Air crescent sign

Not to be confused with the crescent sign.

In radiology, the air crescent sign (also called the Monad sign^[1]) 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.^[2] Classically, it is due to an aspergilloma, a form of aspergillosis, that occurs when the fungus Aspergillus grows in a cavity in the lung.^[3]

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
Differential diagnosis	Aspergilloma

Additional images

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.