In a normal chest x-ray, the tracheobronchial tree is not visible beyond 4th order. As the bronchial tree branches, the cartilagineous rings become thinner and eventually disappears in respiratory bronchioles. The lumen of bronchus contains air and the surrounding alveoli contain air. Thus there is no contrast to visualize bronchi.
The air column in bronchi beyond 4th order becomes recognizable if the surrounding alveoli is filled, providing a contrast or if the bronchi get thickened. The term air bronchogram is used for the former state and signifies alveolar disease.
Note branching radioluscent columns of air corresponding to bronchi, in RUL consolidation in the adjacent CXR.