Critical Chest Radiographs

Pneumomediastinum 

Pneumomediastinum is free air in the mediastinal structures. It most commonly occurs following trauma or iatrogenic injury to the esophagus  or adyacent alveoli. On chest radiography,  free air may outline the anatomic structures. Common findings are a line of radiolucency that outlines the cardiac silhouette  (arrow), vertically orientated streaks of air in the mediastinum,  a double bronchial wall sign, or lucency around the pulmonary artery - the "right around artery" sign. Air is more easily detected retroesternally on lateral chest radiographs. Air is fixed in a pneumomediastinum and does not rise to the highest point.