Air crescent sign
![The arrows denote an ill-defined nodular opacity in medial aspect of right upper lobe with ill-defined rim of lucency surrounding it.](/Images/godic/202412/20/Air_crescent_sign1209.jpg")
![Sagittal reformat from a CT scan of the chest showing air crescent sign in a patient with invasive fungal infection. There is a rounded cavity in the apical right upper lobe, with a non-dependant soft-tissue nodule within it. Also there is some subtle ground-glass opacity surrounding the lesion.](/Images/godic/202412/20/Invasive_fungus1209.JPG")
In radiology, the air crescent sign is a finding on chest radiograph and computed tomography that is crescenteric and radiolucent, due to a lung cavity that is filled with air and has a round radiopaque mass. Classically, it is due to an aspergilloma, a form of aspergillosis, that occurs when the fungus Aspergillus grows in a cavity in the lung. It is also referred as Monad sign.