <a title="Hounsfield unit" href="/articles/hounsfield-unit">HU</a>. 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.</p><h4>History and etymology</h4><p>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 <sup>1,2</sup>.</p>- +<p>The <strong>air bubble artifact</strong> on <a href="/articles/computed-tomography">CT</a> 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.</p><h4>Cause</h4><p>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 <a href="/articles/hounsfield-unit">HU</a>. 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.</p><h5>Formation of air bubbles</h5><p>Gas/air bubbles can arise within the coolant oil via several different mechanisms <sup>3</sup>:</p><ul>
- +<li>during CT service/repair, e.g. oil changes/top-ups</li>
- +<li>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</li>
- +<li>spontaneous formation of gas within the oil when in situ, due to vaporisation in the system, increasingly common as the tube ages</li>
- +</ul><h5>Detecting the artifact</h5><p>It can be difficult for radiologists/radiographers to realise that this artifact is present, as true subtle low density on CT brain is often seen in early ischaemia and other pathologies. Radiologists or clinical referrers may become suspicious when consistent subtle findings on CT are not visible on repeat CT or MRI. </p><p>If there is suspicion that the artifact might be present then the vendor needs to be contacted for testing of the scanner by qualified personnel. The service engineer is able to perform an 'air detection test' to determine how many gas bubbles are in the coolant. </p><h5>Removing the artifact</h5><p>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.</p><h4>History and etymology</h4><p>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 <sup>1,2</sup>.</p>
References changed:
- 3. Trieu N, Xia R, Loneragan R, et al. Artifact on CT brain images caused by the presence of air bubbles in the cooling oil of the X-ray tube. (2017) Journal of medical imaging and radiation oncology. 61 (2): 197-203. <a href="https://doi.org/10.1111/1754-9485.12545">doi:10.1111/1754-9485.12545</a> - <a href="https://www.ncbi.nlm.nih.gov/pubmed/27753267">Pubmed</a> <span class="ref_v4"></span>
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