Dental Radiology and Diagnostic Imaging
This domain tests justification and optimisation of radiographic examinations, the physics behind image quality, and the interpretation of normal and abnormal appearances on intraoral, panoramic and cone-beam images.
Physics and dose
X-rays are produced when accelerated electrons strike a metal target. Tube voltage (kVp) governs beam quality and penetration and therefore subject contrast; tube current and exposure time (mAs) govern quantity and density. Filtration of at least 2.5 mm aluminium removes low-energy photons, and collimation limits the field. Dose is measured in gray for absorbed dose and sievert for equivalent or effective dose; a panoramic radiograph delivers far more than a single intraoral film, and cone-beam CT more again.
Justification and optimisation
The governing principles are ALARA/ALADA and, in European regulation, the requirement that every exposure be justified by a clinical question and optimised. There is no justification for routine radiographs at fixed intervals; the recall interval should be set by caries risk and clinical findings. Use rectangular collimation, thyroid shielding where appropriate, and digital receptors with the lowest dose that answers the question. Pregnancy is not an absolute contraindication to a necessary dental radiograph, but justification becomes stricter.
Image interpretation
- Periapical: use the paralleling technique; the bisecting-angle method distorts. Look at the periodontal ligament space, lamina dura, apical region, caries and restorations.
- Bitewing: best for interproximal caries and alveolar crest height.
- Panoramic: has a focal trough within which structures are sharp, magnification of roughly 20–30%, and produces ghost images of structures on the opposite side, appearing higher.
- CBCT: three-dimensional, voxel-based, indicated for impacted teeth, canal proximity, endodontic anatomy, implant planning and complex trauma - not for caries detection. Beam-hardening and metal artefacts degrade images.
Worked example
A 24-year-old is referred before lower third molar surgery because the panoramic image shows the roots overlapping the mandibular canal. Reasoning: the panoramic radiograph suggests, but cannot prove, proximity. A small-field CBCT is justified because it changes surgical risk assessment and may indicate coronectomy; a CBCT purely to look for caries would not be justified.
Traps that cost marks
- Requesting a panoramic radiograph for every new patient without a clinical indication.
- Confusing ghost images with genuine pathology.
- Using CBCT to diagnose caries.
- Forgetting that the lamina dura is lost in periodontal disease and in some systemic conditions, and that widening of the ligament space may be the earliest sign of apical pathology.