Screen-Film Radiography

Base

The base is the foundation of radiographic film. Its primary purpose is to provide a rigid structure onto which the emulsion can be coated. The base is flexible and fracture resistant to allow easy handling but is rigid enough to be snapped into a viewbox.

Conventional photographic film has a much thinner base than radiographic film and therefore is not as rigid. Can you imagine attempting to snap a 35 × 43 cm photographic negative into a viewbox?

The base of radiographic film maintains its size and shape during use and processing so that it does not contribute to image distortion. This property of the base is known as dimensional stability. The base is of uniform lucency and is nearly transparent to light.

During manufacturing, however, dye is added to the base of most radiographic film to slightly tint the film blue. Compared with untinted film, this coloring reduces eyestrain and fatigue, enhancing radiologists’ diagnostic efficiency and accuracy.

The original radiographic film base was a glass plate. Radiologists used to refer to radiographs as x-ray plates. During World War I, high-quality glass became largely unavailable while medical applications of x-rays, particularly by the military, were increasing rapidly.

A substitute material, cellulose nitrate, soon became the standard base. Cellulose nitrate, however, had one serious deficiency: It was flammable. Improper storage and handling of some x-ray film files resulted in severe hospital fires during the 1920s and early 1930s.

By the mid-1920s, film with a “safety base,” cellulose triacetate, was introduced. Cellulose triacetate has properties similar to those of cellulose nitrate but is not as flammable.

In the early 1960s, a polyester base was introduced. Polyester has taken the place of cellulose triacetate as the film base of choice. Polyester is more resistant to warping from age and is stronger than cellulose triacetate, permitting easier transport through automatic processors. Its dimensional stability is superior. Polyester bases are thinner than triacetate bases (≈175 µm) but are just as strong.

Emulsion

The emulsion is the heart of the radiographic film. It is the material with which x-rays or light photons from radiographic intensifying screens interact. The emulsion consists of a homogeneous mixture of gelatin and silver halide crystals. It is coated evenly with a layer that is 3 to 5 µm thick.

The gelatin is similar to that used in salads and desserts but is of much higher quality. It is clear, so it transmits light, and it is sufficiently porous for processing chemicals to penetrate to the crystals of silver halide. Its principal function is to provide mechanical support for silver halide crystals by holding them uniformly dispersed in place.

The silver halide crystal is the active ingredient of the radiographic emulsion. In the typical emulsion, 98% of the silver halide is silver bromide; the remainder is usually silver iodide. These atoms have relatively high atomic numbers (Z_Br = 35; Z_Ag = 47; Z_I = 53) compared with the gelatin and the base (for both, Z ≈ 7). The interaction of x-ray and light photons with these high-Z atoms ultimately results in the formation of a latent image on the radiograph.

Depending on the intended imaging application, silver halide crystals may have tabular, cubic, octahedral, polyhedral, or irregular shapes. Tabular grains are used in most radiographic films.

Tabular silver halide crystals are flat and typically 0.1 µm thick, with a triangular, hexagonal, or higher-order polygonal cross section. The crystals are approximately 1 µm in diameter. The arrangement of atoms in a crystal is cubic, as shown in Figure 12-2.

The crystals are made by dissolving metallic silver (Ag) in nitric acid (HNO₃) to form silver nitrate (AgNO₃). Light-sensitive silver bromide (AgBr) crystals are formed by mixing silver nitrate with potassium bromide (KBr) in the following reaction:

The entire process takes place in the presence of gelatin and with precise control of temperature, pressure, and the rate at which ingredients are mixed.

The shape and lattice structure of silver halide crystals are not perfect, and some of the imperfections result in the imaging property of the crystals. The type of imperfection thought to be responsible is a chemical contaminant, usually silver sulfide, which is introduced by chemical sensitization into the crystal lattice, usually at or near the surface.

This contaminant has been given the name sensitivity center. During exposure, photoelectrons and silver ions are attracted to these sensitivity centers, where they combine to form a latent image center of metallic silver.

Differences in speed, contrast, and spatial resolution among various radiographic films are determined by the process by which silver halide crystals are manufactured and by the mixture of these crystals into the gelatin. The number of sensitivity centers per crystal, the concentration of crystals in the emulsion, and the size and distribution of the crystals affect the performance characteristics of radiographic film.

Direct-exposure film contains a thicker emulsion with more silver halide crystals than screen film. The size and concentration of silver halide crystals primarily affect film speed. The composition of the radiographic emulsion is a proprietary secret that is closely guarded by each manufacturer.

Radiographic film is manufactured in total darkness. From the moment the emulsion ingredients are brought together until final packaging, no light is present.

