During which step of the processing procedure are the exposed silver halide crystals reduced to metallic silver?

chapter 7 Film Processing

THE DARKROOM

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.

FILM-PROCESSING SOLUTIONS

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).