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Analog Imaging III By Professor Stelmark
Many types of radiographic intensifying screens are available, and each manufacturer uses different names to identify them. Collectively, however, screens usually are identified by their relative speed expressed numerically. Screen speeds range from 100 (slow, detail) to 1200 (very fast).
Screen speed is a relative number that describes how efficiently x-rays are converted into light. Par-speed calcium tungstate screens are assigned a value of 100 and serve as the basis for comparison of all other screens. Calcium tungstate screens seldom are used anymore. High-speed rare Earth screens have speeds up to 1200; detail screens have speeds of approximately 50 to 80. These and other characteristics are summarized in.
The speed of a radiographic intensifying screen conveys no information regarding patient dose. This information is related by the intensification factor (IF). The IF is defined as the ratio of the exposure required to produce the same optical density with a screen to the exposure required to produce an optical density without a scree
Radiographers often use the term image detail or visibility of detail when describing image quality. Radiographic intensifying screens have the disadvantage of lower spatial resolution compared with direct-exposure radiographs.
Cassette The cassette is the rigid holder that contains the film and radiographic intensifying screens. The front cover, the side facing the x-ray source, is made of material with a low atomic number such as plastic. It is thin, yet sturdy. The front cover of the cassette is designed for minimum attenuation of the x-ray beam.
Attached to the inside of the front cover is the front screen, and attached to the back cover is the back screen. The radiographic film is sandwiched between the two screens. Between each screen and the cassette cover is some sort of compression device, such as radiolucent plastic foam, which maintains close screen-film contact when the cassette is closed and latched.
The back cover is usually made of heavy metal to minimize backscatter. The x-rays transmitted through the screen-film combination to the back cover more readily undergo photoelectric effect in a high-Z material than in a low-Z material. X-rays can be transmitted through the entire cassette, and some might be scattered back to the film by the cassette-holding device or a nearby wall. This is called backscatter radiation and results in image fog.
For mammography, the single screen is on the far side of the emulsion to reduce screen blur.
Carbon Fiber Material One of the materials developed early in the space exploration program was carbon fiber. This material was developed for nose cone applications because of its superior strength and heat resistance. It consists principally of graphite fibers (ZC =6) in a plastic matrix that can be formed to any shape or thickness. In radiology, this material now is used widely in devices designed to reduce patient exposure. A cassette with a front that consists of carbon fiber material absorbs only approximately half the number of x-rays that an aluminum or plastic cassette does. Carbon fiber also is used as pallet material for fluoroscopic examination tables and computed tomography beds. Carbon fiber not only reduces patient exposure, it also may produce longer x-ray tube life because of the lower-demand radiographic techniques required.
Characteristics of a Good Cassette • Lightweight • Durable • Smooth, washable surface • Lightproof ( analog system) • Capable of maintaining good film-screen contact • Opened and closed easily • Smooth, rounded edges
Problems with cassettes • Light leaks • Poor film/screen contact – transient and permanent • Artifact presence
Radiographic intensifying screens also can be cleaned with mild soap and water. The screens should be carefully rinsed and thoroughly dried. If the screen is damp, the film emulsion layer may stick to it, possibly causing permanent damage
An equally important requirement in caring for radiographic intensifying screens is maintaining good screen-film contact. Screen-film contact can be checked by radiographing a wire mesh. If darker areas of blurring are seen, as then screen-film contact is poor and should be corrected, or the cassette should be replaced.
PROCEDURE • 1. Choose a clean location to clean screens and cassettes. Wipe the outside of cassettes and clean the countertop before cleaning screens. • 2. Moisten a lint-free wipe with a small amount of commercially available screen cleaner and antistatic solution. A mild soap and water solution or a 70% solution of isopropyl alcohol may be used as an alternative (check with your screen manufacturer to see if this is acceptable), but a screen cleaner and antistatic solution must be used afterwards. • 3. Clean and dry the screen. Be sure to avoid excessive pressure or rubbing on the screen surface. • 4. Use a second lint-free wipe to clean the frame and inside cover (for single-screen cassettes). • 5. Stand the cassettes on edge to dry. • 6. Once the screen and cassette cover are dry, inspect them for any particles of dust. Use a UV light if necessary. If they are clean, reload with fresh film. For mammography cassettes, the screen surface should be carefully brushed with an antistatic brush and then inspected before loading with fresh film
