Air crescent sign

In radiology, the air crescent sign is a finding on chest X-ray and/or computed tomography that is crescenteric and radiolucent, due to a lung cavity that is filled with air and has a round radioopaque mass. Classically, it is due to aspergilloma (a type of aspergillosis) in the lung. This occurs when Aspergillus colonizes a cavity in the lung.