Procedures in Dental Imaging

Procedures in Dental Imaging

Jackie Brown and Jonathan Davies

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

Action Rationale
Only take radiographs if clinically justified. This is a fundamental principle of radiation protection and is the first principle of radiation protection laid down by the International Commission on Radiation Protection (ICRP).
Maintain x‐ray equipment regularly; ideally service x‐ray equipment every year. To ensure correct working; to detect x‐ray leakage from the tube head and to ensure accurate timing and x‐ray output.
Tube output should be monitored regularly and doses compared with similar machines. This allows the development of a ‘dose reference level’ (DRL), a dose which should not be exceeded for any particular radiographic examination.
Aluminium filtration, totalling 1.5 mm for dental x‐ray tubes operating up to 70 kV, must be included to filter the x‐ray beam as it leaves the x‐ray set. Filtration ensures that damaging, low energy x‐ray photons are removed from the beam before it reaches the patient.
Use a modern dental intraoral x‐ray set operating at between 60 and 70 kV and using, where possible, direct current (DC). An operating kilovoltage in the region of 60–70 kV will deliver a reduced radiation dose compared with a machine operating at 50 kV, and direct current (DC) units also deliver a reduced radiation dose due to a shorter exposure time (National Radiological Protection Board/Department of Health, 2001).
Use a rectangular collimator for intraoral radiography. This reduces the x‐ray exposure field to match the size of the film, therefore reducing extraneous exposure. The dose to the patient may be reduced by 30–50%.
Use a 200 mm focus‐to‐skin distance. This distance ensures a narrow, parallel beam and eliminates scatter arising at the tube port.
Apply ALARA. This is the use of the lowest exposure which will achieve a diagnostic image.
Use paralleling technique for intraoral periapical radiography. Advantages:

  • It is the most accurate technique and therefore avoids retakes.
  • The beam is directed across the dental arch and not normally down towards the trunk of the body.
  • It allows reproducible imaging, therefore allowing better comparison with previous films.
For intraoral radiography:
  • Use the fastest conventional film (F speed), or
  • Use a digital sensor which delivers a diagnostic image.
The most sensitive image receptors reduce exposure time. F‐speed film gives a 20% reduction in exposure time in comparison with E‐speed film. Digital intraoral sensors may potentially reduce exposure time (solid state sensors including CCD/CMOS slightly more than PSPs) but this effect is counter‐balanced by a reported increase in re‐takes and the smaller image area of CCD/CMOS sensors which requires more images to cover a region of interest (Wenzel and Møystad, 2010).
  • Use film holders and beam‐aiming devices for intraoral radiography.
These help to correctly centre the beam and stabilise the image receptor during exposure, ensuring a well‐positioned radiograph. This also avoids the patient holding the film with their fingers.
For extraoral radiography:
  • Use suitably matched conventional film and intensifying screens.
Conventional extraoral film has a specified colour sensitivity spectrum and should be matched with intensifying screens of the same colour emission spectrum for proper exposure.
  • Use a digital imaging system where the dose is optimised to match or improve on conventional extraoral film.
Extraoral digital radiographic systems may use direct exposure sensors – CCD/CMOS or PSPs. When using PSPs the intensifying screens are removed from a cassette and therefore the dose‐saving advantage may be lost.
For panoramic radiography:
  • Use machines equipped with constant potential generators.
Constant potential (DC) x‐ray machines deliver a lower radiation dose compared with conventional AC (alternating current) units.
  • Use field limitation devices where possible.
If only the areas of interest can be exposed, this will significantly reduce radiation dose. Selecting a ‘dentition only’ mode can reduce the radiation exposure by up to 50% (Lecomber et al., 2000).
Ensure a quality assurance programme is in place. This will monitor and help maintain good quality radiographs and reduce retakes.
Avoid radiography of the most vulnerable groups where possible; these include children and pregnant women.
If a dental radiograph must be taken of a pregnant woman, a lead apron may provide psychological reassurance; a lead apron must be provided if the x‐ray beam passes towards the abdomen.
The risk of harm to children is greater than for adults:

Age group (years) Multiplication factor for risk of cancer
<10 ×3
10–20 ×2
20 –30 ×1.5
30 ×1
30 –50 ×0.5
50 –80 ×0.3
80+ Negligible risk

(Faculty of General Dental Practitioners, 2013)
The risk to the pregnant woman is in the potential for harm to the developing foetus. This is thought to be greatest in the first and third trimesters.

