Films, cassettes, intensifying screens and processing

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Chapter 12 Films, cassettes, intensifying screens and processing

Brian Channon

KEY POINTS

Radiographic film is composed of a film base with an active emulsion layer adhered using a subbing layer, with a protective super-coat.

A cassette must be robust and light-tight.

Intensifying screens convert the X-rays received into light, which forms the image on the film. This reduces the exposure needed to provide a diagnostic image.

The latent image is converted into a permanent visible image through the use of developer and fixer.

The chemicals used in processing should be replenished and replaced on a regular basis.

Maintenance and quality assurance are essential to maintain high quality diagnostic images.

INTRODUCTION

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.

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).

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.

CONSTRUCTION OF INTENSIFYING SCREENS

The detailed construction of an intensifying screen can vary widely; it is, however, closely related to its planned use in clinical practice and comprises a number of discrete layers (Fig. 12.7).

Figure 12.7 Construction of an intensifying screen.

Supercoat

A thin, transparent waterproof layer of cellulose acetobiturate extends around the sides (as an edge seal) and the back of the screen, encasing it completely. It forms an effective seal against a variety of fluids, any of which could seriously damage the screens. It also provides a limited degree of physical protection to the delicate phosphor layer. Light from the phosphor layer is able pass through this thin transparent layer easily, with minimal distortion, thus helping to minimise unsharpness. It is easy to clean and a poor generator of static electricity.

Phosphor layer

This layer consists of the phosphor crystals; suspended in a transparent binding material such as polyurethane. The nature of the phosphor crystals will vary depending upon the planned spectral emission of the intensifying screen. The speed of the intensifying screen will depend upon both the type of phosphor crystal used and the density to which they are packed. Within high definition intensifying screens the use of coloured pigment or carbon granules in the binder material tends to absorb laterally scattered light within the phosphor layer. This minimises photographic unsharpness but requires an increase in radiation exposure to the patient.

Substratum

This forms the bonding layer between the base and the phosphor layer. It will vary depending on the intended use for the screens in that it may be absorptive, reflective or just transparent in nature.

When absorptive in nature a coloured dye is added to the substratum, which tends to prevent light that is travelling away from the film emulsion layer from being reflected at the base–phosphor interface back onto the film. This aids in the reduction of photographic unsharpness and is found in some high-definition type (slow) intensifying screens.

The reflective function is used in some high speed (faster) intensifying screens. Titanium dioxide or a similar white pigment is incorporated into the substratum. This acts to reflect light that is travelling away from the film emulsion layer back towards the film emulsion, reducing the radiation dose received by the patient but increasing photographic unsharpness.

Base

Frequently made of polyester, the base acts as a smooth and strong, yet flexible, support for the phosphor layer. It must be uniformly radioparent, chemically inert, moisture resistant (enhanced by coating) and ideally not discolour with age.

PHOSPHOR TYPES

It is essential that the materials used as phosphors are very efficient at absorbing X-ray photons and have high quantum detection efficiency. The phosphors should also be efficient at converting X-ray photons into light and exhibit minimal afterglow.

Modern rare earth phosphors, such as gadolinium oxysulphide, lanthanum oxybromide, yttrium oxysulphide and others, generally have both a higher detection and conversion efficiency than the older mostly blue/violet-light emitting materials such as calcium tungstate or barium lead sulphate. Rare earth phosphors are normally used in conjunction with a small amount of an activator, the combination of which determines both the spectral emission, colour of the emitted light, and its intensity of luminescence. For example a combination of gadolinium oxysulphide with terbium as an activator emits green light whilst lanthanum oxybromide with thulium emits blue light. Rare earth phosphors tend to emit the majority of light produced at discrete wavelengths and are referred to as ‘line emitters’, whilst older type materials, such as calcium tungstate, emit light continuously between specific wavelengths and hence are ‘broad spectrum emitters’ (Fig. 12.8).

Figure 12.8 Light emissions for a broad spectrum emitter (calcium tungstate screens) and a line emitter (rare earth screens).

MATCHING FILM SPECTRAL SENSITIVITY AND SPECTRAL EMISSION

It is essential that the film’s sensitivity is matched directly to the spectral emission of the intensifying screens in order to achieve maximum film blackening from a given radiation exposure to the patient (Fig. 12.9). When an orthochromatic-type film is used with intensifying screens emitting green light it can be seen that the majority of the light emitted by the intensifying screens lies within the spectral sensitivity curve of the film. This ensures optimal performance of the system. However, if a monochromatic-type film is used with the same intensifying screens, the majority of the light emitted by the screens lies outside the film’s spectral sensitivity curve. Therefore, most of the emitted light will have minimal impact on the silver halide crystals within the film’s emulsion.

Figure 12.9 Diagram to show emission and absorption spectrums for rare earth and calcium tungstate intensifying screens, demonstrating the need to match light emission colour with film sensitivity.

TYPES OF INTENSIFYING SCREEN

Variations in the construction of the intensifying screen will produce screens with different characteristics for specific use.

