Ionising radiation is used in medicine and dentistry to visualise dense internal structures. In dentistry, the potential problems are stochastic tumour-inducing effects. This chapter discusses the doses and risks in different types of dental radiography and the indications for the use of different views. Methods to protect both the patient and the dentist are discussed, together with the components of a quality assurance programme.
The use of ionising radiation in medicine and dentistry is governed by the statutory requirements laid down in the Ionising Radiations Regulations 1999 (which deals with protection of workers and the public) and the Ionising Radiation (Medical Exposure) Regulations 2000 (which specifically addresses patient protection). Guidance notes on the use of radiation in general dental practice, summarising this legislation, are available to all dentists.
Ionising radiation includes X-rays, gamma rays and cosmic rays. These are all high-energy, short-wavelength, high-frequency electromagnetic radiations. They exist as tiny packets of energy called photons. While gamma rays are used in hospital practice in nuclear medicine, X-rays are the only usual concern of dentists.
X-rays are produced by an electrical process in an X-ray tube (Fig. 16.1). Electrons, released from a heated tungsten filament, are accelerated in a vacuum by the application of a high voltage (typically 50–75 kV (kilovolts)) and strike a positively charged target. The sudden halt of the electrons releases energy, mainly as heat but also as X-rays. The X-rays are a mixture of photons of different energies, but low-energy (dose-producing) photons predominate.
The X-rays are filtered, usually using aluminium, to remove the low-energy photons (Fig. 16.2). The photons are then shaped into an appropriately sized beam by collimation using steel diaphragms or cylinders (Fig. 16.3).
Fig. 16.2 The effect of filtration on the X-ray beam. Low-energy photons predominate in the unfiltered spectrum.
Filtration removes proportionately more of these ‘weak X-rays’, resulting in a filtered beam with a higher mean energy at the expense of a loss of intensity.
In a living cell, ionisation can have damaging effects. We are particularly concerned if the DNA of a cell is damaged. This may occur by a direct interaction of X-ray photons with the DNA or indirectly when a photon disrupts a water molecule into reactive radicals that go on to damage DNA.
The irradiation of cells can result in somatic effects (i.e. those occurring in the irradiated somatic cells of an individual) or genetic effects (i.e. those occurring in the germ cells and transmitted to the offspring of the irradiated individual) because of gonadal exposure. In properly conducted dental radiography, genetic effects are not usually considered because the gonads should not be irradiated.
All tissue effects have threshold doses below which they do not occur. Above the threshold dose, the effect is certain to occur. In dental radiography, these thresholds should never be reached. The risk from dental radiography is for stochastic effects.
‘Radiation dose’ is a measure of the energy imparted by X-ray exposure and its biological effect. At a simple level, we can measure the energy imparted per unit mass (joules/kg), but using established methods we can calculate ‘effective dose’. This is a ‘whole body equivalent’ value which can be directly related to stochastic effects. Doses and risks vary enormously according to the type of equipment used, so it is hard to give firm figures, but recent estimations of risk (and radiation dose) are given in Table 16.1.
|Technique||Effective dose (microsieverts)||Risk of cancer (per million)|
|Intraoral (bitewing, periapical)||<2||<0.1|
|CBCT (‘dento-alveolar’)||11–674 (median = 61)||0.6–37 (median = 3.4)|
|CBCT (‘craniofacial’)||30–1073 (median = 87)||1.6–59 (median = 4.7)|
|Computed tomography (as used for dental implant planning)||280–1410||15–77|
These risks are calculated for a 30-year-old adult, using the nominal risk coefficient for cancer of 5.5 × 10−2/Sv. Risks for children are two to three times greater, while for older patients risk falls until, at 80 years, they are virtually negligible.
There are legal and ethical requirements that no radiological examination should be used unless there is likely to be a benefit in terms of improved prognosis or management of the patient. This implies that no X-ray examination is ever ‘routine’ and that radiographic ‘screening’ is unacceptable. Instead, radiographs should be prescribed according to the clinical needs of the patient.
