Deep sulcus sign depicts pneumothorax in the supine position as air ascends to the non-dependent pleural space outlining the anterior costophrenic recess/sulcus. If the volume of pneumothorax is sufficiently large it can also outline the costocardic angle and the pericardial fat and additional "pericardial fat pad sign" may also be observed.

A prudent radiologist will need to asses the costophrenic and pericardial region closely when inspecting supine trauma and ICU radiographs as small pneumothorax can be a subtle finding.