air crescent sign air crescent sign 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. not to be confused with the crescent sign. air crescent signthe arrows denote an ill-defined nodular opacity in medial aspect of right upper lobe with ill-defined rim of lucency surrounding itdifferential diagnosisaspergilloma 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.