Screen-Film

Screen film is the type of film that is used with radiographic intensifying screens. Several characteristics must be considered when one is selecting screen-film: contrast, speed, spectral matching, anticrossover or antihalation dyes, and requirement for a safelight.

Direct-Exposure Film

The use of radiographic intensifying screens with film allows reduced technique and therefore reduced patient radiation dose. However, the image is more blurred than it would be after exposure without screens. In the past, certain films were manufactured for use without screens; they were used to image thin body parts, such as hands and feet, that have high subject contrast and present a low radiation risk.

Most extremity examinations now use fine-grain, high-detail screens and double-emulsion film as the IR. The emulsion of a direct-exposure film is thicker than that of screen film, and it contains higher concentrations of silver halide crystals to improve direct x-ray interaction.

Mammography Film

Mammography was originally performed with an industrial-grade, double-emulsion, direct-exposure film. The radiation doses associated with such a technique were much too high; consequently, specialty films were developed.

Mammography film is single-emulsion film that is designed to be exposed with a single radiographic intensifying screen. All currently available mammography screen-film systems use green-emitting terbium-doped gadolinium oxysulfide screens with green-sensitive film.

The surface of the base opposite the screen is coated with a special light-absorbing dye to reduce reflection of screen light, which is transmitted through the emulsion and base. This effect is called halation, and the absorbing dye is an antihalation coating. Such an antihalation coating is used on all single-emulsion screen film, not just mammography film. The coating is removed during processing for better viewing.

Heat and Humidity

Radiographic film is sensitive to the effects of elevated temperature and humidity, especially for long periods. Heat increases the fog of a radiograph and therefore reduces contrast. Radiographic film should be stored at temperatures lower than approximately 20°C (68°F).

Storage under conditions of elevated humidity (e.g., >60%) also reduces contrast because of increased fog. Consequently, before use, radiographic film should be stored in a cool, dry place, ideally in a climate-controlled environment. Storage in an area that is too dry can be equally objectionable. Static artifacts are possible when the relative humidity dips to below about 40%.

Light

Radiographic film must be stored and handled in the dark. Any light at all can expose the emulsion before processing. If low-level, diffuse light exposes the film, fog is increased. If bright light exposes or partially exposes the film, a gross, obvious artifact is produced.

Control of light is ensured by a well-sealed darkroom and a light-proof storage bin for film that has been opened but not clinically exposed. The storage bin should have an electrical interlock that prevents it from being opened while the door to the darkroom is ajar or open.

Radiation

Ionizing radiation, other than the useful beam, creates an image artifact by increasing fog and reducing contrast. Film fog is the dull, uniform OD that appears if the film has been inadvertently exposed to light, x-rays, heat, or humidity.

Darkrooms usually are located next to radiographic imaging rooms and are lined with lead. However, this is not always necessary. It is usually acceptable to lead-line only the storage shelf and the film bin.

Radiographic film is far more sensitive to x-ray exposure than are people; therefore, more lead is required to protect film than people. The thickness of the lead barrier is designed to keep the total exposure of unprocessed film below 2 µGy_a. This, of course, requires some assumptions about the storage time of the film.

Care should be taken to ensure that the receiving area for radiographic film is not the same as that for the radioactive material used in nuclear medicine. Even though the packaging of radioactive material ensures the safety of those who handle it, the low-level radiation emitted can fog radiographic film if the radioactive material and film are stored together for even a short time.

Silver Halide Crystal

The silver, bromine, and iodine atoms are fixed in the crystal lattice in ion form (Figure 12-7). Silver is a positive ion, and bromide and iodide are negative ions. When a silver halide crystal is formed, each silver atom releases an outer-shell electron, which becomes attached to a halide atom (either bromine or iodine).

The silver atom is missing an electron and therefore is a positively charged ion, identified as Ag⁺. The bromine and iodine atoms each have one extra electron and therefore are negatively charged ions, identified as bromide and iodide (Br⁻ and I⁻), respectively.

The silver halide crystal is not as rigid as some crystals such as diamonds. Under certain conditions, atoms and electrons are free to migrate within the silver halide crystal.

The halide ions, bromide and iodide, are generally found in greatest concentration along the surface of the crystal. Therefore, the crystal takes on a negative surface charge, which is matched by the positive charge of the interstitial silver ions, the silver ions inside the crystal. An inherent defect in the structure of silver halide crystals, the Frankel defect, consists of interstitial silver ions and silver ion vacancies. Figure 12-8 presents a model of the silver halide crystal.

