• Chemical processing

• Computer digital processing.

Chemical processing

It is CRUCIAL that the stages involved in chemical processing are performed under controlled, standardized conditions with careful attention to detail. Strict quality assurance procedures must be applied (see Ch. 17). Unfortunately, all too often in dental practice poor chemical processing is the cause of radiographic films being of inadequate diagnostic quality, irrespective of how reliable and expensive the X-ray equipment or how accurate the operator’s radiographic techniques.

Theory

A detailed knowledge of the chemistry involved in processing is not essential. However, a working knowledge and understanding of the theory of processing is necessary so that processing faults can be identified and corrected. A simplified approach to the stages involved in converting the green film emulsion into the black/white/grey radiograph is shown in Fig. 5.1 and outlined below:

Stage 1: Development

The sensitized silver halide crystals in the emulsion are converted to black metallic silver to produce the black/grey parts of the image.

Stage 2: Washing

The film is washed in water to remove residual developer solution.

Stage 3: Fixation

The unsensitized silver halide crystals in the emulsion are removed to reveal the transparent or white parts of the image and the emulsion is hardened.

Stage 4: Washing

The film is washed thoroughly in running water to remove residual fixer solution.

Stage 5: Drying

The resultant black/white/grey radiograph is dried.

Practical methods

There are three practical chemical processing methods available:

• Manual or wet processing

• Automatic processing

• Using self-developing films.

Manual processing

Manual processing is usually carried out in a dark-room, the general requirements of which should include:

• Absolute light-tightness

• Adequate working space

• Adequate ventilation

• Adequate washing facilities

• Adequate film storage facilities

• Safelights – positioned 1.2 m from the work surfaces with 25 W bulbs and filters suitable for the type of film being used (see Ch. 17)

• Processing equipment (see Fig. 5.2):

– Tanks containing the various solutions

– Thermometer

– Immersion heater

– An accurate timer

– Film hangers.

Manual processing cycle

1. The exposed film packet is unwrapped and the film clipped on to a hanger.

2. The film is immersed in developer and agitated several times in the solution to remove air bubbles and left for about five minutes at 20 °C.

3. The residual developer is rinsed off in water for about 10 seconds.

4. The film is immersed in fixer for about 8–10 minutes.

5. The film is washed in running water for about 10–20 minutes to remove any residual fixer.

6. The film is allowed to dry in a dust-free atmosphere.

Processing solutions

Two different processing solutions are required, the developer and the fixer. The typical constituents of these solutions are shown in Tables 5.1 and 5.2.

Table 5.1

The typical constituents of developer solution and their functions

Constituents	Functions
Phenidone	Helps bring out the image
Hydroquinone	Builds contrast
Sodium sulphite	Preservative – reduces oxidation
Potassium carbonate	Activator – governs the activity of the developing agents
Benzotriazole	Restrainer – prevents fog and controls the activity of the developing agents
Glutaraldehyde	Hardens the emulsion
Fungicide	Prevents fungal growth
Buffer	Maintains pH (7+)
Water	Solvent

Table 5.2

The typical constituents of fixer solution and their functions

Constituents	Functions
Ammonium thiosulphate	Removes unsensitized silver halide crystals
Sodium sulphite	Preservative – prevents deterioration of the fixing agent
Aluminium chloride	Hardener
Acetic acid	Acidifier – maintains pH
Water	Solvent

Important points to note regarding development

• The alkaline developer solution should be made up to the concentration recommended in the manufacturer’s instructions.

• The developer solution is oxidized by air and its effectiveness decreased. Solutions should be used for no more than 10–14 days, irrespective of the number of films processed during that time.

• If the development process is allowed to continue for too long, more silver will be deposited than was intended and the radiograph will be too dark. Conversely, if there is too short a development time the radiograph will be too light.

• Development time (in fresh solutions) is dependent on the temperature of the solution. The usual value recommended is five minutes at 20°C.

• If the temperature is too high, development is rapid, the film may be too dark and the emulsion may be damaged. If the temperature is too low, development is slowed and a pale film will result.

Important points to note regarding fixing

• Fixer solution should be made up to the concentration recommended by the manufacturer. It/>

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