• High-speed screens are used to image large or dense body areas or in circumstances where there is a risk of voluntary or involuntary patient movement. Increased speed is achieved by increasing the thickness, within limits, of the phosphor layer, by including a reflective layer and by increasing the size of the phosphor crystals. Therefore, for a given exposure greater film blackening occurs but image resolution is reduced.

• High definition or detail screens are most commonly utilised for extremity imaging when a high tube loading is not necessary and fine detail is a prerequisite.

Enhanced image resolution may be achieved by the use of absorptive material within the substratum and inclusion of coloured dyes within the binding material of the phosphor layer. Except in special circumstances, intensifying screens are paired facilitating the use of duplitised film emulsions (see p. 136). In such circumstances the back intensifying screen will receive slightly fewer X-ray photons than the front screen due to absorption within both the front screen and the film itself. Thus the two images may be of a slightly different density. Manufacturers may either choose to ignore this and produce a pair of screens of identical speed. Alternatively they may opt to increase the speed of the back screen by the use of a reflective layer or greater coating weight or use a pigment to reduce the speed of the front screen.

RADIOGRAPHY WITH A SINGLE INTENSIFYING SCREEN

Mammography is the exception to the general norm that screens are always used in pairs. In this case a single-sided film emulsion is used in conjunction with a single intensifying screen with the prime aim being to reduce photographic unsharpness. The normal arrangement is for the intensifying screen to be positioned behind the film. This helps to reduce photographic unsharpness, because the light emission from the phosphor crystals has a shorter distance to travel prior to interacting with the silver halides within the film emulsion.

INTENSIFYING SCREENS AND IMAGE RESOLUTION

The use of screens plays a significant role in reducing the radiation dose received by patients. The following features of their construction make a significant contribution to the final image resolution:

• Presence of a reflective layer as part of the substratum degrades resolution.

• Presence of an absorptive layer as part of the substratum enhances resolution.

• Presence of carbon granules or coloured dye within the binder enhances resolution.

• Crossover effect. When light arising from one intensifying screen interacts with silver halides in an emulsion layer remote from it there is enhanced potential for spread due to the longer track taken.

• Size of crystals (larger crystals produce more light).

• Thickness of the phosphor layer. The potential spread of light can be greater when there is an increased distance between the phosphor and emulsion layers. (Obviously there is an overall limit to the thickness of emulsion.)

As the speed of the system increases then in general terms resolution is reduced. However, as with many aspects of imaging, a balance has to be struck between the competing considerations and the need to produce an image that is of a diagnostic quality.

QUANTUM MOTTLE

This arises when a very fast imaging system is utilised and a relatively small radiographic exposure is required. It occurs when a relatively small number of X-ray photons in the beam transmitted by the object are available to interact with the phosphor crystals. Thus the number of light photons available to interact with silver halides declines and the resultant image may be grainy, mottled or even unrecognisable.

CARE OF INTENSIFYING SCREENS

Intensifying screens are relatively delicate and as such must be handled with care. Most opening/closing occurs within film loaders, reducing the opportunity for damage to screens. If a need to open a cassette arises then this should occur in areas away from dust or liquid, which may cause contamination.

Internal cleaning is necessary to remove small artefacts that often appear as small white spots on the subsequent image and have a detrimental effect on image quality. The manufacturer’s instructions relating to the procedure and cleaning agent should be followed. It is essential that the cassette is not reloaded and closed until both screens are completely dry. If a screen becomes damaged then it is irreparable and both screens within the cassette will need to be replaced, which is time consuming and expensive.

PROCESSING

The final stage in the production of a hardcopy X-ray image is processing. Automatic processing is often linked to a daylight handling system for the loading and unloading of cassettes.

Whilst passing through the automatic processor the film is subjected to a number of processes during which the latent image is changed into a visible format.

DEVELOPMENT

Development is the initial stage in the processing cycle which converts the latent image into a visible form. This involves a process of electron donation by which the exposed silver halide crystals are reduced to metallic silver whilst the unexposed silver halides remain unchanged. An exposed silver halide crystal possesses a weakness in its negatively charged ion barrier caused by a collection of silver atoms at the crystal’s sensitivity centre. Electrons from the developing agent are able to penetrate the exposed silver halide and convert it into silver. Unfortunately, the developer is not entirely effective at differentiating between exposed and unexposed silver halide grains. The development of unexposed crystals contributes to the overall image density whilst reducing the contrast of the film.

The developer solution has various constituents including:

• Developing agent, which supplies electrons for the process of reduction. It is normally a combination of two specific developing agents – phenidone and hydroquinone – producing a PQ developer. They are used in precise proportions in order to utilise the specific features of each.

• Accelerator provides an alkaline environment, pH range 9.8–11.4, to allow the developer to function effectively. This is achieved by the use of potassium carbonate or potassium hydroxide. The pH of the developer, along with solution temperature, plays a major role in controlling the activity of the developing agent. The activity of developer is greater at higher pH levels.

• Restrainer (anti-fogging), usually benzotriazole, acts to aid the selectivity of the developing agent, helping to prevent conversion of unexposed silver halides. This is achieved by strengthening the negatively charged barrier that surrounds the silver halide. Potassium bromide, an effective restrainer, is produced as a by-product of the development process.

