15.2 Dental Radiology & Imaging

Imaging Modalities — an Overview

CBCT should be justified and restricted to the smallest volume that answers the question; the principle is the lowest dose yielding the necessary diagnostic information.

Intra-oral Techniques

Paralleling (Long-cone) Technique

• Image receptor placed parallel to the long axis of the tooth; X-ray beam at 90° to both tooth and receptor.
• Uses a film holder (beam-aiming device) to reproduce geometry.
• Best image: minimal distortion, accurate root length, reproducible — preferred technique.
• Requires more space in the palate/floor of mouth.

Bisected-angle Technique

• Receptor placed against the tooth (not parallel); the angle between tooth and receptor is bisected; beam aimed at 90° to this bisecting plane.
• Used where paralleling is impossible (shallow palate, children, gagging).
• More distortion (foreshortening/elongation) and less reproducible.

Bitewings

• Receptor parallel to the teeth; beam horizontal at 5–8° to the occlusal plane.
• Horizontal bitewings for caries; vertical bitewings for periodontal bone loss in the posterior.

The ALARP Principle and Selection Criteria

Radiography must be justified (a clinical indication) and optimised (lowest dose for the information needed) — the ALARP (as low as reasonably practicable) principle, formalised in the Ionising Radiation (Medical Exposure) Regulations (IRMER).

The FGDP/RCP Selection Criteria for Dental Radiography gives evidence-based guidance:

• No routine (screening) radiography without a clinical indication.
• Bitewings: recall intervals based on caries risk (low risk 2-yearly; high risk 6–12 monthly).
• PA radiographs: symptomatic tooth, endodontics, trauma, periapical pathology.
• Panoramic: broad survey when localised views are inadequate; jaw pain of unknown origin; impacted thirds; large lesions.
• CBCT: small field, specific question, when 2D imaging is inadequate.

IRMER roles: the referrer (justifies referral), the practitioner (justifies the exposure), and the operator (carries it out) — duties defined in law.

Radiographic Appearances of Disease

Caries

• Radiolucency at the contact area (proximal caries) or cervical (CEJ) (root caries) — best seen on bitewings.
• Bitewing detection is more sensitive than PA for proximal lesions.
• E2/E3/E4 (or 0–4) classification by depth into enamel/dentine.

Periapical Pathology

• Early: widened periodontal ligament (PDL) space.
• Progression: loss of the lamina dura and an apical radiolucency (periapical periodontitis → radicular cyst or granuloma).
• A radiolucency at the apex of a non-vital tooth most often indicates apical periodontitis.

Periodontal Disease

• Horizontal bone loss: generalised, even crest height.
• Vertical (angular) defects: infrabony pockets; furcation involvement visible radiographically.
• Radiographs underestimate true bone loss (they show only mineralised tissue).

Other Pathology

• Radiolucent — cysts (odontogenic keratocyst, radicular cyst, dentigerous cyst), granulomas, malignancy.
• Radiopaque — odontomes, osteomas, sclerotic bone, retained root, foreign body.
• Mixed — fibro-osseous lesions, calcifying odontogenic cyst, some odontogenic tumours.

Common Technique Errors

Digital Radiography

Digital sensors (phosphor plates, solid-state sensors) reduce dose (up to 50%), allow image manipulation (enhancement, measurement), and enable storage, sharing, and audit. They also support CBCT. The same geometric and selection principles apply; dose reduction does not justify unnecessary exposures.

Typical Effective Doses and Natural Background

UK average natural background radiation is about 2.7 mSv per year (roughly 7 µSv per day). Dental doses are best understood against this comparator:

CBCT dose is highly field-size-dependent: a small volume for a single impacted tooth is dramatically lower than a full craniofacial acquisition, so the field of view must be the smallest that answers the clinical question.

Radiation and Pregnancy

Dental radiography is not an absolute contraindication in pregnancy. Exposures must still be justified and optimised (ALARP): use a lead apron where it does not compromise the image, prefer the lowest-dose technique, and avoid non-urgent exposures in early pregnancy where possible. The fetal dose from a single dental radiograph is negligible, so a justified urgent film (trauma, acute infection) should not be withheld.

Dose Reduction is Not a Justification

The dose saving from digital sensors never justifies an exposure that is not clinically indicated. Justification precedes optimisation — a smaller dose is still a dose, and an unjustified exposure remains unjustified however low it is.

Grading the Diagnostic Usefulness of Images (Outcome C8.8)

Outcome C8.8 asks candidates to grade the diagnostic usefulness of images. Every radiograph must receive a clinical evaluation under IR(ME)R, and UK practice also requires ongoing quality assurance in which images are graded and the results audited.

The current framework grades each image as diagnostically acceptable or not acceptable:

The recommended standard is that not less than 95% of radiographs are diagnostically acceptable, and a rising rate of unacceptable images triggers investigation of technique, equipment or processing.

Older schemes graded images as 1 (excellent), 2 (diagnostically acceptable) and 3 (unacceptable), with a target of no more than 10% grade 2 and no more than 5% grade 3; candidates may still encounter that terminology in local protocols.

Common causes of an unacceptable image and their remedies:

• Cone cutting — receptor and beam not aligned; use a beam-aiming device.
• Elongation or foreshortening — vertical angulation error in the bisected-angle technique; use the paralleling technique.
• Horizontal overlap of contacts — horizontal angulation error; re-angle the beam through the contact points.
• Movement blur — patient or tube head movement; stabilise and reassure.
• Too dark or too light — exposure factor or processing error; check settings and processing chemistry or sensor calibration.
• Bending of a phosphor plate — creases and artefacts; support the plate.

A repeat exposure is itself a dose to the patient and must be justified afresh; a technically imperfect image that still answers the clinical question should not be repeated.