On a normal CXR, we can visualize the air-filled trachea, mainstem bronchi and initial portions of the lobar bronchi. Further branchings should only be seen with the injection of contrast during a bronchogram. When air can be visualized in the more peripheral intrapulmonary bronchi, this is known as the ‘air-bronchogram sign’. This abnormality is usually caused by an infiltrate/consolidation that surrounds the bronchi.
Air Bronchogram #1