Seen on a supine chest radiograph, it is a sign, and may be the only sign, of a pneumothorax
In the supine position, air rises to the anterior and inferior portion of the thorax, first medially and then laterally
The air may cause the costophrenic angle on the side of the pneumothorax to project more inferiorly than the costophrenic angle on the opposite side
Imaging Findings
Deep, lucent, ipsilateral costophrenic angle on supine chest radiograph
Double diaphragm sign may also be seen on supine radiographs in patients with pneumothorax due to an interface of the dome (highest part of the hemidiaphragm) and the anterior costophrenic sulcus and the hemidiaphrag