Air Crescent Sign

This febrile neutropaenic patient had pulmonary nodules surrounded by ground-glass opacity consistent with angioinvasive fungal infection. One week after antifungal treatment his follow-up CT chest showed
signs of healing with one of the nodules demonstrating the air crescent sign.
In angioinvasive fungal infection, the nodules are composed of infected haemorrhagic and infarcted lung tissue. As the neutrophil count recovers and the patient mounts an immune response, peripheral reabsorption of necrotic tissue causes the retraction of the infarcted centre and air fills the space in between. This creates an air crescent within the nodules and is a good prognostic finding because it marks the recovery phase of the infection. This sign is seen in approximately 50% of patients.
Reference: Abramson S. The Air Crescent Sign. Radiology. 2001; 218:230-232
Credit: Dr Donna D'Souza
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