Air bubble sign (pulmonary hydatid)

The air bubble sign is seen in CT of complicated (ruptured or infected) pulmonary hydatid cyst and refers to small bubbles of gas within the periphery of pulmonary mass and is helpful, particularly in endemic areas, in suggesting the diagnosis over other masses (e.g. metastases or primary lung tumors) . It is thought to represent air tracking between the pericyst and the hydatid cyst membrane .

It should not be confused with the similarly named sign of tension pneumocephalus: see air bubble sign (tension pneumocephalus).