Air does not absorb X-ray beams, so areas filled with air, such as the lungs and bronchial tree, appear dark on a CXR. Notice that there are white "streaks" in the lungs, especially near the hilum. These streaks are formed by the branches of the bronchial tree and pulmonary vessels. Notice that in a normal chest film, the right hemidiaphragm is slightly higher than the left hemidiaphragm.
