General considerations when taking x-rays
Diagnostically useful radiographs have specific visual qualities. The x-ray image is a combination of black, white, and shades of gray. Contrast is the difference between the colors and shows definition of the items contained in the picture. The denser an object is, the more it blocks the x-ray beam, and it appears white on the x-ray picture. The contrast depends on the type of film used, the processing of the film, and the density of the film. Extraoral films have their own inherent contrast. Incorrect developing can ruin the contrast of a film by lightening or darkening the subtle shades of gray. Overdevelopment will make the film too dark, and underprocessing will make the film too light. A film with good contrast will show the darkness of soft tissue, the lightness of amalgam, and the subtle shades of gray in the nerve canal and trabecular bone. The contrast in the film is determined by:
- thickness of the patient’s tissues
- the density of his or her hard tissue
- the anatomic number of the tissue of the subject
The operator should consider these variables and adjust the time to compensate for each if possible.
Density is another important characteristic of an x-ray. The density is dependent on the amount of radiation that penetrates the tissues and reaches the film, the distance from the x-ray tube head to the patient, the subject tissue thickness, and the way the film is developed.
Different dental x-ray units will be set for different kV and mA, so the time settings for the same film on the same patient may vary from machine to machine:
- The higher the amperage (mA) the more intense the x-ray beam, and the darker the film will be.
- The higher the voltage peak (kVp) the more energy is produced, the more intense the x-ray beam, and the darker the film will be.
- The farther the patient is from the source of the x-ray beam, the less intense the x-ray beam, and the lighter the film will be. This varies with the design of the head of the x-ray unit being used.
- The longer the film is exposed, the darker the film will be. This is the one variable that the operator can always adjust to compensate for the other three variables.
The quality of detail on a radiograph is called resolution. low resolution films look “fuzzy”, and high resolution films are sharp and crisp. Films can lose resolution if the patient or film moves during exposure. Tubehead movement will not cause blurry distortion. This is because the x-rays emerging from a long cone are all parallel, and the tube head tends to swing or drop in such a way that the angle of the beam does not change. As long as the angle of the beam remains the same, parallel x-rays do not cast a moving shadow.
A good radiograph will contain the following characteristics: (From “Radiographic Imaging for Dental Auxiliaries” by Miles, Van Dis, Jensen, and Ferretti)
- The correct anatomic area should be represented
- At least 3-4 mm (1/4 inch) of alveolar bone should be visible beyond the apex.
- The image should not be elongated or foreshortened
- The radiograph should have acceptable density
- The radiograph should be free of film-handling or processing errors
- The interproximal contacts should not overlap
- There should be no cone cuts
- The embossed (raised) dot should appear at the incisal or the occlusal edge
- In a complete mouth radiograph series, the apex of each tooth should be visible at least once, preferably twice
- The interproximal contacts should not be overlapped from the distal surface of the canine to the mesial surface of the third molar
- The crowns of the maxillary and mandibular teeth should be centered in the image from top to bottom
- The crest of the alveolar bone should be visible with no superimposition of the crowns of the adjacent teeth
- The occlusal plane should be as horizontal as possible