3: Dental X-ray equipment, image receptors and image processing

Dental X-ray equipment, image receptors and image processing

This chapter summarizes the more important points of the equipment and the other practical aspects involved in the production of the final radiographic image, namely:

Dental X-ray generating equipment

There are several conventional dental X-ray units available from various manufacturers. They vary in appearance, complexity and cost, but all consist of three main components:

These dental units can either be fixed (wall, floor or ceiling mounted) or mobile (attached to a sturdy frame on wheels), as shown in Figs 3.1A and B. A recent development has been the production of hand-held dental units (Fig. 3.1C), particularly useful for domiciliary and forensic radiology.

Main components of the tubehead

A diagram of a typical tubehead is shown in Fig. 3.2. The main components include:

• The glass X-ray tube, including the filament, copper block and the target (see Ch. 2)

• The step-up transformer required to step-up the mains voltage of 240 volts to the high voltage (kV) required across the X-ray tube

• The step-down transformer required to step-down the mains voltage of 240 volts to the low voltage current required to heat the filament

• A surrounding lead shield to minimize leakage

• Surrounding oil to facilitate heat removal

• Aluminium filtration to remove harmful low-energy (soft) X-rays (see Fig. 3.3)

• The collimator – a metal disc or cylinder with central aperture designed to shape and limit the beam size to a rectangle (the same size as an intraoral image receptor) or round with a maximum diameter of 6 cm (see Figs 3.3 and 3.4)

• The spacer cone or beam-indicating device (BID) – a device for indicating the direction of the beam and setting the ideal distance from the focal spot on the target to the skin. The required focus to skin distances (fsd) are:

It is the length of the focal spot to skin distance (fsd) that is important NOT the physical length of the spacer cone. Various designs are illustrated in Fig. 3.4.

Focal spot size and the principle of line focus

As stated in Chapter 1, the focal spot (the source of the X-rays) should be ideally a point source to reduce blurring of the image – the penumbra effect – as shown in Fig. 3.5A. However, the heat produced at the target by the bombarding electrons needs to be distributed over as large an area as possible. These two opposite requirements are satisfied by using an angled target and the principle of line focus, as shown in Fig. 3.5B.

Main components of the control panel

Examples of three typical control panels are shown in Fig. 3.6. The main components include:

Circuitry and tube voltage

The mains supply to the X-ray machine of 240 volts has two functions:

However, the incoming 240 volts is an alternating current with the typical waveform shown in Fig. 3.7. Half the cycle is positive and the other half is negative. For X-ray production, only the positive half of the cycle can be used to ensure that the electrons from the filament are always drawn towards the target. Thus, the stepped-up high voltage applied across the X-ray tube needs to be rectified to eliminate the negative half of the cycle. Four types of rectified circuits are used:

The waveforms resulting from these rectified circuits, together with graphical representation of their subsequent X-ray production, are shown in Fig. 3.8. These changing waveforms mean that equipment is only working at its optimum or peak output at the top of each cycle. The kilovoltage is therefore often described as the kVpeak or kVp. Thus a 50 kVp half-wave rectified X-ray set only in fact functions at 50 kV for a tiny fraction of the time of any exposure.

Modern designs favour constant potential circuitry, often referred to as DC units, which keep the kilovoltage at kVpeak throughout any exposure, thus ensuring that:

Image receptors

In dentistry 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

The film packet contents: The contents of a film packet are shown in Fig. 3.10. 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. 14).

The radiographic film: The cross-sectional structure and components of the radiographic film are shown in Fig. 3.11. It comprises four basic components:

Indirect-action film

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 silver halide emulsion is designed to be sensitive primarily to light rather than X-rays.

• Different emulsions are manufactured which are sensitive to the different colours of light emitted by different types of intensifying screens (see later). These include:

• It is essential that the correct combination of film and intensifying screens is used.

• There is no orientation dot embossed in the film so some form of additional identification is required, e.g. metal letters, L or R, placed on the outside of the cassette or electronic marking.

• Indirect action film has a resolution of about 5 lp per mm.

Jan 12, 2015 | Posted by in Oral and Maxillofacial Radiology | Comments Off on 3: Dental X-ray equipment, image receptors and image processing
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