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.
D003964:Diaphragm / Q000000981:diagnostic imaging; D006321:Heart / Q000000981:diagnostic imaging; D006801:Humans; D008168:Lung / Q000000981:diagnostic imaging; D011030:Pneumothorax / Q000000981:diagnostic imaging*; D011187:Posture; D011859:Radiography