Air crescent sign and Monod sign

 Air crescent sign also known as Meniscus sign or Cap sign, appears on xray or CT of the chest as air interposed between an intracavitary ball like mass and the cavity wall.

Most common cause is fungal ball of invasive aspergillosis. Other causes are pulmonary hydatid cyst, other fungi, blood clot or Rasmussen aneurysm in a tuberculous cavity, lung abcess with inspissated pus, staphylococcal pneumonia, nocardial infection, carcinoma of the lung, pulmonary gangrene or hematoma.

In aspergilloma, this mass usually moves within the cavity when the patient changes position and the sign is called Monod sign. A CT scan of the chest can be performed in prone position and if the mass moves to dependent area, diagnosis can be confirmed.