5: Image processing

Image processing

Processing is the general term used to describe the sequence of events required to convert the invisible latent image, contained in the sensitized film emulsion or in the solid-state or phosphor layer of the digital sensors, into the visible black and white radiographic film or digital image. This chapter summarizes the two methods involved, namely:

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:

Practical methods

There are three practical chemical processing methods available:

Manual processing

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

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.

Jan 12, 2015 | Posted by in Oral and Maxillofacial Radiology | Comments Off on 5: Image processing
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