The CT fallen-lung sign.

On chest radiograph, the diagnosis of tracheobronchial tear is usually suspected because of the persistence of a pneumothorax after chest tube insertion. Since this radiographic pattern is nonspecific, the diagnosis is usually made by bronchoscopy and delayed. The fallen-lung sign consists in the fall of the collapsed lung away from the mediastinum occurring when the normal central bronchial anchoring attachment of the lung is disrupted. In contrast to the persistent pneumothorax, this sign is specific but rarely observed. Our purpose is to present the corresponding CT patterns observed in two cases of right stem bronchus tear, consisting in a caudal-dependent displacement of the right upper lobe bronchus which becomes obliquely oriented.