#DIRECT EXPOSURE X RAY FILM PLUS#

Some manufacturers do not provide sensitivity values for their receptor systems, but most provide speed values such as 100, 200, 400, etc.

#DIRECT EXPOSURE X RAY FILM PLUS#

The sensitivity of a receptor, such as an intensifying screen-film combination, is expressed in terms of the exposure required to produce a film density of 1 unit above the base plus fog level.

If we assume a 5% energy conversion efficiency, then one 50-keV x-ray photon can produce 1,000 blue-green light photons with an energy of 2.5 eV each. This percentage is the conversion efficiency of the screen, and depends on the type of material used in the screen.Д¬onversion of X-Ray Energy in an Intensifying ScreenДЄlthough the total energy of the light emitted by a screen is much less than the total x-ray energy the screen receives, the light energy is much more efficient in exposing film because it is 'repackaged' into a much larger number of photons. The intensifying screen is an energy converter it converts approximately 5 to 20% of the absorbed x-ray energy into light. In the intensifying screen, the fluorescent process creates visible light when such material is exposed to high-energy x-ray photons. The figure below illustrates what happens to the x-ray energy that is absorbed by an intensifying screen. Materials that glow, or emit visible light, when exposed to high-photon energy ultraviolet light have this property. Fluorescence is the property of a material that enables it to absorb radiation energy in one portion of the photon-energy spectrum and emit some of the energy in the form of lower energy photons. The second function performed by the intensifying screen is to convert a portion of the absorbed x-ray energy into light. Absorption efficiency is primarily determined by three factors: (1) screen material, (2) screen thickness, and (3) the photon energy spectrum. An ideal screen would have a 100% absorption efficiency actual screens generally have absorption efficiencies The absorption efficiency is the percentage of incident radiation absorbed by the screen material. This is wasted radiation since it does not contribute to image formation and film exposure. In most cases, a significant portion of x-ray energy is not absorbed by the screen material and penetrates the receptor. As we discuss later, increasing the thickness of an intensifying screen to increase its absorption capabilities degrades image quality. The ideal intensifying screen would absorb all x-ray energy that enters it real intensifying screens are generally not thick enough to absorb all of the photons. The first function performed by the intensifying screen is to absorb the x-ray beam (energy) emerging from the patient's body. In some procedures that require high image detail, such as mammography, one intensifying screen is used in conjunction with a single-emulsion film. Using two screens in this manner increases x-ray absorption and receptor sensitivity The receptor used for most radiographic procedures contains two intensifying screens mounted on each side of double-emulsion film. The selection of a screen for a specific procedure is usually based on a compromise between the requirements for image detail and patient exposure. Imaging process and places a limit on the visibility of detail that mustД«e considered when selecting screens for specific clinical applications.ДЄ Conventional Screen/Film Radiographic ReceptorДifferent types of intensifying screens are available for clinical use. Unfortunately, intensifying screens introduce blurring into the X-radiation would be required to expose a film without using intensifying Sensitive to light than to x-radiation approximately 100 times as much Intensifying screens are used because film is much more The x-ray energy is absorbed by the intensifying screen material,ДЄnd a portion of it is converted into light. The screen-film combination is housed in either a cassette or a filmД¬hanger. Intensifying screens are thin sheets, or layers, of fluorescent materials. In screen/film radiography, the receptor consists of the film mounted inД¬ontact with either one or two intensifying screens, as shown below. The section titled, 'Fluoroscopic Imaging Systems.' Processing,' and the fluoroscopic systems that can also produce radiographs as described in The section titled, 'Digital Imaging Systems and Image Screen/film type described in this chapter there are the digital radiographic receptors described in There are three basic types of radiographic receptors.

Direct exposure x-ray film