Photon Interaction with Silver Halide Crystal

When light photons from the radiographic intensifying screen interact with film, it is the interaction with the silver and halide atoms (Ag, Br, I) that forms the latent image. This interaction releases electrons in the crystal (Figure 12-9, A).

These electrons are released with sufficient energy to travel a large distance within the crystal (Figure 12-9, B). While crossing the crystal, the electrons may have sufficient energy to dislodge additional electrons from the crystal lattice.

Secondary electrons liberated by the absorption event migrate to the sensitivity center and are trapped. After a sensitivity center captures an electron and becomes more negatively charged, the center is attractive to mobile interstitial silver ions (Figure 12-9, C). The interstitial silver ion combines with the electrons trapped at the sensitivity center to form metallic silver atoms.

Most of these electrons come from the bromide and iodide ions because these negative ions have one extra electron. These negative ions therefore are converted to neutral atoms, and the loss of ionic charge results in disruption of the crystal lattice.

The bromine and iodine atoms are now free to migrate because they no longer are bound by ionic forces. They migrate out of the crystal into the gelatin portion of the emulsion.

Latent Image

The concentration of electrons at the sensitivity center produces a region of negative electrification. As halide atoms are removed from the crystal, the positive silver ions are electrostatically attracted to the sensitivity center. After migrating to the sensitivity center, the silver ions are neutralized by electrons and are converted to metallic silver.

In an optimally exposed film, most developable silver halide crystals have collected 4 to 10 silver atoms at a sensitivity center (Figure 12-9, D). Consequently, this silver deposition is not observable, even microscopically.

This group of silver atoms is called a latent image center. It is here that visible quantities of silver form during processing to create the radiographic image (Figure 12-9, E).

Crystals with silver deposited at the sensitivity center are developed into black grains (Figure 12-9, F). Crystals that have not been irradiated remain crystalline and inactive. The unobservable information contained in radiation-activated and -inactivated silver halide crystals constitutes the latent image.

Protective Coating

The layer of the radiographic intensifying screen closest to the radiographic film is the protective coating. It is 10 to 20 µm thick and is applied to the face of the screen to make the screen resistant to the abrasion and damage caused by handling. This layer also helps to eliminate the buildup of static electricity and provides a surface for routine cleaning without disturbing the active phosphor. The protective layer is transparent to light.

Phosphor

The active layer of the radiographic intensifying screen is the phosphor. The phosphor emits light during stimulation by x-rays. Phosphor layers vary in thickness from 50 to 300 µm, depending on the type of screen. The active substance of most phosphors before about 1980 was crystalline calcium tungstate embedded in a polymer matrix. The rare Earth elements gadolinium, lanthanum, and yttrium are the phosphor material in newer, faster screens.

The action of the phosphor can be seen by viewing an opened cassette in a darkened room through the protective window of the control booth. The radiographic intensifying screen glows brightly when exposed to x-rays.

Many materials react in this way, but radiography requires that materials possess the characteristics given in Box 12-1. Through the years, several materials have been used as phosphors because they exhibit these characteristics. These materials include calcium tungstate, zinc sulfide, and barium lead sulfate, as well as oxysulfides of the rare Earths gadolinium, lanthanum, and yttrium.

Roentgen discovered x-rays quite by accident. He observed the luminescence of barium platinocyanide, a phosphor that was never successfully applied to diagnostic radiology. Within 1 year of Roentgen’s discovery of x-rays, the American inventor Thomas A. Edison developed calcium tungstate. Although Edison demonstrated the use of radiographic intensifying screens before the beginning of the 20th century, screen-film combinations did not come into general use until about the time of World War I. With improved manufacturing techniques and quality control procedures, calcium tungstate proved superior for nearly all radiographic techniques and, until the 1970s, was used almost exclusively as the phosphor.

Since then, rare Earth screens have been used in diagnostic radiology. These screens are faster than those made of calcium tungstate, rendering them more useful for most types of radiographic imaging. Use of rare Earth screens results in a lower patient dose, less thermal stress on the x-ray tube, and reduced shielding for x-ray rooms.

Reflective Layer

Between the phosphor and the base is a reflective layer, approximately 25 µm thick, that is made of a shiny substance such as magnesium oxide or titanium dioxide (Figure 12-11). When x-rays interact with the phosphor, light is emitted isotropically.

Less than half of this light is emitted in the direction of the film. The reflective layer intercepts light headed in other directions and redirects it to the film. The reflective layer enhances the efficiency of the radiographic intensifying screen, nearly doubling the number of light photons that reach the film.

