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Pneumomediastinum may result from a variety of causes that may be either intrathoracic (eg, narrowed or plugged airway, straining against a closed glottis, blunt chest trauma, alveolar rupture) or extrathoracic (eg, sinus fracture, iatrogenic manipulation in dental extraction, perforation of a hollow viscus [corrected]. The radiographic signs of pneumomediastinum depend on the depiction of normal anatomic structures that are outlined by the air as it leaves the mediastinum. These signs include the thymic sail sign, "ring around the artery" sign, tubular artery sign, double bronchial wall sign, continuous diaphragm sign, and extrapleural sign. In distal esophageal rupture, air may migrate from the mediastinum into the pulmonary ligament. Pneumomediastinum may be difficult to differentiate from medial pneumothorax and pneumopericardium. Occasionally, normal anatomic structures (eg, major fissure, anterior junction line) may simulate air within the mediastinum. Iatrogenic entities that may simulate pneumomediastinum include helium in the balloon of an intraaortic assist device. In addition, pneumomediastinum may be simulated by the Mach band effect, which manifests as a region of lucency adjacent to structures with convex borders. The absence of an opaque line, which is typically seen in pneumomediastinum, can aid in differentiation. Computed tomographic (CT) digital radiography and conventional CT can also be helpful in establishing or confirming the diagnosis.

Encountering a developmental lung anomaly in the adult can be a challenge, as the abnormality may be mistaken for something more sinister. The common anomalies encountered are classified into three broad categories: bronchopulmonary (lung bud) anomalies, vascular anomalies, and combined lung and vascular anomalies. The imaging features of these developmental anomalies at conventional radiography, ventilation-perfusion lung nuclear scanning, angiography, computed tomography, and magnetic resonance imaging are useful in differential diagnosis of thoracic lesions. Lung bud anomalies include agenesis, congenital bronchial atresia, congenital lobar emphysema, congenital cystic adenomatoid malformation, pulmonary bronchogenic cysts, tracheal or pig bronchus, and accessory cardiac bronchus. Vascular anomalies include interruption or absence of a main pulmonary artery, anomalous origin of the left pulmonary artery from the right, anomalous pulmonary venous drainage (partial or complete), and pulmonary arteriovenous malformation. Combined lung and vascular anomalies include the hypogenetic lung (scimitar) syndrome and bronchopulmonary sequestration, both intralobar and extralobar.

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