Deep sulcus sign
The deep sulcus sign on a supine chest radiograph is an indication of a pneumothorax.
In a supine film (common in the ICU), it may be the only indication of a pneumothorax because air collects anteriorly and basally, within the nondependent portions of the pleural space, as opposed to the apex when the patient is upright.
The costophrenic angle is abnormally deepened when the pleural air collects laterally, producing the deep sulcus sign.
Patients with chronic obstructive pulmonary disease (COPD) may exhibit deepened lateral costophrenic angles due to hyperaeration of the lungs and cause a false deep sulcus sign.