Base

The layer farthest from the radiographic film is the base. The base is approximately 1 mm thick and serves principally as a mechanical support for the active phosphor layer. Polyester is the popular base material in radiographic intensifying screens, just as it is for radiographic film.

Screen Speed

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 50 (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. 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 Table 12-4.

The speed of a radiographic intensifying screen conveys no information regarding patient dose. This information is related by the IF. The IF is defined as the ratio of the exposure required to produce the same OD with a screen to the exposure required to produce an OD without a screen.

The OD chosen for comparison of one radiographic intensifying screen versus another is usually 1.0. The value of the IF can be used to determine the dose reduction accompanying the use of a screen.

Several factors influence radiographic intensifying screen speed; some of these are controlled by the radiologic technologist. Ultimately, the screen speed is determined by the relative number of x-rays that interact with the phosphor and how efficiently x-ray energy is converted into the visible light that interacts with the film.

Box 12-2 gives the properties of radiographic intensifying screens that affect screen speed and cannot be controlled by the radiologic technologist. They are listed in their relative order of importance.

Several conditions that affect radiographic intensifying screen speed are controlled by the radiologic technologist. These include radiation quality, image processing, and temperature.

Image Noise

Image noise appears on a radiograph as a speckled background. It occurs most often when fast screens and high-kVp techniques are used. Noise reduces image contrast.

Rare Earth radiographic intensifying screens have increased speed because of two important characteristics, both of which are higher compared with other types of screens. The percentage of x-rays absorbed by the screen is higher. This is detective quantum efficiency (DQE). The amount of light emitted for each x-ray absorbed also is higher. This is conversion efficiency (CE).

Figure 12-14 illustrates why an increase in CE increases image noise but an increase in DQE does not. In Figure 12-14, A, a calcium tungstate screen has a DQE of 20% and a CE of 5%. A radiographic technique of 10 mAs results in 1000 x-rays incident on the screen, 200 of which are absorbed, resulting in light photons equivalent to 10 x-rays. We could say that this system has a speed of 100.

If phosphor thickness is doubled as in Figure 12-14, B, the DQE increases to 40%, so the mAs can be reduced to 5 mAs. The speed is now 200, but there is no increase in noise because the same number of x-rays is absorbed.

However, if the phosphor is changed to one with a CE of 10%, the speed is doubled at the expense of increased noise (Figure 12-14, C,). A 200-speed screen is attained because twice as much light is emitted per x-ray absorption. Only half as many x-rays are required, and this results in increased quantum mottle, a principal component of image noise.

Spatial Resolution

Radiographers often use the term image detail or visibility of detail when describing image quality. These qualitative terms combine the quantitative measures of spatial resolution and contrast resolution. Spatial resolution refers to how small an object can be imaged. Contrast resolution refers to the ability to image similar tissues, such as the liver and pancreas or gray matter and white matter.

The use of radiographic intensifying screens adds one more step to the process of imaging with x-rays. Radiographic intensifying screens have the disadvantage of lower spatial resolution compared with direct-exposure radiographs.

Spatial resolution is measured in a number of ways and can be assigned a numeric value. Spatial resolution in screen-film radiography is limited principally by effective focal spot size. For our purposes, a general description should be sufficient.

A photograph in focus shows good spatial resolution; one that is out of focus shows poor spatial resolution and therefore much image blur. Figure 12-15 shows the differences in spatial resolution between a direct-exposure film and a par-speed screen-film combination obtained when an x-ray test pattern is imaged.

Such a test pattern is called a line-pair test pattern. It consists of lead lines separated by interspaces of equal size. Spatial resolution is expressed by the number of line pairs per millimeter (lp/mm) that are imaged. The higher this number, the smaller is the object that can be imaged and the better is the spatial resolution.

Very fast screens can resolve 7 lp/mm, and fine-detail screens can resolve 15 lp/mm (see Table 12-4). Direct-exposure film can resolve 50 lp/mm. The unaided eye can resolve about 10 lp/mm.

When x-rays interact with the screen’s phosphor, the area of the film emulsion that is activated by the emitted light is larger than it would be with direct x-ray exposure. This situation results in reduced spatial resolution or increased image blur.

High-speed screens have low spatial resolution, and fine-detail screens have high spatial resolution. Spatial resolution improves with smaller phosphor crystals and thinner phosphor layers. Figure 12-16 shows how these factors affect image resolution. Unfortunately, these factors are not controlled by the radiologic technologist.

In both parts of Figure 12-16, the x-ray is shown to interact with the phosphor soon after entry; this results in screen blur. Screen blur is reduced in thinner screens.

In mammography, spatial resolution is improved by placing the single-emulsion film on the tube side of the cassette.

