In cases of trauma, with internal damage to lungs and airways, air can leak out into various anatomic spaces. Most commonly, air from tears in the visceral pleura will leak into the pleural space giving a pneumothorax (PTX, for short). But air can also leak from the airways into the adjacent connective tissue of the mediastinum (called 'pneumomediastinum', abbreviated 'pneumomed' above). Air can track along connective tissues into the subcutaneous regions, from tears in the parietal pleura, producing subcutaneous emphysema ('subQ air' above). This air can be dramatic on imaging but does not typically cause much problem for the patient. It can be palpated, causing swelling of the tissues and a crackling sensation.

Images A and B are axial CT images in lung windows, which are useful for lung detail and abnormal air, but not good for soft tissues.

CASE 4 followup