What is the use and effect of intensifying screen in radiographic density?

The experimental method is given in detail elsewhere (1). The exposures are made by means of a sensitometer, which is a mechanical device used to produce a series of exposures of increasing time. A piece of film is partially covered with intensifying screens, and the whole is exposed in the sensitometer, all factors being maintained constant during the exposure. After development, two series of densities are thus produced on the film, one resulting from the action of X-rays alone, and the other from the combined action of X-rays and fluorescent light from the intensifying screens. Such a strip of film is shown in Figure 1. Numerical values are obtained for these densities on a densitometer, and the results shown graphically by plotting the density as a function of the common logarithm of the exposure time. This is the usual Hurter and Driffield method of plotting curves used in photographic sensitometry. A pair of such curves is shown in Figure 2.

The intensification factor is defined as the ratio of two exposures required to produce the same density, one exposure being made with the aid of screens and the other without. This quantity is readily derived from the above described curves. Since the curve representing the exposure by X-rays has a different shape from that of the screen exposures, the intensification factor is dependent upon the density for which it is calculated, as described previously (2). For the sake of simplicity, the results which follow are calculated at a density of 1.0 (10 per cent transmission of light through the radiograph), which is assumed to be the average density of a radiograph. It should be recognized that when the very light portions of the radiograph are being considered these values are too high.

The results to follow were obtained with a Coolidge broad focus universal tube, energized by a mechanically rectified machine. A filter of 2 mm. of aluminum was used in all of the exposures. Freshly prepared X-ray developer was used, at a temperature of 18.4° C. (65° F.). All exposures were made on films from the same emulsion, as indicated by the maker's number.

It has been previously determined (3, 4) that with increasing tube potential there is a corresponding increase in the intensification factor, and the measurements shown in Figure 3 confirm this observation.

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.

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

Production of emulsion layer

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.

Finally, as a result of these processes the emulsion layer, a precipitate of silver bromide within gelatine, is produced.

Spectral sensitivity

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.

• Monochromatic emulsions – are blue sensitive (480 nm).

• Orthochromatic emulsions – have an extended sensitivity to include the green aspect of visible spectrum to approximately 620 nm.

• Panchromatic emulsions – have an extended sensitivity to cover all of the visible spectrum (675 nm) and thus must be handled in complete darkness. They are of limited use in diagnostic imaging.

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.

FILM CONSTRUCTION

Duplitised emulsion

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.

Figure 12.1 Cross-sectional diagram of duplitised emulsion.

Film base

This is a thin layer of polyester (polyethylene teraphthalate), which transmits light and provides a support for the other layers.

It is essential that the base should be:

• Thin (0.08–0.18 mm depending on nature of film) – this assists in the reduction of image unsharpness caused by the parallax effect.

• Strong and flexible – to withstand stresses it will receive in film loaders and automatic processors.

• Chemically inert – so it does not affect either processing solution or sensitive emulsion.

• Impermeable to water – to aid in the reduction of processing time (and remains firm to facilitate transportation through automatic processors).

• Uniform thickness – to ensure maximal light transmission.

• Safety-base (non-flammable).

If colour tone is added it should be consistent between batches and not change tone with age. If either were to occur then density and contrast would change.

Substratum or subbing layer

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.

Emulsion layer

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:

• increased speed and sensitivity of film due to a larger surface for interaction with light.

• grains lie in closer proximity, reducing the crossover effect.

Supercoat/anti-abrasive layer

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:

1 It must not be overly smooth as a specific amount of grip or roughness is required for the film to be transported through automatic processors.

2 If overly hard, processing fluid would be unable to penetrate it.

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 ().

Figure 12.2 Cross-sectional diagram of single-sided emulsion.

Figure 12.3 Halation.

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.

Advantages of using duplitised emulsions

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:

• the radiation dose to the patient and the amount of scattered radiation produced is reduced – and reduction in scatter produces a safer working environment for staff

• owing to the decreased exposure, shorter exposure times can be used – providing a possible reduction in patient movement

• reduced exposures facilitate the use of a smaller focal spot size, reducing geometric unsharpness.

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.

Irradiation

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.

Figure 12.4 Irradiation.

Halation

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

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).

Figure 12.5 Crossover in an intensifying screen–film system.

Parallax

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.

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.

Figure 12.6 Cassette.

CONSTRUCTION

• The frame is either synthetic or metal to provide structural strength and support to internal features.

• Internal aspects are blackened to reduce risk of internal light reflections.

• The front recessed plate is made of carbon fibre, plastic or even aluminium to form the cassette well. Attenuation of the X-ray beam should be minimal and even across the plate; thus it is essential that the material used is of even density and thickness.

• The plate is firmly attached to the frame, providing support for the front intensifying screen.

• It may contain a small lead block, which prevents exposure reaching the film in the area designated for patient identification.

• The back is hinged to one side of the frame. This supports the back intensifying screen, which is often mounted on a foam or plastic pressure pad.

• A layer of thin lead foil may be located behind the pressure pad to help reduce backscatter.

• If the cassette can be used with a specific patient identification camera then it will include a recessed and sliding area, which should only open when within the camera device.

• Clips need to be strong and are thus frequently made of metal.

• The hinges are either plastic or metal.

The criteria for an effective cassette include:

• Light in weight, yet robust and durable.

• Rounded corners.

• Provides intimate contact between film and intensifying screen. The use of pressure pads, strong clips and hinges combined with other specific aspects used by individual manufacturers, such as a curved design or magnetic contact, help to achieve this.

• Must be individually identifiable.

• Must clearly indicate the type of intensifying screen contained within it.

CARE OF CASSETTES

Cassettes should be stored upright and away from heat. They should be cleaned and inspected on a regular basis. The outsides should be cleaned, following departmental protocols, after direct patient contact to prevent cross-infection.

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.

Luminescence constitutes two effects:

• Fluorescence – occurs when the light emission commences (when exciting radiation starts) and terminates when exciting radiation stops.

• Phosphorescence (afterglow) – occurs when the light emission continues for more than 10−8 s after the exciting source has been withdrawn.

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