What is the name of the fixing agent that removes unexposed silver halide crystals from the emulsion?

chapter 7 Film Processing

Three qualities constitute a good darkroom. A darkroom must be (1) clean, (2) organized, and (3) lightproof. Although individual darkrooms may vary in design, all should possess the same qualities. A darkroom should be separate from the radiographic suite and should be used for only one purpose: processing exposed radiographs. Ideally, the room dimensions should be no less than 6 × 8 feet (2.6 × 2 m), and the layout should reduce the possibility of film damage. Most of the work in the darkroom is performed with minimal illumination. Therefore it is important that the darkroom be organized so that all of the equipment can be located quickly and easily. And, of course, cleanliness is crucial. This is the only room where both the intensifying screens and the x-ray film are exposed to the air. If the countertops are dirty and soiled with chemicals, it is easy for both to be sucked into the cassette as it is opened, possibly causing damage to the intensifying screens.

Another factor that is often overlooked in the darkroom is climate control. Because the film emulsion is extremely sensitive to heat and humidity, good ventilation and temperature control are mandatory. A darkroom should be relatively cool and should have low humidity. The specific temperatures and humidity for proper film care are described in Chapter 6.

There should be essentially two sides to the darkroom: a dry side and a wet side (Fig. 7-1).

Figure 7-1 A sample dark room layout showing a wet side and a dry side.

The dry side of the darkroom is where the cassettes are unloaded and reloaded. A countertop or tabletop large enough to accommodate the largest cassette in the open position should be available. The tabletop should be constructed of a material that allows frequent cleaning, which is necessary to reduce the source of darkroom artifacts that can potentially get on the film. It must be impossible for chemicals to splash into the dry side. At no time should anything “wet” be brought to the dry side. It is customary to store film under the dry table, either in a cupboard or in a film bin, to allow easy access for reloading cassettes (Fig. 7-2). Film hangers for each size of film should be hung above the table on the dry side on an appropriate bracket. Brackets can be purchased commerially or constructed inexpensively using large hooks found at any hardware store.

Film hangers are available in two designs: channel hangers and clip hangers (Fig. 7-3). Channel hangers tend to retain water and chemicals and need special cleaning and drying to prevent contamination of the dry side. Films must also be removed from the channel hangers to be dried. However, clip hangers are more fragile than the channel type. When the clips are used frequently over a period of time, they become weak and lose the ability to “stretch” the film. The clips also puncture the four corners of the film, which, when filed, can scratch other films in the same envelope. It is important to cut off the corners of films processed with clip hangers before filing to prevent this. When more than one film is processed at the same time in the tanks, the clips on the hangers can scratch neighboring films.

Figure 7-3 A clip film hanger (left) versus a channel film hanger (right).

As mentioned earlier, one criterion of a good darkroom is that it be lightproof. Light leaks in a darkroom can cause significant film fog; therefore taking appropriate measures to lightproof the darkroom is imperative. Lightproofing a room is more difficult than may be expected. The first step is locating the light leaks. Small light leaks may not be perceptible until the eyes have acclimated to the dark, and it may be necessary to spend 5 minutes waiting for the eyes to adapt. To achieve a truly lightproof room, a number of tasks may be necessary.

The entrance to the darkroom is a common site for light leaks. A double-door system or revolving door is preferred but not always practical in a veterinary practice (Fig. 7-4). The first step in lightproofing a standard door is to fit it tightly into its frame against strips of felt or rubber molding. Weather stripping is also useful around doors to prevent the entrance of light. Light entering from underneath the door can be prevented by a vapor seal designed specifically for the bottom of a door. A sliding bolt lock or doorknob lock prevents someone from accidentally entering the darkroom at an inopportune time. A suspended ceiling can be a radiographer’s nightmare. It may be necessary to place a large black sheet of plastic above the ceiling tiles to prevent light in adjacent rooms from entering through the seams.

It is a common fallacy that the walls of a darkroom should be “dark.” The opposite is true. The walls of the darkroom should be painted white or cream with a good-quality, washable paint. By painting the walls a light color, more reflection of the safelight is produced, providing a more visible work environment. If the quality and intensity of the light are “safe,” the illumination reflected from any surface also is “safe,” regardless of the color of that surface.

Film processing, whether it is manual or automatic, comprises five basic steps: (1) developing, (2) rinsing or stop bath, (3) fixing, (4) washing, and (5) drying. The first step in learning how to process a film is a basic understanding of the processing solutions. The chemical solutions can be purchased in a number of forms. Powders and liquid concentrates are those most commonly used in veterinary practice. Water is added to the concentrates according to the manufacturer’s instructions to produce the proper amount of solutions for the processing tanks. Preparing the chemicals correctly is important or the resulting solution may adversely affect the radiographic product.

Every effort should be made to keep the chemical solutions at a specified temperature—any variance may adversely affect the radiographic product. At temperatures below those recommended, some of the chemicals may become sluggish in action and may produce an underdeveloped or underfixed radiograph. At temperatures much above those recommended, the chemical activity is too high for manual control.

Keep in mind also that all of the chemical solutions should be the same temperature. If the chemicals vary greatly in temperature, film reticulation can result. Reticulation appears as a mottled density on a finished radiograph and is caused by irregular expansion and contraction of the film emulsion.

A quality assurance program should be established and maintained in the veterinary practice. This program allows reproducibility, and it gives the radiographer confidence in the exposures used on each radiograph (see Chapter 10).