9.3
Extraoral Film Types
There are two extraoral film types: screen film and non-screen film. The majority of extraoral films are screen films.
Screen Film
Screen film requires the use of a screen in a cassette with phosphors for exposure. The film is sensitive to fluorescent light rather than direct exposure to x-radiation. Some screen films are sensitive to blue light and some are sensitive to green light; the film-screen combinations must match.
In the Iannucci & Howerton, Dental Radiography Principles & Techniques, 6th Edition textbook on page 85, refer to Figure 9-17.
Non-Screen Film
Nonscreen film does not require the use of a screen; it is exposed directly to x-rays. It is not recommended for dental use, and the emulsion is sensitive to direct x-ray exposure rather than fluorescent light. This type of film does require more exposure time than a screen film and is not recommended for use in dental radiography.
Extraoral Film Equipment
Extraoral film equipment includes intensifying screens and cassettes. In extraoral radiography, screen films are used in combination with these two special equipment items.
Intensifying Screens
Intensifying screens is a device that transfers x-ray energy into visible light; the visible light then exposes the screen film. This lowers the radiation dose to the patient. A screen film is secured in a cassette between two intensifying screens of matching size and the intensifying screen is a smooth plastic sheet coated with minute fluorescent crystals known as phosphors.
There is a slight loss of image detail as a result of the intensified x-ray beam because the light produces a halo effect at the edge of the image field. This loss of detail makes panoramic x-rays unsuitable for caries diagnosis. For this reason, pediatric dentists take bite-wings in conjunction with the panoramic x-ray to make a pediatric full set.
In the Iannucci & Howerton, Dental Radiography Principles & Techniques, 6th Edition textbook on page 85, refer to Figure 9-18.
The two types of screens are calcium tungstate screens, which have phosphors that emit blue light, and rare earth screens, which have phosphors that emit green light, and these require less radiation than calcium tungstate screens.
Cassette
A cassette is used to hold the film and the intensifying screens, and it may be flexible or rigid. Most are rigid, except the panoramic cassette, which may be flexible. The cassette must be light-tight and hold the intensifying screen in contact with the film; otherwise, the lack of contact between the film and cassette will result in a lack of sharpness of the image in that area. A rigid cassette is more expensive but usually lasts longer than a flexible cassette.
In the Iannucci & Howerton, Dental Radiography Principles & Techniques, 6th Edition textbook on page 86, refer to Figure 9-20.
A rigid cassette has a plastic front cover to allow the photons to pass through and the back cover is metal to reduce scatter radiation. The cassette is marked with an L and an R for orientation, and the front of the cassette must always face the patient during exposure. There is no identification dot on the extraoral film.
Duplicating Film
Duplicating film is a different type of film than radiographic film. It is a special film, or duplicating film, which is used to make copies or duplicate images. The longer it is exposed, the lighter the film will be and this is good for referrals and for the submission of insurance claims. It also has an emulsion on one side only and it is not exposed to x-rays; it must be used with a film duplicator.
Duplicating film packaging is available in periapical sizes and 5×12-inch, 6×12-inch, and 8×10-inch sheets in boxes of 50 sheets.
Film Storage and Protection
Film is negatively affected by heat, humidity, and radiation and should be kept in a cool, dry place. It’s also important to remember that film has an expiration date.
Media Attributions
- Iannucci & Howerton: Dental Radiography Principles and Technique , 6th Edition, Chapter 9, CC BY-NC-ND
