Abnormal pulmonary thoracic imaging finding Air crescent 2021-02-27T14:34:38Z An air crescent is a collection of air in a crescentic shape that separates the wall of a cavity from an inner mass. The air crescent sign is often considered characteristic of either Aspergillus colonization of preexisting cavities or retraction of infarcted lung in angioinvasive aspergillosis. However, the air crescent sign has also been reported in other conditions, including tuberculosis, Wegener granulomatosis, intracavitary hemorrhage, and lung cancer. The crescent can be seen in both plain X-ray and CT scan. HP:0032172 Air crescent sign See Figure 3 of PMID:181953762. The air crescent sign is said to be characteristic of invasive pulmonary aspergillosis, but it can also be seen in cavitating neoplasms, lung abscesses, and infections. This sign is considered a good marker of immune activity as it is seen when the necrotic tissue gets invaded by leukocytes and replaced by air in invasive pulmonary aspergillosis. Other conditions in which air crescent sign can be seen are pulmonary gangrene, pulmonary hematoma, Rasmussen aneurysm in a tuberculous cavity, and lung hydatid.