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.

Carbon Fiber

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 (Z_C = 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 couches 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.

Screen-Film Radiographic Exposure

From its introduction in 1896 by Thomas Edison until the 1970s, calcium tungstate (CaWO₄) was used almost exclusively as the phosphor for radiographic intensifying screens. Such screens, however, exhibit only 5% CE.

One reason why calcium tungstate is a useful screen phosphor is that it emits light in the violet-to-blue region. The sensitivity of conventional radiographic film is highest in the violet-to-blue region of the spectrum. Consequently, the light emitted by calcium tungstate screens is readily absorbed in radiographic film (Figure 12-18).

If the screen phosphor emitted green or red light, its IF would be greatly reduced because it would require a greater number of light photons to produce a latent image. The light of the screen emission would be mismatched to the light sensitivity of the film.

Rare Earth Screens

Newer phosphor materials have become the material of choice for most radiographic applications. Table 12-5 lists these phosphors and the general identification of screens into which they have been incorporated. Except for barium- and zinc-based phosphors, the other new phosphors are identified as rare Earth; therefore, all of these screens have come to be known as rare Earth screens.

The term rare Earth describes those elements of group IIIa in the periodic table (see Figure 2-4) that have atomic numbers of 57 to 71. These elements are transitional metals that are scarce in nature. Those used in rare Earth screens are principally gadolinium, lanthanum, and yttrium. The compositions of the four principal rare Earth phosphors are terbium-activated gadolinium oxysulfide (Gd₂O₂S: Tb), terbium-activated lanthanum oxysulfide (La₂O₂S: Tb), terbium-activated yttrium oxysulfide (Y₂O₂S: Tb), and lanthanum oxybromide (LaOBr).

Rare Earth radiographic intensifying screens are manufactured to perform at several speed levels, up to 1200. This increase in speed is attained without loss of spatial or contrast resolution; however, with the fastest rare Earth screens, the effects of quantum mottle (image noise) are noticeable and can become bothersome.

Rare Earth radiographic intensifying screens obtain their increased sensitivity through higher x-ray absorption (DQE) and more efficient conversion of x-ray energy into light (CE). The light emitted by these screens, however, differs from that emitted by other screens; therefore, rare Earth screens require specially matched film.

• Wear a proper mask that reduces inhalation of fumes—not the standard surgical mask that only guards against particles and bugs.

• Wear nitrile gloves. Do not use surgical gloves; they only protect against biologic matter. Remember that photographic chemicals are designed to penetrate, and thin rubber gloves provide no guarantee of safety.

• Wear protective glasses. Chemical splashes in the eyes are painful.

Wetting

For these chemicals to penetrate the emulsion, the radiograph must first be treated by a wetting agent. The wetting agent is water, and it penetrates the gelatin of the emulsion, causing it to swell. In automatic processing, the wetting agent is in the developer.

Development

The principal action of the developer is to change the silver ions of exposed crystals into metallic silver. The developer provides electrons to the sensitivity center of the crystal to change the silver ions to silver.

In addition to the solvent, the developer contains a number of other ingredients. The composition of the developer and the function of each ingredient are outlined in Table 12-8.

For the ionic silver to be changed to metallic silver, an electron must be supplied to the silver ion. Chemically, the reaction is described as follows:

When an electron is given up by a chemical, in this case the developer, to neutralize a positive ion, the process is called reduction. The silver ion is said to be reduced to metallic silver, and the chemical responsible for this is called a reducing agent.

The opposite of reduction is oxidation, a reaction that produces an electron. Oxidation and reduction occur simultaneously and are called redox reactions. To help recall the proper association, think of EUR/OPE: electrons used in reduction/oxidation produce electrons.

The principal component of the developer is hydroquinone. Secondary constituents are Phenidone and Metol.

Usually, hydroquinone and Phenidone are combined for rapid processing. As reducing agents, each of these molecules has an abundance of electrons that can be easily released to reduce silver ions.

The optical density of a processed radiograph results from the development of crystals that contain a latent image (Figure 12-27).

The characteristic curve of a radiograph is shaped by the synergistic action of developing agents. Hydroquinone acts rather slowly but is responsible for the very blackest shades. Phenidone acts rapidly and influences the lighter shades of gray. Phenidone controls the toe of the characteristic curve, and hydroquinone controls the shoulder (Figure 12-28).

An unexposed silver halide crystal has a negative electrostatic charge distributed over its entire surface. An exposed silver halide crystal, on the other hand, has a negative electrostatic charge distributed over its surface except at the sensitivity center. The similar electrostatic charges on the developing agent and the silver halide crystal make it difficult for the developing agent to penetrate the crystal surface except in the region of the sensitivity center in an exposed crystal.

