Q: What is "Air-Cresent" sign?

Answer:  An air crescent sign is a crescent of air usually seen in invasive aspergillosis or any disease process that cause pulmonary necrosis. It is due to increased granulocyte activity. This is regarded as a good sign as it may herald recovery. As patient recovers necrotic tissues starts getting absorbed from periphery and retraction of the infarcted center creates air filled crescent area. Though it is considered as a hallmark sign of aspergillosis, it may also be seen in pulmonary tuberculous cavity, pulmonary abscess, bronchogenic carcinoma, pulmonary haematoma and Pneumocystis jirovecii pneumonia (PJP).