Extraoral x-ray film continues to evolve, improve.
To understand how researchers continue to perfect this technology, it's helpful to review its history.
Extraoral x-ray systems don't rely on exposing x-ray film directly. Instead, they are created by exposing x-ray film to visible light, in much the same way regular 35 mm film is exposed.
With x-rays, however, the light is created through a process called fluorescence. Fluorescence occurs when molecules struck by radiation of one wavelength become excited and emit radiation of a different wavelength.
Fluorescence created by x-ray radiation was discovered in 1895 in Wurzburg, Germany, by physicist Wilhelm Conrad Roentgen. Roentgen was experimenting with vacuum tubes in a dim room when he noticed that a nearby plate, that had been coated with barium platinocyanide, began to glow. When he moved the vacuum tube nearer to the plate, the material glowed more brightly, even if he covered the tube with black paper.
It was this experiment that led Roentgen to the discovery of x-rays, because he knew that some invisible energy was causing the fluorescent effect.
The First Screen Radiograph
It wasn't long after Roentgen's discovery that the first film/screen radiograph was made by Professor Michael Putin of Columbia University. In 1896, Putin sandwiched a film against a fluorescent intensifying screen and x-rayed a hand.
Figure 1: First radiograph made using a fluorescent screen in combination with a film, 1896. (Image originally published in the 1997 book, The Basics of Film Processing in Medical Imaging, by Art G. Haus and Susan Jaskulski ISBN 0-944838-78-2. Reprinted with permission.)
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The process Putin used is identical to the way extraoral radiographs are exposed today: fluorescent light generated by the screen amplified the remnant x-rays that passed through the tissues and other structures of the hand. Most significantly, by using an intensifying screen, Putin was able to create the image with an exposure of only several seconds--a far shorter exposure than would have been required to expose the film using direct x-rays.
Further research on fluorescent materials was conducted by Thomas A. Edison that same year. Edison and his associate tested 8500 different materials, identifying about 1800 as fluorescent. One of these materials, calcium tungstate, was found to fluoresce with approximately six times the intensity of barium platinocyanide.
In 1918, the first commercial x-ray film was introduced, followed shortly by a double emulsion product. This was the foundation of modern extraoral radiography.
How Screen Cassettes Work
Extraoral radiograph screen-film systems are constructed using a piece of double-coated x-ray film: the film is sensitized to light on both sides. This film is placed between two layers of phosphor material (Fig. 2). When the x-ray radiation strikes the object to be imaged, some of it is absorbed, some of it is blocked and some passes through, depending on how thick the object is and its other qualities. Amalgam, for example, blocks most radiation that strikes it, whereas soft tissue permits most radiation to pass through unobstructed.
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The radiation that passes through the object strikes the phosphor layers of the screen, causing them to fluoresce, which in turn exposes the film. The more radiation striking the cassette, the greater the density or darkness created on the corresponding section of the film.
Using screens for extraoral radiography allows an image to be formed using far less radiation than would be required to expose the film directly. In fact, to get the same density without screens, exposure would have to be increased by 200-600 times.
Manufacturers that design film/screen systems have learned a great deal about how to deliver the highest possible image quality. Great care must be taken, for example, to coat the screen phosphors uniformly during manufacture. The film itself must be designed so that it is precisely sensitized to the light emitted by the chosen phosphorous material.
In addition, the cassette system itself must do two things. It must prevent any ambient "room" light from reaching the film after it's been loaded. Second, it must exert uniform contact between the screen and the film. It's a good practice to check screen/film contact annually using a wire mesh test imaging protocol.
Good quality cassettes also have a lead foil backing (Fig. 3). This helps reduce a phenomenon called "backscatter." Backscatter is caused when x-ray radiation passes through the cassette, bounces and reenters the cassette. It forms unwanted, nondiagnostic density on the film that manifests as a veil of fog. Some cassettes sold as "lightweight" unfortunately are built without a lead backing. While they are lighter, they sacrifice image quality and should be avoided.
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Cassettes must also be handled carefully. Cleaning screens with the wrong solvent, exposing them to nail polish, or even scratching them, can render them unusable. With care, however, a screen can last for five years or more.
"Blue" and "Green" Screens
Today there are two types of extraoral film-screen systems, commonly referred to according to the color of the visible light emitted by their fluorescent materials. Blue-emitting screens use materials such as calcium tungstate and have been in use since the 19th century. When exposed to x-ray radiation, these screens give off visible blue light.