In such an exposed crystal, the developing agent penetrates the crystal through the sensitivity center and reduces the remaining silver ions to atomic silver. The sensitivity center can be considered a metallic conducting electrode through which electrons are transferred from the developing agent into the crystal. Development of exposed and unexposed crystals results in the types of differences illustrated in Figure 12-29.

The reduction of a silver ion is accompanied by the liberation of a bromide ion. The bromide ion migrates through the remnant of the crystal into the gelatin portion of the emulsion. From there, the ion is dissolved into the developer and is removed from the film.

The developer contains alkali compounds, such as sodium carbonate and sodium hydroxide. These buffering agents enhance the action of the developing agent by controlling the concentration of hydrogen ions: the pH.

These alkali compounds are caustic, that is, they are very corrosive and can cause a skin burn. Sodium hydroxide, the strongest alkali, is commonly called lye. Be very cautious if you mix a developer solution that contains sodium hydroxide. You should wear rubber gloves and, of course, never let it get near your mouth or eyes.

Potassium bromide and potassium iodide are added to the developer as restrainers. Restrainers restrict the action of the developing agent to only those silver halide crystals that have been irradiated. Without the restrainer, even those crystals that have not been exposed are reduced to metallic silver. This results in an increased fog that is called development fog.

A preservative is also included in the developer to control the oxidation of the developing agent by air. Air is introduced into the chemistry when it is mixed, handled, and stored; such oxidation is called aerial oxidation. By controlling aerial oxidation, the preservative helps maintain the proper development rate.

Mixed chemicals last only a couple of weeks; thus, replenishment tanks require close-fitting floating lids for the control of aerial oxidation. Hydroquinone is particularly sensitive to aerial oxidation. It is easy to tell when the developing agent has been oxidized because it turns brownish. The addition of a preservative, usually sodium sulfite, causes the developer to remain clear.

Developers used in automatic processors contain a hardener, usually glutaraldehyde. If the emulsion swells too much or becomes too soft, the film will not be transported properly through the system because of the very close tolerances of the transport system.

The hardener controls swelling and softening of the emulsion. When films that drop from the processor are damp, the usual cause is depletion of the hardener.

The developer may contain metal impurities and soluble salts. Such impurities can accelerate the oxidation of hydroquinone, rendering the developer unstable. Chelates are introduced as sequestering agents that form stable complexes with these metallic ions and salts.

With proper development, all exposed crystals that contain a latent image are reduced to metallic silver, and unexposed crystals are unaffected. The development process, however, is not perfect: Some crystals that contain a latent image remain undeveloped (unreduced), but other crystals that are unexposed may be developed. Both of these actions reduce the quality of the radiograph.

Film development is basically a chemical reaction. Similar to all chemical reactions, it is governed by three physical characteristics: time, temperature, and concentration (of the developer). Long development time increases reduction of the silver in each grain and promotes the development of the total number of grains. High developer temperature has the same effect.

Similarly, silver reduction is controlled by the concentrations of developing chemicals. With increased developer concentrations, the reducing agent becomes more powerful and can more readily penetrate both exposed and unexposed silver halide crystals.

Manufacturers of x-ray film and of developing chemicals have very carefully determined the optimal conditions of time, temperature, and concentration for proper development. Optimal conditions of contrast, speed, and fog can be expected if the manufacturer’s recommendations for development are followed.

The image on a fogged film is gray and lacks proper contrast. The causes of fog are many, but perhaps the most important are those just mentioned—time, temperature, and developer concentration. An increase in any of these factors beyond manufacturer recommendations results in increased development fog.

Fog also can be produced by chemical contamination of the developer (chemical fog), unintentional exposure to radiation (radiation fog), or improper storage at an elevated temperature and humidity.

Fixing

When development is complete, the film must be treated so that the image will not fade. This stage of processing is fixing. The image is said to be fixed on the film, and this produces film of archival quality.

When the film is removed from the developer, some developer is trapped in the emulsion and continues its reducing action. If developing is not stopped, development fog results. As discussed earlier, the step in manual processing that follows development is called stop bath, and its function is just that—to neutralize the residual developer in the emulsion and stop its action. The chemical used in the stop bath is acetic acid.

In automatic processing, a stop bath is not used because the rollers of the transport system squeeze the film clean. Furthermore, the fixer contains acetic acid that behaves as a stop bath. This acetic acid, however, is called an activator. An activator neutralizes the pH of the emulsion and stops developer action. Table 12-9 lists the chemical components of the fixer.

