1. Gas collections in the lung tend to be spherical, while gas collections in the pleura tend to be longer in one dimension.2. Pneumopericardium appears as “clean” gas that is bounded by the superior pericardial recess, while pneumomediastinum is “dirty” gas that can track superiorly into the neck.3. It is critically important to scan the upper abdomen for incidental abnormal gas, including pneumatosis and pneumoperitoneum.

1. Abnormal air in the lungsa. Bullab. Cavitary consolidation2. Abnormal air in the pleuraa. Pneumothorax, including deep sulcusb. Hydropneumothorax - distinguish from intraparenchymal gas3. Abnormal air in the mediastinuma. Pneumopericardiumb. Pneumomediastinumc. Signs: ring-around-the-artery, continuous diaphragmd. Herniae. Post-surgical4. Abnormal air in the abdomena. Pneumoperitoneumb. Pneumobiliac. Portal venous gas and pneumatosis