Image receptors

Published on 13/06/2015 by admin

Filed under Radiology

Last modified 13/06/2015

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Image receptors

This chapter summarizes the various image receptors used in dentistry to detect X-rays. These include:

Radiographic film

Radiographic film has traditionally been employed as the image receptor in dentistry and is still widely used. There are two basic types:

Direct-action (non-screen) film


Direct-action film is used for intraoral radiography where the need for excellent image quality and fine anatomical detail are of importance.

The film packet contents

The contents of a film packet are shown in Fig. 4.2. It is worth noting that:

• The outer packet or wrapper is made of non-absorbent paper or plastic and is sealed to prevent the ingress of saliva.

• The side of the packet that faces towards the X-ray beam has either a pebbled or a smooth surface and is usually white.

• The reverse side is usually of two colours so there is little chance of the film being placed the wrong way round in the patient’s mouth and different colours represent different film speeds.

• The black paper on either side of the film is there to protect the film from:

• A thin sheet of lead foil is placed behind the film to prevent:

• The sheet of lead foil contains an embossed pattern so that should the film packet be placed the wrong way round, the pattern will appear on the resultant radiograph. This enables the cause of the resultant pale film to be easily identified (see Ch. 17).

The radiographic film

The cross-sectional structure and components of the radiographic film are shown in Fig. 4.3. It comprises four basic components:

Indirect-action film


Film/screen combinations are used as image detectors whenever possible because of the reduced dose of radiation to the patient (particularly when very fine image detail is not essential). The main uses include:

Indirect-action film construction

This type of film is similar in construction to direct-action film described above. However, the following important points should be noted:

The relative spectral sensitivity of these four different film emulsions is shown in Fig. 4.4.

Characteristics of radiographic film

This section summarizes the more important theoretical terms and definitions used to describe how radiographic film responds to exposure to X-rays.

Film speed

This is the exposure required to produce an optical density of 1.0 above background fog (see Fig. 4.6). Thus, the faster the film, the less the exposure required for a given film blackening and the lower the radiation dose to the patient.

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