The terms clearing agent, hypo, and thiosulfate often are used interchangeably in reference to the fixing agent. Fixing agents remove unexposed and undeveloped silver halide crystals from the emulsion. Sodium thiosulfate is the agent classically known as hypo, but ammonium thiosulfate is the fixing agent that is used in most fixer chemistries.

Hypo retention is the term used to describe the undesirable retention of the fixer in the emulsion. Excess hypo slowly oxidizes and causes the image to discolor to brown over a long time. Fixing agents retained in the emulsion combine with silver to form silver sulfide, which appears yellow-brown.

The fixer also contains a chemical called a hardener. As the developed and unreduced silver bromide is removed from the emulsion during fixation, the emulsion shrinks. The hardener accelerates this shrinking process and causes the emulsion to become more rigid or hardened.

The purpose of hardeners is to ensure that the film is transported properly through the wash-and-dry section and that rapid and complete drying occurs. The chemicals commonly used as hardeners are potassium alum, aluminum chloride, and chromium alum. Normally, only one is used in a given formulation.

The fixer also contains a preservative that is of the same composition and that serves the same purpose as the preservative in the developer. The preservative is sodium sulfite, and it is needed to maintain the chemical balance because of the carryover of developer and fixer from one tank to another.

The alkalinity and acidity—the pH—of the fixer must remain constant. This is helped by adding a buffer, usually acetate, to the fixer.

In the same way that metallic ions are sequestered in the developer, so must they be sequestered in the fixer. Aluminum ions represent the principal impurity at this stage. Boric acids and boric salts are used for sequestering.

Finally, the fixer contains water as the solvent. Other chemicals might be applicable as a solvent, but they are thicker and are more likely to gum up the transport mechanism of the automatic processor.

Washing

The next stage in processing is to wash away any residual chemicals remaining in the emulsion, particularly hypo that clings to the surface of the film. Water is used as the wash agent. In automatic processing, the temperature of the wash water should be maintained at approximately 3°C (5°F) below the developer temperature.

In this way, the wash bath also serves to stabilize developer temperature. Inadequate washing leads to excessive hypo retention and the production of an image that will fade, turn brown with time, and be of generally poor archival quality.

Drying

For the final step in processing, drying the radiograph, warm dry air is blown over both surfaces of the film as it is transported through the drying chamber.

The total sequence of events involved in manual processing takes longer than 1 hour to be completed. Most automatic processors are 90-second processors and require a total time from start to finish—the dry-to-drop time—of just that, 90 seconds.

The process of converting the latent image to a visible image can be summarized as a three-step process within the emulsion (Figure 12-30). First, the latent image is formed by x-ray exposure of silver halide grains. Next, the exposed grains and only the exposed grains are made visible by development. Finally, fixing removes the unexposed grains from the emulsion and makes the image permanent.

Transport System

The transport system begins at the feed tray, where the film to be processed is inserted into the automatic processor in the darkroom. There, entrance rollers grip the film to begin its trip through the processor. A microswitch is engaged to control the replenishment rate of the processing chemicals.

Always feed the film evenly using the side rails of the feed tray and alternate sides from film to film (Figure 12-32). This ensures even wear of the transport system components. From the entrance rollers, the film is transported by rollers and racks through the wet chemistry tanks and the drying chamber and is finally deposited in the receiving bin.

The transport system not only transports the film; it also controls processing by controlling the time the film is immersed in each wet chemical. Timing for each step in processing is governed by careful control of the rate of film movement through each stage. The transport system consists of the following three principal subsystems: rollers, transport racks, and drive motor.

Three types of rollers are used in the transport system. Transport rollers, with a diameter of 1 inch, convey the film along its path. They are positioned opposite one another in pairs or are offset from one another (Figure 12-33).

A master roller, with a diameter of 3 inches, is used when the film makes a turn in the processor (Figure 12-34). A number of planetary rollers and metal or plastic guide shoes are usually positioned around the master roller.

Except for the entering rollers at the feed tray, most of the rollers in the transport system are positioned on a rack assembly (Figure 12-35). These racks are easily removable and provide for convenient maintenance and efficient cleaning of the processor.

When the film is transported in one direction along the rack assembly, only 25-mm (1-inch) rollers are required to guide and propel it. At each bend, however, a curved metal lip with smooth grooves guides the film around the bend. These are called guide shoes. For a 180-degree bend, the film is positioned for the turn by the leading guide shoe, is propelled around the curve by the master roller and its planetary rollers, and leaves the curve by entering the next straight run of rollers through the trailing guide shoe.