Selection of bitewing radiographs: The nearest we come to ‘routine’ radiography in dentistry is with the bitewing radiograph. For dentate patients who are new to the practice (and partially dentate patients where films can be supported in the mouth), most authorities agree that a posterior bitewing examination is justified. Thereafter, the intervals between bitewing examinations should be determined by assessment of caries risk. Current UK guidelines are shown in Table 16.2, although other guidelines with slightly different intervals are available and can be found internationally. Obviously bitewing frequency for an individual patient may change if the patient changes caries risk category.
If the dentist feels that a radiographic examination is of help in assessment of bone loss in periodontal disease, then bitewing radiographs will provide the necessary information in the premolar and molar regions, providing geometrically accurate images. Where periodontal probing depths exceed 5 mm, then vertical bitewing radiographs are appropriate.
• Prior to the extraction of erupted third molars, retained roots, lone-standing upper molars or where there is reasonable clinical suspicion that problems may arise. The fashion of routine pre-extraction radiographs has arisen in the absence of any scientific evidence of benefit.
Selection of panoramic radiographs: In terms of image quality, panoramic radiography is inferior to good intraoral radiographs. Consequently, for most dental diagnostic uses, it is a ‘second best’ imaging technique. Possible situations where it may be useful include:
Routine ‘screening’ of all new patients is never justifiable; research has shown that the majority of patients who receive a ‘screening’ panoramic radiograph receive no diagnostic benefit from the examination.
Selection of CBCT examinations: Cone beam computed tomography (CBCT) is a relatively new technology and its capabilities are changing as equipment is refined. Despite this, one general principle can be stated: that CBCT should only be used when the question for which imaging is required cannot be answered adequately by lower-dose conventional (traditional) radiography. This view reflects the status quo in which radiation doses and economic costs associated with CBCT are usually higher than with conventional radiography.
Artefact, arising from most metals (usually dental restorations) in the scan degrade the image quality significantly throughout that axial plane, producing radiating dark bands. This is one reason why CBCT should not be used as a method of caries detection, as the artefacts can mimic radiolucency. Similarly, metal posts in roots may produce the same effect. Detailed, evidence-based selection criteria for CBCT have been developed recently for Europe (see ‘Further reading’ section).
• Operating potential (kilovoltage): for intraoral radiography, a minimum of 50 kV is set and 65–70 kV is recommended. This is often fixed on dental intraoral X-ray sets but is usually used to control exposures on panoramic X-ray equipment.
• Collimation: on intraoral X-ray sets, the beam can be restricted to a rectangle of 4 cm by 3 cm, leading to a substantial dose reduction over the conventional 6-cm-diameter round beam; all new equipment should be fitted with rectangular collimation and it should be retrofitted on older equipment; on panoramic machines, selective field size collimation facilities may be available. For CBCT equipment, there should be a choice of fields of view and examinations must use the smallest that is compatible with the clinical situation if this provides less radiation dose to the patient.
• Image receptor speed: for intraoral radiography, digital systems may offer some reduction in mAs (and hence dose) compared with film. Where film is used, E- or F-speed films should be used. For digital panoramic equipment, this factor is out of the control of the operator, but for film-based panoramic radiography, a rare-earth screen/film combination should be used (a combination of ISO speed 400 or better is used). For CBCT equipment, the choice of image receptor is out of the control of the operator, although this will influence radiation dose.
• Lead shielding of patients: the only requirement to use a lead apron is when a pregnant woman is being examined using a technique involving a beam that would pass through the fetus. Thyroid shielding can be used if that organ lies in the primary beam of X-rays.
A poor-quality image means that the patient receives reduced, or no, benefit from the risk of the X-ray examination. Even in the best hands, radiographs may be produced that are ‘rejects’. A quality standard of no greater than 10% of radiographs (5% for CBCT images) being non-diagnostic has been set for general dental practice. Good quality of radiographs can be addressed by attention to all of the criteria listed in Table 16.3.
|Radiographic technique||Use of film-holding/beam-aiming devices for intraoral radiography
Careful positioning for panoramic radiography
Careful selection and instruction of patients
|X-ray set||Regular maintenance and servicing, as recommended by the manufacturer