Introduction

Radiographs are an important diagnostic tool in dentistry. Unlike other diagnostic tools, radiography involves a risk of harm to the patient from radiation exposure and therefore the benefits of gaining valuable diagnostic information need to be balanced against the risk of harm from the radiation used to create the image. Radiation protection is therefore a central principle of radiography and is underpinned by a regimen of legislation, selection criteria, justification, quality assurance and audit.

Procedures in Radiation Protection for Dental Imaging

Exposure to x‐rays, at levels used in diagnostic radiology, has the potential to cause harm to the exposed individual through damage to DNA and to cell structures which may lead to the induction of tumours. These are known as somatic stochastic effects, and it is understood that there is no threshold dose beneath which these effects do not occur. Therefore any dose of ionising radiation has the potential to induce a tumour, but the level of risk is proportional to the level of exposure and for this reason exposures are kept as low as possible. This principle of radiation protection is described by the ALARA (As Low As Reasonably Achievable) principle which advocates the lowest possible exposure to ionising radiation, consistent with producing a diagnostic radiograph, in order to reduce the risk of harm to its lowest possible level. The net detriment to the patient must be outweighed by the net benefit from the diagnostic information obtained from the radiograph.

Dental radiographs normally expose the individual patient to relatively low doses of ionising radiation, but unfortunately dental radiographs are used much more frequently than medical radiographs and tend to be used on a generally younger population of patients who are more at risk of harm from ionising radiation. It is therefore important to understand how to implement radiation protection measures during dental radiography in order to reduce the level of risk to the patient.

Justification and Selection Criteria

A fundamental principle of radiation protection is that radiographs should only be taken when clinically necessary, and when their benefit to the patient outweighs the harm from exposure to ionising radiation. The aims of the investigation, and any other imaging methods which may yield the information but avoid or reduce x‐ray exposure, must always be considered. This process is known as ‘justification’ and is required under UK and European guidelines (Faculty of General Dental Practitioners, 2013; European Commission, 2004). The responsibility for undertaking this decision, through a weighing of the potential benefit against detriment, belongs normally to the dentist who requests the radiograph.

Selection criteria have been developed to assist this decision‐making process. Selection Criteria for Dental Radiography, published by the Faculty of General Dental Practitioners (2013) and the European Commission Guidelines on radiation protection in dental radiology, The Safe Use of Radiographs in Dental Practice (2004), identify evidence‐based indications for commonly taken dental radiographs in a range of clinical scenarios, such as in endodontics, periodontics, orthodontics, implantology and the heavily restored adult dentition. Specialist societies including the British Society of Orthodontics have also produced their own selection criteria in relation to orthodontic imaging (BOS, 2015), and the American Academy of Oral and Maxillofacial Radiology has implemented similar guidance in North America (Isaacson et al., 2008).

Levels of panoramic radiography have increased significantly in the past 20 years and concerns have been raised over its use as a ‘screening’ radiograph. Evidence shows that these concerns are unjustifiable (Rushton, Horner and Worthington, 2002a, b). Indications for panoramic radiography include:

• Pathology extending outside the alveolus.
• Prior to dental surgery under general anesthetic (GA).
• Orthodontic assessment.
• Assessment of third molars.
• Mandibular fractures.
• Periodontal assessment if pocketing >5 mm in >1 quadrant and when other views are unavailable.
• Prior to dental clearance or multiple extractions.
• Prior to implant placement.

Selection criteria for orthodontic radiography are also available (Faculty of General Dental Practitioners, 2013) and include advice specific to the use of radiographs in orthodontic diagnosis and treatment, and flow charts for decision‐making in prescribing lateral cephalometric radiographs.

Techniques for radiation protection of patients

CCD/CMOS, charge‐coupled device/complementary metal oxide semiconductor; PSPs, photo‐stimulable phosphor plates; QA, quality assurance.

Techniques for radiation protection of staff

Procedures in Dental Radiography

Long Cone Periapical Radiography

This radiographic technique is undertaken to image one or two teeth and their immediate supporting structures, and is designed, by optimising imaging geometry, to give as accurate and undistorted an image as possible. It is used to detect apical pathology, caries and periodontal bone loss.

A similar methodology is used in endodontic imaging where, by using a specialised film holder, the endodontic instrumentation within the tooth and rubber dam can be contained within a basket while an intraoral radiograph is undertaken.

Periapical radiography is achieved by placing the long axis of the image receptor and the tooth under investigation parallel with each other, with the x‐ray beam aligned at right angles to the two objects.

Equipment

• Intraoral film (film size 0–2) or digital image receptor that is barrier wrapped using a manufacturer‐recommended bag.
• Periapical film/sensor holder.
• Intraoral x‐ray tube with a rectangular collimator.

Alternative Technique for Lower Third Molars

Tissue forceps may be used to insert the image receptor into the posterior lingual sulcus and to hold it in place. The x‐ray beam is centred approximately 1 cm above the lower border of the mandible and on a vertical line dropped from the outer corner of the eye.

Digital image receptors such as photo‐stimulable phosphor plates (PSPs) may be easily damaged when gripped by forceps. Clear tubing may be placed on forceps teeth, or additional protective covers used to protect the image receptor from damage.

Image Quality Criteria

• The tooth and surrounding periodontal tissues are clearly defined.
• At least 2 mm of bone is seen beyond the tooth apex, although ideally 4–5 mm should be demonstrated.
• There is no overlap of adjacent teeth. The margins of the interdental alveolar bone should be clearly visible.
• There is minimal elongation/foreshortening of the tooth.
• There should be no geometrical distortion of the image due to incorrect beam angulation or image receptor position or because of image receptor bending.
• The contrast and density should be such that the enamel, dentine and pulp can be readily differentiated from each other.

An alternative technique that may be undertaken in periapical radiography is the bisecting angle technique. Here, using the principle of similar triangles, the tooth is imaged by placing the image receptor as close to the tooth as possible but with an unavoidable angle between the long axis of receptor and of tooth. A line bisecting the angle between long axis of tooth and image receptor is envisaged, and the x‐ray beam aimed perpendicular to this imagined bisecting plane. One inherent problem is that the roots may be foreshortened or elongated due to incorrect angulation of the x‐ray beam to the bisecting plane and there may be lack of the detail required for periodontal and endodontics diagnosis, particularly a lack of definition of the alveolar crest.

Bitewing Radiography

This radiographic technique is undertaken to image the crowns, interproximal spaces and immediate supporting structures of the posterior maxillary and mandibular premolar and molar teeth. It is not intended to image the full root and apical tissues of any individual tooth. This technique is designed, by optimising imaging geometry, to give as accurate and undistorted an image as possible. It is used to detect coronal caries and interproximal caries and can be used to assess periodontal bone loss. The technique can be undertaken with the film in a portrait (vertical) position but is more commonly used in a landscape (horizontal) position. The vertical bitewing technique may have more precedence in periodontal diagnosis as it demonstrates fewer teeth but a greater depth of alveolar bone; however, the more prevalent technique is the horizontal bitewing.

This bitewing technique is achieved by placing image receptor parallel with the teeth of the dental arch, and with the x‐ray beam aligned at right angles to these two objects. It is undertaken on both the patient’s left and right sides.

Equipment

• Intraoral film (film size 0–2) or digital image receptor that is barrier wrapped using a manufacturer‐recommended bag.
• Bitewing film/sensor holder.
• Intraoral x‐ray tube using a rectangular collimator.

Image Quality Criteria