Ensure adequate training and regular updating for staff in radiographic techniques, processing, QA and radiation protection. This ensures that quality is maintained and that staff use optimal radiographic techniques, avoiding unnecessary re‐takes and applying sound radiation protection.

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

Techniques for radiation protection of staff

Action Rationale
Ensure dental x‐ray equipment is properly installed and within a suitable protected environment. It is very important to protect both staff and patients. Staff outside the immediate x‐ray cubicle should not receive more than 1 mSv per year additional exposure.
No person, other than the patient, should go within 1.5 m of the dental x‐ray set, and nowhere within the primary beam, while it is in operation, unless this is essential (e.g. a mother reassuring a child). Any person admitted to the controlled area must be adequately protected, i.e. the comforter should wear lead protection. Record details of any persons assisting in radiography. The area immediately around the dental x‐ray tube head contains both primary and scattered radiation.
The operator should not, in any circumstances, hold the image receptor, spacer cone or x‐ray tube head during an exposure. This would expose the operator to unacceptable levels of scattered radiation.
The operator of the dental x‐ray set should stand away from the primary beam and ideally behind a protective screen equivalent to at least 0.25 mm lead, where they may operate the equipment. Exposure to primary and scattered radiation can be avoided by increasing the distance between yourself and the x‐ray tube, because radiation obeys the inverse square law. A lead screen can be used where a suitable distance cannot be achieved.
Avoid aligning the beam so that it passes into adjacent rooms or into adjoining premises. The walls may not prevent x‐rays passing through to the neighbouring room, and therefore it is prudent to avoid irradiating adjacent areas. The Radiation Protection Adviser (RPA) should be consulted regarding protection afforded by walls of the dental surgery.
A warning sign should be placed at the entry point to an x‐ray room. This is to prevent people from entering the area when an x‐ray exposure is being made.
If an additional person is needed to stand with the patient during a radiograph, to help or support the patient, then this person must be protected with a lead apron. The helper is receiving no personal benefit from the radiographic exposure and must therefore be protected as much as possible from radiation exposure.
Personal monitoring, in the form of a film badge or thermo‐luminescence dosemeter (TLD), may be worn by staff undertaking dental radiography. This may be recommended if the radiographic workload is high, e.g. more than 100 periapicals per week, more than 40 panoramics per week or if cone beam CT examinations are undertaken in the practice.
Dose limits must be applied to staff who are occupationally exposed to radiation, e.g.:
Total annual exposure limit = 20 mSv (Ionising Radiation (Medical Exposure) Regulations, 2018)
By exercising good radiation protection practice in a dental surgery the operator should not exceed more than 1 mSv.
This will protect staff from excessive radiation exposure and identify cases where this may be a risk.
Ensure the x‐ray set is switched off after use. This will ensure that no accidental expose occurs.

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.

