Dental X-ray equipment, image receptors and image processing
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. 3.3 (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:
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.
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.
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.
• Indirect-action or screen film, so-called because it is used in combination with intensifying screens in a cassette. This type of film is sensitive primarily to light photons, which are emitted by the adjacent intensifying screens by the photoelectric effect (see Ch. 2). They respond to shorter exposure of X-rays, enabling a lower dose of radiation to be given to the patient.
• 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:
• The emulsion on both sides of the base – this consists of silver halide (usually bromide) crystals embedded in a gelatin matrix. The X-ray photons sensitize the silver halide crystals that they strike and these sensitized silver halide crystals are later reduced to visible black metallic silver in the developer (see later)
Film orientation: The film has an embossed dot on one corner that is used to help orientation. Its position is marked on the back of the packet or can be felt as a raised dot on the front. The side of the film on which the dot is raised is always placed towards the X-ray beam. When the films are mounted, this raised dot is towards the operator and the films are then arranged anatomically and viewed as if the operator were facing the patient.
Film speed: The speed of the film determines how quickly it reacts to X-rays. Thus the faster the film, the less the exposure required for a given film blackening and the lower the radiation dose to the patient. It is determined by the number and size of the silver halide crystals in the emulsion and designated by the letters D, E and F. The larger the crystals, the faster the film, but the poorer the image quality.
Resolution: Resolution, or resolving power, is a measure of the radiograph’s ability to differentiate between different structures that are close together. Factors that can affect resolution include penumbra effect (image sharpness), silver halide crystal size and contrast. It is measured in line pairs (lp) per mm. Direct-action film has a resolution of approximately 10 lp per mm.
Uses: 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: