Image receptors
Radiographic film
• Direct-action or non-screen film (sometimes referred to as wrapped or packet film). This type of film is sensitive primarily to X-ray photons.
• 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.
Direct-action (non-screen) film
Uses
The film packet contents
The contents of a film packet are shown in Fig. 4.2. It is worth noting that:
• The outer packet or wrapper is made of non-absorbent paper or plastic and is sealed to prevent the ingress of saliva.
• The side of the packet that faces towards the X-ray beam has either a pebbled or a smooth surface and is usually white.
• The reverse side is usually of two colours so there is little chance of the film being placed the wrong way round in the patient’s mouth and different colours represent different film speeds.
• The black paper on either side of the film is there to protect the film from:
• A thin sheet of lead foil is placed behind the film to prevent:
– Some of the residual radiation that has passed through the film from continuing on into the patient’s tissues
– Scattered secondary radiation, from X-ray photon interactions within the tissues beyond the film, scattering back on to the film and degrading the image.
• 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 radiographic film
The cross-sectional structure and components of the radiographic film are shown in Fig. 4.3. It comprises four basic components:
• A plastic base, made of clear, transparent cellulose acetate – acts as a support for the emulsion but does not contribute to the final image
• A thin layer of adhesive – fixes the emulsion to the base
• 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)
• A protective layer of clear gelatin to shield the emulsion from mechanical damage.
Indirect-action film
Uses
Indirect-action film construction
• The silver halide emulsion is designed to be sensitive primarily to light rather than X-rays.
• Different emulsions are manufactured which are sensitive to the different colours of light emitted by different types of intensifying screens (see later). These include:
The relative spectral sensitivity of these four different film emulsions is shown in Fig. 4.4.
Characteristics of radiographic film
Characteristic curve
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.
Film speed
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.
