Film/screen imaging

Chapter 2 Film/screen imaging



Barry Carver



Introduction


In Western Europe and North America in particular the advance of digital imaging technologies would appear to be irresistible. Indeed, although film/screen technology had been argued to offer some advantages in mammography,1 digital imaging has now been shown to be at least comparable.2 Consequently, film/screen systems are rapidly being replaced by digital technologies; indeed, in the UK, film/screen systems are largely a thing of the past.


This chapter is required for those regions in which this is not yet the case, and in the UK there is still a requirement for the teaching of this technology. It is helpful in order to evaluate digital technologies to have an understanding of the contribution of film/screen technology to medical imaging during the last century.



Imaging plates


The first medical radiographic image receptors were silver halide-coated glass plates, which were placed in light-tight envelopes or cassettes. Junior staff often had the task of waxing the edges of the plates to prevent the emulsion from slipping off!3


Although the value of photographic film was recognised, it was used sparingly prior to the 1920s. Once in regular use, however, the X-ray film soon proved its worth. It was quickly recognised that, unlike the early glass plates, a film could be coated on both sides. This had obvious advantages, particularly when used with intensifying screens. Because only about 1–2% of incident radiation was absorbed by the X-ray film alone, it was soon apparent that this wastefulness could be reduced by using light rather than X-rays to create the latent image on the film.4



Intensifying screens and film emulsion technology


The introduction of fluorescent intensifying screens proved to be a significant development, enabling more of the incident X-rays to be absorbed by the phosphor material and emitted as light. In addition, the use of two intensifying screens meant that double emulsion films could be used, thereby instantly doubling the light absorption. However, the increase in density and contrast was partially counterbalanced by a decrease in resolution, and an increase in quantum noise in faster film/screen combinations. As always in radiography, there is a choice to be made when balancing image quality and patient dose.


During the remainder of the 20th century, film/screen technology continued to develop. Intensifying screens became more efficient when ‘rare earth’ phosphors were introduced in the 1970s, and the familiar globular silver halide crystals in the film emulsion were superseded by the ‘tabular’ variety.


The introduction of asymmetric film screen combinations with anti-crossover features provided greater visualisation with reduced image blur. In more recent years there were further developments in emulsion technology, but the undoubted success of the new digital technologies has mounted a serious challenge to traditional practices.



The X-ray cassette


The cassette is essentially a light-tight protective container for the film and intensifying screens. It is also designed to maintain uniform contact between the film and screens. A foam pressure pad behind the back screen helps to ensure this.


Various cassette materials, such as aluminium and plastic laminate, have been used. However, the ideal low-attenuation material for the cassette front is carbon fibre, as it represents a considerable reduction in patient dose. It is lightweight, durable, and relatively comfortable for the patient, but rather more expensive than other materials. The cassette back is lined with lead foil to reduce scattered radiation. A sliding aperture and lead blocker is incorporated into the design for use with patient identification systems.


Although the film/screen cassette is still relatively commonplace, older cassette types are less familiar sights in a modern imaging department. These include the multisection cassette, the formatter cassette and the photofluorographic cassette.5



Radiographic film


Film technology depends upon certain materials undergoing changes when subjected to electromagnetic radiation such as visible light or X-rays. The main light-sensitive materials used are the halogens, e.g. bromine, iodine or chlorine. In radiographic film these are combined with silver to form, for example, silver bromide or silver idobromide.



Film manufacture


The manufacturing process is extremely stringent, as there must be no variation between batches of film. Solutions of silver nitrate (AgNO3) and potassium bromide (KBr) are added to liquid gelatine. Potassium nitrate, which is soluble, is washed away in the process.6


There are usually four stages in the preparation of the emulsion layer. It is during the latter stages that the characteristics of the film are determined. For example, the speed and contrast of the film depend on the size of the silver halide grains. A high-contrast narrow-latitude film has a narrow range of grain sizes, whereas relatively large grains will produce a film of greater speed. In the final stage various additives are introduced, such as sensitisers, colour sensitisers, hardeners, plasticisers, fungicides, antistatic agents, wetting agents and anti-foggants.5


Impurities such as sulphur are deliberately added during the process in order to create imperfections in the crystal lattice. These imperfections create areas known as electron traps or sensitivity centres. These centres, coupled with excess bromine added to the mix, create the conditions necessary for the formation of the latent image.



Film construction (Fig. 2.1)


So that it can be used as a photographic material, the silver halide needs to be prepared in a form that can be coated on to a support or base.


image

Figure 2.1 Diagrammatic representation of film structure (not to scale).



Base


The material for the base is usually polyester, which has all the necessary characteristics required:



• strong but flexible


• dimensionally stable


• non-flammable


• unaffected by processing chemicals and high temperatures


• impermeable to water


• uniform colour tone and thickness.



Photographic emulsion


The silver halide crystals have to be suspended in a suitable binder to form a photographic emulsion. Gelatin has the properties required to act as a binding agent and suspension medium. It allows the silver halide crystals to grow. Gelatin is transparent and can exist as either a liquid or a solid, thereby allowing the crystals to be suspended evenly within the emulsion. It does not react chemically with the silver, but it allows the processing chemicals to penetrate the emulsion.

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Mar 3, 2016 | Posted by in GENERAL RADIOLOGY | Comments Off on Film/screen imaging

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