The air crescent sign refers to a radiolucent rim around a soft tissue density and is characteristic of invasive pulmonary aspergillosis (or other angioinvasive fungal infections) in immunocompromised patients. It results when Aspergillus invades the pulmonary vasculature and causes hemorrhage with subsequent infarction of pulmonary tissue. Over time, the infarcted tissue retracts and is absorbed along its periphery by leukocytes which is what gives rise to the air crescent appearance. The visualization of the air crescent sign indicates a favorable outcome because patients without leukocyte function cannot form this consolidation.
The image above is from a patient with prior history of tuberculosis who presented with a cough. The air crescent sign is seen in the left upper lobe.
REFERENCES
Abramson S. The air crescent sign. Radiology 2001;18:230-2.
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