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X-ray film displays the radiographic image. It is made up of several layers. The emulsion layer is the layer that produces the latent image. The latent image later developed into a visible film during film processing.
base: cellulose triacetate or polyester for support
substratum: an adhesive layer containing gelatin and solvents that bind emulsion and base
emulsion: silver halide and gelatin. Gelatin is used to support the silver halide crystals 5. Some hardening agents is the main layer where latent images are stored
protective layer: gelatin, protects emulsion from damage
The total thickness of the film is about 0.25 mm.
screen type films: faster when used with intensifying screen
orthochromatic (green sensitive)
direct exposure type: used for dental exposures
The emulsion layer consists of silver halide crystals where silver bromide (AgBr) is the most common type of crystal at 95% while silver iodide is at 5% 6. The silver iodide ions functioned to introduce defect within the silver bromide crystal because a perfect crystal does not have photographic sensitivity. A sensitivity speck is introduced into the silver halide crystal by silver sulfide in order to trap electrons. When exposed to light or X-rays, a bromide ion releases an electron that is attracted to a sensitivity speck. In this process, the bromide ion becomes a bromine atom and diffuses into the gelatin. The electron gets attached to the sensitivity speck and attracts a silver ion. The silver ion attaches to the sensitivity speck and forms a silver atom. The silver atom continues to attract other silver ions to form more silver atoms. At least two to six silver atoms are required for a latent image to be processed into a visible image. During film processing, more silver atoms are produced, thus metallic silver (black) image is formed 1,2.
Silver halide crystals are inherently sensitive to blue and ultraviolet light. Only with the addition of dyes, then the crystals can become more sensitive to other spectrums of light such as green and red 3.
Fogging is the development of unexposed silver halide crystals. In darkrooms, safelights are used that enable a radiographer to see the film he is working on but does not expose the film. For example, a red safelight is used during the processing of films that are sensitive to green lights. However, this does not totally protect the film from fogging, especially under prolonged exposure under safelight. The film is most sensitive to safelight after the latent image is formed and before it is processed 4.
The silver can be reclaimed from old x-ray film, in a process known as silver recovery.
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- 3. Bailey J & Clark B. Uses in Photographic and Reprographic Techniques. Comprehensive Heterocyclic Chemistry. 1984;:361-92. doi:10.1016/b978-008096519-2.00014-x
- 4. Perry Sprawls. Physical Principles of Medical Imaging. (1993) ISBN: 9780834203099 - Google Books The Photographic Process and Film Sensitivity
- 5. Lestra Yves. The importance of photographic gelatin. Nature. 1985;313(24):333.Nature
- 6. American Association of Physicists in Medicine. AAPM