X-ray images rely on the physical attenuation of the x-ray beam to form an image on film. The denser the tissue, the more radiation it will absorb. The more radiation absorbed by the tissues, the less reaches the film. The more radiation that passes through the tissue and reaches the film, the darker the area will be on the image.
The most dense area of a normal tooth is the enamel cap, which typically appears more radiopaque (white) than the other tissues. The dentin is less dense and appears as a uniform grey area. The junction between the enamel and dentin is very distinct. The layer of cementum on the root surface is nearly the same density as the dentin, thus it is usually not apparent radiographically. The soft tissues of the pulp are much less dense than the other tooth structures and typically appear radiolucent. In normal, fully-formed teeth the root canal may be apparent extending to the apex of the root with a recognizable apical foramen.
Supporting structures of the tooth that are visible radiographically include the lamina dura, the alveolar crest, the periodontal ligament space, and the cancellous bone. When the x-ray beam is projected directly through the long axis of the lamina dura, it is seen clearly as a thin, white line. If the beam passes through at an angle, the lamina dura may appear more diffuse or not be visible at all. The radiographic appearance of the alveolar crest varies from a dense layer of cortical bone to a smooth surface without cortical bone. The level of the bony crest is considered normal when it is not more than 1.5 mm from the cementoenamel junction of the adjacent teeth. The periodontal ligament space appears as a radiolucent space between the root and lamina dura, beginning at the alveolar crest, extending around the portion of the root within the alveolus, and returning to the alveolar crest on the opposite side. The width of the PDL varies from tooth to tooth, although it is typically thinner in the middle of the root and wider near the alveolar crest and root apex. Cancellous bone lies between the cortical plates of both jaws and shows many small radiolucent pockets of marrow which create the trabecular pattern we see on the film. The trabecular pattern varies considerably from patient to patient and even within the same patient.