Chapter 12 Films, cassettes, intensifying screens and processing
Within the imaging department the use of automatic wet film processors to produce a visible image on a conventional film, which is exposed to light originating from intensifying screens held within a cassette, is declining. However, films exposed to a combination of light and X-radiation, using screen film, or directly to X-radiation alone using direct-exposure film, may still be encountered.
The production of an X-ray image depends upon the existence of materials that are unstable and, when exposed to light or electromagnetic radiation, change their nature. Halogens such as bromine or iodine are combined with silver to produce silver bromide or silver idobromide.
It is essential that film manufacture is stringent and that films of the same type produced in different batches are identical. There are several stages in the formation of emulsion during which the grain size distribution and therefore the contrast and speed characteristics of film are determined. Initially silver nitrate and potassium bromide are added to a gelatin solution. Impurities are then added to create imperfections, known as electron traps or sensitivity centres, within the silver halide crystal lattice. In the latter stages of the process sensitisers that increase responses to specific colours of light or radiation and other agents, such as hardeners, bactericides, fungicides anti-foggants and wetting agents, are added.
The spectral sensitivity of a specific emulsion is the range of wavelengths of the electromagnetic spectrum to which it will respond. Silver bromide crystals are inherently sensitive to the electromagnetic spectrum up to and including blue light, with other colours having a minimal impact.
During the manufacturing process the inherent sensitivity of the emulsion can be extended to other wavelengths by adding a suitable dye, usually to the surface of the crystal. The spectral sensitivity of the film emulsion can be arranged to fall into one of three categories: monochromatic, orthochromatic and panchromatic.
It is essential that the colour of spectral sensitivity of the emulsion and the colour of spectral emission of the intensifying screen be matched in order to obtain maximum film blackening for the minimum exposure.
The majority of screen-type film is ‘duplitised’. This type of film has two sensitive emulsion layers – one on each side of base (Fig. 12.1). It is used for most general applications. However duplitised emulsions are also used for intra-oral dental film, although in this instance the film is exposed directly to X-radiation alone.
This is a thin, strong adhesive layer that binds the base to emulsion. It plays a vital role in ensuring that these do not separate whilst processing, as the emulsion layer absorbs warm chemicals and swells. This layer is usually a mixture of the film base solvent and gelatin. A coloured dye may be included within this layer to reduce the amount of light transmitted from one emulsion layer to the other, reducing the crossover effect.
This is a suspension of light/radiation-sensitive silver halides suspended within a gelatin binder. The use of tabular (flat-shaped) silver halide crystals with a larger surface area–volume ratio, provides significant advantages including:
This is a very thin coating of hardened gelatin. It protects the sensitive emulsion layer against mechanical damage that can arise from handling and transport within manual and automatic film loaders and processors. However, two issues arise:
Single-sided film, with one emulsion layer, may be used when a single intensifying screen is used; for example in mammography where high resolution is imperative and in instances when an image of a light source (laser source, photofluorographic) is required. All films consist of a number of discrete layers.
This is similar in construction to duplitised film; however, the second emulsion layer is replaced with an anti-curl/halo backing (Fig. 12.2). Curl may occur during processing as the emulsion layer absorbs processing chemicals and water and expands to a certain degree. To avoid this a layer of gelatin of identical thickness to the emulsion layer is applied to the non-emulsion aspect of the film. During processing this will expand to the same degree as the emulsion, ensuring that the dry film will lie flat. In single-sided emulsions light can be reflected at the base–air interface, back towards the sensitive emulsion layer, thus creating a halo effect (Fig. 12.3).
To minimise the halo effect a coloured dye is incorporated within the gelatin of the anti-curl backing. This acts as a colour filter and absorbs light of specific wavelengths, increasing the resolution of the image. The dye colour utilised is always the opposite colour to the exposing light source; for example yellow dye to absorb blue light. The anti-halation dye is bleached out in the fixer during the processing cycle. Processors that process large numbers of single-sided films require a higher fixer replenishment rate than those that primarily process duplitised films, as the removal of anti-halation dye utilises more fixer energy.
The use of ‘duplitised’ emulsions results in increased film speed and blackening for a given exposure because the amount of emulsion available for exposure enhances sensitivity. This effect is enhanced further when the film is sandwiched between a pair of intensifying screens and provides several potential benefits in that:
This is the sideways scattering of light within the emulsion layer as a consequence of light striking the silver halide crystals (Fig. 12.4). This is a cause of image unsharpness, as the scattered light does not contribute to the primary image.
Halation occurs when an image is formed by light and some of this incident energy passes through the emulsion to the base. On reaching the base–air interface this light either passes out of the film or is reflected back towards the emulsion layer where it creates unsharpness by interacting with silver halide crystals.
Crossover creates an increase in image unsharpness because light that is not completely absorbed in the emulsion layer nearest to source of light passes through the film base and subsequently interacts with silver halide crystals in the opposite emulsion layer, creating a wider and thus less sharp image (Fig. 12.5).
Parallax is unsharpness caused by the separation of the two images recorded on duplitised film. There is an element of spatial separation between these images, and when subsequently viewed this separation creates a small degree of blur. In reality, the distance involved in image separation is very small, thus the blurring effect is negligible.
In radiographic terms a cassette normally houses and provides a physically safe and light-tight environment for both the film and the intensifying screens in which the processes associated with fluorescence and the formation of the latent image can occur (Fig. 12.6). Cassettes are available in various sizes and with detailed differences between specific manufactures.
Intensifying screens operate by converting X-ray energy into light photons. This occurs within the phosphor layer of the intensifying screen where the X-ray photons are absorbed by the phosphor crystals. This causes the crystals to become excited and luminescence occurs. Luminescence is the ability of a material to absorb short wavelength energy (X-radiation) and emit longer wavelength radiation (light). This process facilitates a gain within the imaging procedure as each X-ray photon that is absorbed releases many light photons, thus allowing the radiation dose to the patient to be reduced. In reality, approximately 95% of film blackening is created by light emitted from the phosphor layer and 5% by the direct effect of X-radiation.