Films, cassettes, intensifying screens and processing

Chapter 12 Films, cassettes, intensifying screens and processing





KEY POINTS










FILMS


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.



FILM MANUFACTURE AND SENSITIVITY





FILM CONSTRUCTION








Single-sided emulsion



Single emulsion


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.




Image resolution and use of films


No radiographic image is truly sharp and all images are to some extent blurred as a result of imperfections within the imaging system itself.







CASSETTES


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.




CONSTRUCTION
















INTENSIFYING SCREENS


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.



Feb 20, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Films, cassettes, intensifying screens and processing
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