The suprasternal fossa.
A well-demarcated U or V-shaped gas density superimposed over the trachea on posteroanterior chest radiographs is due to air in the suprasternal fossa. This is the depression on the skin surface of the neck between the sternal heads of the sternocleidomastoid muscles. The fossa is frequently visualized radiographically in cachectic or very thin people, and in patients with prior laryngectomies. It is also seen in patients with chronic obstructive lung disease and in patients in acute respiratory distress. Because of its typical location and configuration, it should not be confused with an air-fluid level in the esophagus or an upper airway diverticulum.
D002968:Clavicle / Q000000981:diagnostic imaging; D005260:Female; D006801:Humans; D009132:Muscles / Q000033:anatomy & histology; D009333:Neck / Q000033:anatomy & histology / Q000000981:diagnostic imaging; D011859:Radiography; D013249:Sternum / Q000033:anatomy & histology* / Q000000981:diagnostic imaging; D014132:Trachea / Q000000981:diagnostic imaging