Supine subpulmonary pneumothorax.

Intrapleural air often assumes a subpulmonary position in the recumbent patient even in the absence of underlying pulmonary disease. The margins of the collapsed lung may be difficult or impossible to detect, and a poorly defined basilar hyperlucency may be the only clue present. Additional helpful findings include depression and clear visualization of the diaphragm anteriorly (anterior costophrenic sulcus) creating a "double" appearance to the diaphragm, an unusually distinct cardiac apex, and pericardial fat tags. These signs should help alert the viewer to the correct diagnosis of pneumothorax.