• Water is used as a solvent as it is clean and free from chemical deposits and is the medium in which the other developer constituents are mixed.

• The use of potassium sulphite as a preservative acts to reduce the rate of aerial oxidation of the developing agent and to facilitate the regeneration of phenidone by hydroquinone.

• Bactericides and fungicides act to restrain growth of organisms within the solution.

• Hardeners reduce the chances of damage to the emulsion layer during transportation or of the film becoming stuck in the processor. The hardening agents are usually aldehydes or sulphates. In addition, films are pre-hardened during manufacture.

Undesirable changes to the pH level of developer solution may occur as a result of both aerial oxidation and the acid by-products of the development process. Within modern developers the use of carbonates as accelerators and sulphides as preservatives counteracts the potential effects that could arise from changes in the pH of the solution.

Factors influencing development rate include:

• pH of the solution

• solution temperature – developing agent is more active at higher temperatures

• nature of the developing agent (controlled by the manufacturer)

• development time.

FIXATION

Major functions of fixation include:

• continuing the process of film hardening

• terminating further development

• converting undeveloped silver halides into a soluble silver complex

• making the image permanent.

Constituents of fixer

• Ammonium thiosulphate – the fixing agent is rapid acting and combines with undeveloped silver halides to form a soluble silver compound that then migrates through a process of osmosis into the fixing solution. Fixer therefore becomes rich in silver complexes.

• Acetic acid – ensures development is terminated and provides an appropriate environment, pH 4.0–4.5, in which hardener functions. If the pH of the solution is below 4.0 the fixing agent breaks down.

• Aluminium salts are commonly used as hardeners to reduce the drying time and enhance the hardening effect.

• Water as a solvent.

• Preservative reduces the rate at which the fixing agent decomposes. This is known as sulphurisation.

• Boric acid is used as an anti-sludging agent to reduce the rate at which the aluminium salts may precipitate out of solution.

• Buffers act to control the pH of the solution by neutralising the effects of the alkaline developer solution that is carried over within the film emulsion.

Factors affecting fixation rate include:

• presence of hardeners (slows process)

• high concentration of silver complexes in solution (retards process)

• variation in pH (pH should be constant)

• nature and concentration of fixing agent.

WASHING

This stage is designed to remove both residual fixer chemicals and silver salts from the film emulsion. This process is not 100% effective and residue salts can adversely affect the archival permanence of the resultant image, causing a brown stain. Therefore, the aim of washing is to reduce level of the residual salts to such an extent that they will not create staining during the expected life of the film. Washing is most effective when the film is exposed to a continuous flow or spray of uncontaminated water, as the diffusion of residual salts from the emulsion layer is more effective in clean water.

DRYING

If film completely dries it becomes brittle, so approximately 15% of the dry film is actually moisture. Air is used to evaporate excess moisture from the film. Air of low humidity (dry air) accelerates the process, as does air circulation.

AUTOMATIC PROCESSORS

The basic components within an automatic processor are very similar despite the differences that occur between manufacturers products in relation to design and film capacity (Fig. 12.10).

Figure 12.10 Processor types.

MAIN CONTENTS OF THE PROCESSOR

The processor is essentially a light-tight box containing:

• a series of processing tanks and a roller transport mechanism (Fig. 12.11). (Movement of film is achieved by racks of rollers. These are delicate and may be made of various materials and arranged in a variety of formations)

• an electric motor to ensure that all racks are driven at a constant speed. Plastic or stainless steel guide-plates assist movement at top and bottom of tanks where film changes direction

• crossover assemblies, located between adjacent processing tanks, utilise ‘squeegee’ rollers to remove much surface liquid (Fig. 12.12)

• Arrival of a film into the entry roller system can activate a number of processes,including replenishment of processing fluids, activated by either a microswitch situated above the higher entry roller or by an infra-red detector.

Figure 12.11 A: Roller system from a processor. B: Roller from the bottom of the system to allow return of the film.

Figure 12.12 Crossover assembly.

DEVELOPER SECTION

Temperature control

To maintain image quality temperature must be maintained within 0.5 °C. An immersion heater working in conjunction with a thermostat may achieve this. Alternatively, a heat exchange unit may be used.

Drainage system

This facilitates the emptying of the tank for routine cleaning and maintenance. It is usually a length of plastic tubing which is screwed into a drain hole in the base of the tank. It may also act as an overflow pipe.

Replenishment System

This pumps fresh developer solution into the tank, maintaining the activity and quantity of developer within the tank.

Recirculatory system

This system requires inlet and outlet pipes, an electric pump and possibly a filter to ensure agitation and recirculation of the solution.

FIXER SECTION

The fixer section contains drainage, recirculatory and replenisher systems that are similar in function to those within the developer section (Fig. 12.13). The temperature control will be dependent on the design of the processor and may utilise heat exchange from the surrounding warm developer and wash tanks. In the case of cold-water wash units the temperature control is achieved by using an immersion heater thermostat device, with insulation provided in the dividing wall between the fixer and wash sections. The precise control of fixer temperature is not as critical as it is for developer solution.