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:

• X-ray generating equipment – required to produce the X-rays

• Image receptors (film and digital) – required to detect the X-rays

• Image processing (chemical or computer) – required to produce the visual black, white and grey image.

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:

• A tubehead

• Positioning arms

• A control panel and circuitry.

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.

Fig. 3.1 Examples of modern dental X-ray generating equipment. A Wall-mounted Prostyle Intra^® manufactured by Planmeca. B Mobile Kodak 2200. C Hand-held Nomad ™ manufactured by Aribex.

Ideal requirements

The equipment should be:

• Safe and accurate

• Capable of generating X-rays in the desired energy range and with adequate mechanisms for heat removal

• Small

• Easy to manoeuvre and position

• Stable, balanced and steady once the tubehead has been positioned

• Easily folded and stored

• Simple to operate and capable of both film and digital imaging

• Robust.

Main components of the tubehead

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

Fig. 3.2 Diagram of the tubehead of a typical dental X-ray set showing the main components.

• 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)

Fig. 3.3 (i) Examples of adaptors/collimators designed to change the shape of the beam from circular to rectangular: A Sirona Heliodent^® DS collimator; B Dentsply’s Universal collimator. (ii) Aluminium filter (arrowed) viewed from down the spacer cone on the Sirona Heliodent^® DS.

• 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)

Fig. 3.4 A Diagrams showing various designs and shapes of spacer cones or beam-indicating devices. Note: The short plastic pointed spacer cone is NOT recommended. B Diagrams showing (i) the original tubehead design with the X-ray tube at the front of the head, thus requiring a long spacer cone (L) to achieve a near-parallel X-ray beam and the correct focus to skin distance (fsd) and (ii) the modern tubehead design with the X-ray tube at the back of the head, thus requiring only a short spacer cone (S) to achieve the same focus to skin distance (fsd).

• 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:

– 200 mm for sets operating above 60 kV

– 100 mm for sets operating below 60 kV

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.

Fig. 3.5 A Diagrams showing the effect of X-ray beam source (focal spot) size on image blurring (i) a small or point source, (ii) a large source. B The principle of line focus: diagram of the target and focal spot showing how the angled target face allows a large actual focal spot but a small effective focal spot.

Main components of the control panel

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

Fig. 3.6 Examples of modern dental X-ray equipment control panels. A Focus^® manufactured by Instrumentarium Imaging. B Prostyle Intra^® manufactured by Planmeca. C Heliodent^® DS manufactured by Sirona. They are all anatomical timers suitable for film and digital imaging.

• The mains on/off switch and warning light

• The timer, of which there are three main types:

– electronic

– impulse

– clockwork (inaccurate and no longer used)

• An exposure time selector mechanism, usually either:

– numerical, time selected in seconds

– anatomical, area of mouth selected and exposure time adjusted automatically

• Warning lights and audible signals to indicate when X-rays are being generated

• Other features can include:

– Film speed selector

– Patient size selector

– Mains voltage compensator

– Kilovoltage selector

– Milliamperage switch

– Exposure adjustment for digital imaging.

Circuitry and tube voltage

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

• To generate the high potential difference (kV) to accelerate the electrons across the X-ray tube via the step-up transformer

• To provide the low-voltage current to heat the tube filament via the step-down transformer.

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:

Fig. 3.7 Diagram showing the alternating current waveform.

• Half-wave rectified

• Single-phase, full-wave rectified

• Three-phase, full-wave rectified

• Constant potential.

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.