Radiology and Diagnostic Imaging
What this domain tests
Items test image formation and quality, technique and interpretation, dose and protection, and the selection and limitations of panoramic, CBCT and intraoral imaging.
Physics and technique
- Kilovoltage controls contrast and penetration; milliamperage and time control quantity. Filtration of at least 2.5 mm aluminium and rectangular collimation reduce dose.
- Paralleling technique gives less distortion than bisecting-angle; a film holder and beam alignment reduce retakes.
- Panoramic imaging has 20-30% magnification, a focal trough that structures must lie within, ghost images of the opposite side, and cervical spine superimposition; it is poor for caries and fine bone detail.
- CBCT uses a cone beam and voxels, giving three-dimensional bone assessment at higher dose than a panoramic; artefacts arise from metal and movement. Justify every scan.
Interpretation
- Caries: radiolucency at the enamel-dentin junction; bitewings are best for proximal lesions.
- Periapical pathology shows loss of lamina dura and apical radiolucency; widening of the PDL may be early.
- Bone patterns: ground glass in fibrous dysplasia, sunburst in osteosarcoma, soap bubble in ameloblastoma, cotton wool in Paget disease.
- Localisation uses the SLOB rule with two angled views: the object moving in the same direction as the tube head is lingual, the object moving opposite is buccal.
Dose and protection
- Principles: justification, optimisation, ALARA/ALADA. Occupational limit 20 mSv per year averaged; public 1 mSv. Use lead apron and thyroid collar, and monitor staff with dosimeters.
- Approximate effective doses: intraoral radiograph a few microsieverts, panoramic around 10-20 µSv, CBCT tens to hundreds of µSv depending on field of view.
Worked example
A radiograph shows a radiopaque object near the apex of 25, and a second view taken with the tube head shifted mesially shows the object moving distally on the image. Reasoning: opposite movement means buccal. The object lies buccal to the root, which changes the surgical approach; a same-direction shift would have placed it lingual or palatal.
Traps
Using a panoramic film to diagnose proximal caries; forgetting magnification on panoramic images; confusing same and opposite movement in the SLOB rule; repeating an exposure before checking positioning and processing errors.