Image Receptor Film and screens

In 1895, the image receptors for photography included glass plates, flexible films, and papers coated with a light-sensitive emulsion. However, in Rontgen's initial communication to the Wurzburg Physical Medical Society, he stressed the importance of using photographic plates (Gagliardi 1996a). The plates were manufactured in "standardized" sizes, including 14 x 17, 11 x 14, 10 x 12, 8 x 10, and 5 x 7 in. Prior to being exposed, the lightsensitive plate was placed into a "light-tight" envelope. A 14 x 17 in. glass plate weighed approximately 2 lb (4.4 kg), was fragile and, because of its thickness, difficult to handle for direct viewing. Therefore, prior to interpretation, direct contact prints of the images were made on sensitized paper (Gagliardi 1996b). Radiography lexicon still contains an allusion to this original recording media: an anterior-posterior, supine projection of an abdomen is commonly referred to as a flat plate of the abdomen.

A wrist x-ray on a glass plate in the early 1900s required a 30-min exposure. The long exposure was because less than 1% of the x-rays reaching the image receptor contributed to the formation of an image (Bushong 2008c). In February 1896, Michael Pupin, a Columbia University physicist, received a fluorescent screen from Thomas Edison. Since the screen, developed by Edison, fluoresced or converted x-ray to visible light, Pupin theorized he could reduce the exposure time by combining it with a photographic plate. He succeeded in producing the first intensifying screen-film image of a hand demonstrating the location of a shotgun pellet with an exposure of a few seconds (Eisenberg 1992b).

By 1913 the high cost and other problems previously stated regarding the plates led to a search for a replacement film base. Because the finest glass specifically manufactured for x-ray exposure was produced in Belgium, World War I forced many nations, including the United States, to search for another substrate for the photographic emulsion. Cellulose nitrate was one of the first to be marketed with an emulsion coated on a single side. Because cellulose nitrate was flammable, it was soon replaced with cellulose acetate and today with polyester. Glass plates were still available at least until the mid-1980s for special applications (see Chapter 7, Figure 7.26). The photosensitive crystals in the emulsion were considered "ultra-fine grained" and the resulting images could be magnified many times.

By eliminating the glass base, the thinner films had an emulsion that was more sensitive to x-rays and had a more conducive fit into a holder or cassette equipped with an intensifying screen. In 1918, Kodak introduced an x-ray film with an emulsion on both sides that could be placed into a cassette equipped with two screens. This new combination drastically reduced the exposure time and the radiation dose to the patient. In order to acquire a high-resolution image, nonscreen film was still used for some medical procedures, such as mammography, through the 1960s but single-emulsion nonscreen film was primarily relegated to industrial applications. Nonscreen film holders, often referred to as a cardboard holder, were commonly available through the early 1970s for mammography. However, by that time, a single high-resolution screen, single-emulsion film combination was developed for mammography.

Although the single-emulsion film requires more radiation to achieve an acceptable image, it has one advantage over double-emulsion film: a less blurry and sharper image. Double-emulsion film, even though the film base is very thin at 150-300 ^m (Bushong 2004a), provides two images separated by a very small distance. The result is a phenomenon known as parallax. When viewing objects a millimeter or less in size on doubleemulsion film, parallax results in apparent blurring of the object's margins. For situations in which magnification is necessary, such as mammography, a single-emulsion film and single high-resolution screen is employed.

Another benefit of a nonscreen approach is that the resulting image has increased latitude or more shades of gray. If the x-ray unit can produce the high mAs values required for nonscreen imaging, it will provide the best images (Figures 2.4A and 2.4B).

In medical radiography, there has always been a trade-off between producing a diagnostic image and reducing the radiation exposure to the patient. To achieve this goal, films were developed with emulsions that were more sensitive to the light emitted by the intensifying screen. Similar to photography, one method to produce "faster" films was to increase the size of the light-sensitive crystals embedded in the emulsion. With an increase in crystal size, the exposure to the patient was decreased, but there was a corresponding decrease in detail or resolution on the processed film.

A similar process occurred simultaneously in the development of intensifying screens. High-energy x-rays photons interact with the crystalline material in the fluorescent layer of the screen and are converted to many lower-energy photons of visible light. The larger the crystal embedded in the fluorescent layer, the more light photons generated from a single photon of x-ray. A high-speed screen would be very efficient at converting x-ray to light, but at the cost of a loss of detail. However, another consequence of using intensifying screens is an increase in contrast over nonscreen images. Without screens, the image is produced

Figure 2.4A An anterior-posterior, or AP, projection of the abdomen of an Egyptian mummy from the library at Cazenovia, New York, taken with a screen cassette. Note the high-contrast appearance, black and white with few shades of gray, of the image. An organ packet can be easily seen in the abdomen (arrow). Below the packet, the entire area appears as a "black" void.

