• Indirect-action or screen film, so-called because it is used in combination with intensifying screens in a cassette. This type of film is sensitive primarily to light photons, which are emitted by the adjacent intensifying screens. They respond to shorter exposure of X-rays, enabling a lower dose of radiation to be given to the patient.
• The sheet of lead foil contains an embossed pattern so that should the film packet be placed the wrong way round, the pattern will appear on the resultant radiograph. This enables the cause of the resultant pale film to be easily identified (see Ch. 17).
The cross-sectional structure and components of the radiographic film are shown in Fig. 4.3. It comprises four basic components:
• The emulsion on both sides of the base – this consists of silver halide (usually bromide) crystals embedded in a gelatin matrix. The X-ray photons sensitize the silver halide crystals that they strike and these sensitized silver halide crystals are later reduced to visible black metallic silver in the developer (see Ch. 5)
The film has an embossed dot on one corner that is used to help orientation. Its position is marked on the back of the packet or can be felt as a raised dot on the front. The side of the film on which the dot is raised is always placed towards the X-ray beam. When the films are mounted, this raised dot is towards the operator and the films are then arranged anatomically and viewed as if the operator were facing the patient.
Film/screen combinations are used as image detectors whenever possible because of the reduced dose of radiation to the patient (particularly when very fine image detail is not essential). The main uses include:
The relative spectral sensitivity of these four different film emulsions is shown in Fig. 4.4.
Optical density is the term used for describing the degree of film blackening and can be measured directly using a densitometer. In diagnostic radiology the range of optical densities is usually 0.25–2.5. There are no units for optical density.
The characteristic curve is a graph showing the variation in optical density (degree of blackening) with different exposures. Typical characteristic curves for direct-action (non-screen) and indirect-action (screen) film are shown in Fig. 4.5. This curve describes several of the film’s properties.
If the film has been stored correctly (see later), this background fog density should be less than 0.2 (see Fig. 4.5).
This is the exposure required to produce an optical density of 1.0 above background fog (see Fig. 4.6). Thus, the faster the film, the less the exposure required for a given film blackening and the lower the radiation dose to the patient.