In the 1970s, experiments were conducted with "rare-earth" or green-emitting phosphors, including lanthanum and gadolinium. Imaging scientists immediately recognized the value of this type of fluorescent material: for every 100 units of x-ray radiation, the old blue screens emit five units of visible light, while the new materials emit 18 units of visible light (Fig. 4). This increase in efficiency enabled a substantial reduction in x-ray exposure levels without compromising image quality, and x-ray film manufacturers moved quickly to develop x-ray film that was sensitized to green light.
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Today, as a result, dental offices that use a green-emitting screen system expose their patients to one-half the radiation as those that continue to use the older "blue" screen systems (Fig. 5).
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Cutting exposure levels was one important modern innovation in extraoral x-ray film. The other was the development of new barriers in the film/screen system to reduce a phenomenon called "crossover."
To understand what creates crossover, remember that extraoral x-ray film is double-sided. The reason for this is to maximize the benefit of the fluorescence step of the exposure process. Imaging scientists need to balance two factors: the thicker the fluorescent layer, the more light is emitted, which exposes the film faster and more effectively. However, light photons tend to scatter; therefore, thicker fluorescent layers can cause a diffusing of the light which in turn reduces the sharpness of the diagnostic image (Fig. 6).
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Scientists discovered long ago that two thin fluorescent layers deliver the benefit of a single thicker layer, without as much scattering. However, some scattering does occur as light from one fluorescent layer passes through the film base; this creates unwanted exposure and reduces the clarity of the diagnostic image.
In 2001, Kodak introduced a new generation of extraoral film, Kodak Ektavision Extraoral Imaging Film, that uses crossover barriers--an absorbing dye layered in the film--to reduce this unwanted exposure by 97 percent. Since this film was introduced, clinicians have reported a noticeable improvement in image clarity and detail (Fig. 7).
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Switching to Green
With improvements in extraoral film technology continuing into the 21st century, there are more film choices available for today's dental practice than ever before.
If your practice is using a blue screen system, you should seriously consider upgrading to green screen technology. Upgrading involves changing both the screens and the film together since one must be changed at the same time as the other.
Switching is economical and simple. For newer x-ray units, simply reduce exposure times by 50 percent after the new screens have been installed. For older x-ray units that will not allow such a big exposure reduction, purchase an inexpensive beam filter kit and install it in the port of the x-ray tube head. Your exposure will be cut by 50 percent automatically. With these simple steps, you'll reduce radiation exposures significantly, a great enhancement to your practice's standards of care.
In addition, take the time to educate yourself and your office about the newer extraoral film products. Today, dental professionals are becoming aware of the important role extraoral imaging can play in quality patient care. Researchers have shown that extraoral imaging not only allows dentists to visualize dentition, but also to flag a variety of other pathology, ranging from lesions to metabolic disorders, even carotid artery blockages. (1,2,3,4) For this reason, it's more important than ever to select a film-screen system that does the best possible job of producing sharp, high-contrast images.
(1.) Tetradis, S, Kantor, ML: Prevalence of skeletal and dental anomalies and normal variants seen in cephalometric and other radiographs of orthodontic patients. Am J Orthod Dentofacial Orthop, 166(5):572-7, 1999.
(2.) Carillo R, Morales A, Rodriguez, Peralto, JL et al. Benign fibro-osseous lesions in Paget's disease of the jaws. Oral Surg, Oral Med, Oral Pathol, 71:588-92, 1991.
(3.) O'Carroll MK, Krolls SO, Mosca NG, Metastatic carcinoma to the mandible: report of two cases. Oral Surg, Oral Med, Oral Pathol, 76:368-374, 1993.
(4.) Friedlander AH, Gratt BM, Panoramic dental radiography as an aid in detecting patients at risk for stroke, J Oral Maxillofac Surg 1994 Dec;52(12):1257-62.
(Note: Kodak and Ektavision are trademarks of Eastman Kodak Company.)
For more information such as this or on other current topics, visit the dental hygienist/assistants section of the Kodak website at www.Kodak.com/go/dental, or subscribe online to the Kodak dental e-newsletter.
Tony Rostron R.T.C.I.M., is Western U.S. Channel Manager, Dental Business Division, Eastman Kodak Company. He is a graduate of the Medical Radiography Technologist program, McGill University, and of the Canadian Institute of Management, University of Toronto. His background includes the London Polytechnic Institute of Photography, past presidency of the Society for Non Destructive Testing (Canada), former Canadian and U.S. Regional Manager for Eastman Kodak's Medical Imaging Division, and a continuing education provider for the California Dental Association.