Action Rationale
Check identifying data for the patient such as their name, date of birth or address. This ensures that you are about to irradiate the correct patient.
Prepare the patient by requesting removal of dentures and warning them of the need to keep still. This prevents the degradation of the image by artefacts and patient movement.
Insert the image receptor into the film holder. To support the image receptor during radiography and ensure it is in alignment with the x‐ray beam. Check that the image receptor has not slipped during placement.
Insert the film holder into the patient’s mouth, aligning it parallel with the palatal/lingual aspect of the tooth. To ensure that the image receptor is adjacent to the object being imaged.
In the maxilla the image receptor may be placed at the mid palate region, due to the curved shape of the palate. In the mandible, the image receptor may be placed against the tooth and into the lingual sulcus.
Ask the patient to gently close onto the bite block and support this with a cotton wool roll (Figure 8.1). The cotton wool roll enables support of the bite block and may be used to fill any space where teeth may be missing.
Place the locating ring towards the patient’s skin; this ring does not need to be in contact with the patient’s face. To enable the x‐ray cone to be placed close to the object under imaging to reduce magnification of the image and to assist in correct alignment of the x‐ray cone.
Bring the x‐ray cone into close relationship to the locating ring, ensuring the rectangular collimator is correctly aligned with the positioning aid on the locating ring, that the end of the cone is parallel to the locating ring and that the arm of the film holder is parallel with the x‐ray cone. To assist in parallel alignment of the x‐ray cone, to ensure the rectangular collimator is in the correct position in relation to the image receptor, and that the front edge of the cone is parallel to the image receptor to prevent any cone cuts or obliquely positioned images.
Select exposure factors and expose image. The exposure factors are chosen to optimally display the tooth and apical bone tissues. A kilovoltage of 65–70 kV is normally selected with a short exposure time.
Remove the film holder and image receptor from the mouth and wipe the external barrier wrap, then dispose of the barrier wraps. To prevent any contamination with saliva. There is a high risk of transmission of infection through salivary contamination of the film processor/image reading device.
Process the image receptor. To enable display of the final image.
Assess the image for final image quality. To ensure the image is of the correct density and contrast to enable differentiation of enamel, dentine, pulp and supporting bone; correctly positioned with no cone cuts, image faults, no obvious elongation orforeshortening of the image; that the whole image receptor has been exposed; and assess for any error of processing/handling.
Report the image in the clinical notes. To comply with the IRMER – Ionising Radiation (Medical Exposure) Regulations in the UK (2018).
Schematic of the paralleling technique periapical radiography illustrating x-ray tube with rectangular collimator, film holder with centering device, film, tooth and maxillary alveolus, and x-ray beam.

Figure 8.1 Paralleling technique periapical radiography.

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.

Action Rationale
Check identifying data for the patient such as their name, date of birth or address. This ensures that you are about to irradiate the correct patient.
Prepare the patient by requesting removal of dentures and warning them of the need to keep still. This prevents the degradation of the image by artefacts and patient movement.
Insert the film/image receptor into the holder. To support the image receptor during radiography and ensure it is in alignment with the x‐ray beam. Check that the image receptor has not slipped during placement.
Insert the film into the patient’s mouth, aligning it parallel with palatal and lingual aspect of the upper and lower posterior teeth and ask the patient to close tightly together. To ensure adequate coverage, the image receptor should be placed so the front surface can be seen adjacent to the distal aspect of the canine tooth. Check this position when the patient bites together.
Align any locating ring towards the patient’s skin, although this ring does not need to be in contact with the patient’s face. To enable the x‐ray cone to be placed as close as possible to the object under investigation to reduce magnification of the image and to assist in correct alignment of the x‐ray cone.
Bring the x‐ray cone into close relationship with any locating ring or using the arm of the film holder, ensuring the rectangular collimator is correctly aligned with the orientation of the film and any positioning aids, and that the end of the cone is perpendicular to the image receptor. To assist in parallel alignment of the x‐ray cone and to ensure that the rectangular collimator is in the correct position in relation to the image receptor and that the front edge of the cone is parallel to the image receptor to prevent any cone cuts or obliquely positioned images.
Select exposure factors and expose image. Appropriate choice of exposure factors is important; when imaging caries in a bitewing image a lower kV (such as 60 kV, as opposed to 70 kV) may be chosen for greater image contrast.
Remove the film holder and image receptor from the mouth and wipe the external barrier wrap, then dispose of the barrier wraps. To prevent any contamination with saliva. There is a high risk of transmission of infection through salivary contamination of the film processor/image reading device.
Repeat the procedure for the opposite arch. To enable imaging of the left and right sides.
Process the film or image receptor. To enable display of the final image.
Assess the image for final image quality. To ensure the image is of the correct density and contrast to enable differentiation of enamel, dentine, pulp and supporting bone; correctly positioned with no cone cuts, image faults, no obvious elongation or foreshortening of the image; that the whole image receptor has been exposed; and assess for any error of processing/handling.
Report the image in the clinical notes. To comply with the IRMER – Ionising Radiation (Medical Exposure) Regulations in the UK (2018).

Image Quality Criteria

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Jan 22, 2018 | Posted by in General Dentistry | Comments Off on Procedures in Dental Imaging
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