When the film exits the top of the rack assembly, it is guided to the adjacent rack assembly through a crossover rack. The crossover rack is a smaller rack assembly that is composed of rollers and guide shoes.

Power for the transport system is provided by a fractional horsepower drive motor. The shaft of the drive motor is usually reduced to 10 to 20 rpm through a gear reduction assembly.

Temperature Control System

The developer, fixer, and wash require precise temperature control. The developer temperature is most critical, and it is usually maintained at 35°C (95°F). Wash water is maintained at 3°C (5°F) lower. Temperature is monitored at each stage by a thermocouple or thermistor and is controlled thermostatically by a controlled heating element in each tank.

Circulation System

Agitation is necessary to continually mix the processing chemicals, maintain a constant temperature throughout the processing tank, and aid exposure of the emulsion to the chemicals. In automatic processing, a circulation system continuously pumps the developer and the fixer, thus maintaining constant agitation within each tank.

The developer circulation system requires a filter that traps particles as small as approximately 100 µm to trap flecks of gelatin that are dislodged from the emulsion. The particles thus have less chance of becoming attached to the rollers, where they can produce artifacts. These filters are not 100% efficient; therefore, sludge can build up on the rollers.

Filtration in the fixer circulation system is normally unnecessary because the fixer hardens and shrinks the gelatin so that the rollers are not coated. Furthermore, the fixer neutralizes the developer; therefore, the products of this reaction do not affect the final radiograph.

Water must be circulated through the wash tank to remove all of the processing chemicals from the surface of the film before drying; this ensures archival quality. An open system, rather than a closed circulation system, usually is used. Fresh tap water is piped into the tank at the bottom and overflows out the top, where it is collected and discharged directly to the sewer system. The minimum flow rate for the wash tank in most processors is 12 L/min (3 gal/min).

Replenishment System

Each time a film makes its way through the processor, it uses some of the processing chemicals. Some developer is absorbed into the emulsion and then is neutralized during fixing. The fixer, likewise, is absorbed during that stage of processing, and some is carried over into the wash tank.

The replenishment system meters the proper quantities of chemicals into each tank to maintain volume and chemical activity. Although replenishment of the developer is more important, the fixer also has to be replenished. Wash water is not recirculated and therefore is continuously and completely replenished.

When a film is inserted onto the feed tray with its widest dimension gripped by the leading rollers and its narrow side against the side rail, a microswitch is activated and turns on the replenishment for as long as film travels through the microswitch. Replenishment rates are approximately 60 to 70 mL of developer and 100 to 110 mL of fixer for every 35 cm (14 in) of film.

Dryer System

A wet or damp finished radiograph easily picks up dust particles that can result in artifacts. Furthermore, a wet or damp film is difficult to handle in a viewbox. When stored, it can become sticky and may be destroyed.

The dryer system consists of a blower, ventilation ducts, drying tubes, and an exhaust system. The dryer system extracts all residual moisture from the processed radiograph, so it drops into the receiving bin dry.

The blower is a fan that sucks in room air and blows it across heating coils through ductwork to the drying tubes. Therefore, room air should be low in humidity and free of dust. Sometimes as many as three heating coils of approximately 2500 W capacity are used. The temperature of the air entering the drying chamber is thermostatically regulated.

The hot, moist air is vented from the drying chamber to the outside, in much the same way as the air in a clothes dryer is vented. Some fraction of the exhaust air may be recirculated within the dryer system.

When damp films drop into the receiving bin, the radiologic technologist should immediately suspect a malfunction of the dryer system, although developer and fixer replenishment also should be checked. Underreplenishment reduces the concentration of hardener and is a common cause of damp films.

• Contrast. High-contrast film produces black-and-white images. Low-contrast film produces images with shades of gray.

• Latitude. Latitude is the range of exposure techniques (kVp and mAs) that produce an acceptable image.

• Speed. Speed is the sensitivity of the screen-film combination to x-rays and light. Fast screen-film combinations need fewer x-rays to produce a diagnostic image.

• Crossover. When light is emitted from a radiographic intensifying screen, it exposes not only the adjacent film emulsion but also the emulsion on the other side of the base. The light crosses over the base and blurs the radiographic image.

• Spectral matching. The x-ray beam does not directly expose the x-ray film. Radiographic intensifying screens emit light when exposed to x-rays and the emitted light then exposes the radiographic film. The color of light emitted must match the response of the film.

• Reciprocity law. When exposed to the light of radiographic intensifying screens, radiographic film speed is less if the exposure time is very short or very long.

Challenge Questions

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