Figure 2.4A An anterior-posterior, or AP, projection of the abdomen of an Egyptian mummy from the library at Cazenovia, New York, taken with a screen cassette. Note the high-contrast appearance, black and white with few shades of gray, of the image. An organ packet can be easily seen in the abdomen (arrow). Below the packet, the entire area appears as a "black" void.

solely by the differential absorption of x-rays. The visible light emitted from the screen eliminates some of the subtle differences in density that produce the shades of gray on the processed film. The result is fewer shades of gray and more black and white.

Screens are rated according to their ability to convert x-rays to visible light. A slow speed screen may be assigned a value of 100. For the sake of simplicity, let's say it will convert one photon of x-ray to 100 photons of light. This means that the original exposure variables (mAs) without screens can be reduced to 1/100 of the mAs with the intensifying screens. A par speed screen would be rated at approximately 200, a high-speed screen at 400, and an ultrahigh-speed screen at 1200. Shorter exposure times have two advantages: they reduce the radiation dose to the patient and eliminate involuntary movement, which would blur the image. A high-speed film/screen system would be an excellent choice for a chest radiograph when it is important to reduce the exposure time to minimize the effect of heart motion. However, the high-speed system would render very little trabecular detail within the long bones. Orthopedic radiography, on the other hand, would use the slower-speed screens to produce bone images with more detail. Therefore, for optimal results, film and screens of similar speeds would be matched for specific imaging objectives. If film and screens are not matched, the exposure (mAs) required may need to be adjusted and the resulting shades of gray, or latitude, available on the processed image may be compromised.

There are also specialty cassettes for specific applications. Standard x-ray film comes in a 14 x 36 in. (35.5 x 91.4 cm) size and is typically used in orthopedic or chiropractic medicine to image an entire spine. This film size requires a special cassette. These specialized

Figure 2.4B The same region radiographed with a nonscreen film holder. Since an intensifying screen was not used, the film had lower contrast with many shades of gray visible. As in the previous image, the abdominal packet was seen; however, soft tissue structures were noted where there was only a "black" void with the screen image.

screens are also available with varied degrees of intensification, with one end of the cassette being faster than the other.

Xeroradiography, or dry radiography, was a type of radiographic technique in which the image of the body was not recorded on film but on paper, eliminating the need for wet film developers (Selman 1985). In this technique, a plate of selenium, resting on a thin layer of aluminum oxide, was charged uniformly by passing it in front of a screen-controlled corona device termed a scorotron. As x-ray photons interacted with this amorphous coat of selenium, charges diffused out, in proportion to energy content of the x-ray. This process was a result of photoconduction. The resulting imprint, in the form of charge distribution on the plate, attracted toner particles, which were then transferred to reusable paper plates.

By the late 1970s, xeroradiography became an alternative to using film for mammography. There were a number of advantages over the film, including lower patient dose than with nonscreen film mammography, a dry chemical process, margins of varying density materials were enhanced, and wider latitude that demonstrated more materials with similar densities (Gagliardi 1996c). In addition, because it was printed on paper, a view box was not needed to view the image. In 1977, a French team used xeroradiography to obtain a lateral image of Ramses II's skull. The image revealed that the embalmer packed the pharaoh's nasal cavity with peppercorns and employed a small bone to support the tip of the nose (Lang and Middleton 1997). By 1990, xeromammography was replaced by a single-screen film system that provided better images at even lower patient doses (Bushong 2004b). Subsequently, Xerox stopped making the toner required for the processes, and xer-oradiography became another footnote in imaging history.

In the past decade or so, shortening the length of time it takes to complete the patient's examination has also become a consideration. The more patients examined in an hour, the higher the profits to the imaging center. This concept, termed throughput, has been a factor in the development of new hardware and software, and will be discussed in more detail in Chapter 3.

Since the emulsion slowly deteriorates over time, medical imaging film has an expiration date, and medical facilities cannot use outdated film on patients. This expired film can be a tremendous resource for field forensic or anthropological research projects. Outdated film can be obtained either from medical facilities or vendors; however, there will likely be a mismatch between the film and light frequency emitted by the screens. Several test exposures will be necessary to formulate a technique chart for the optimal mAs settings.

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