Pneumothorax on an erect chest radiograph is easy to diagnose, but on supine films it is difficult because air in the pleural cavity takes a non dependent position.

Signs of pneumothorax on supine radiographs are:

• deep sulcus sign: air in the pleural cavity collects in the anterolateral pleural space outlining the deep costophrenic angle
• increased clarity or sharpness of the ipsilateral hemidiaphragm
• double diaphragm sign
• increased sharpness of the mediastinal or cardiac outline