Oral and Maxillofacial Radiology
Radiology in this exam tests image formation physics, technique error recognition and the diagnostic use of radiolucency and radiopacity patterns, not equipment trivia.
Projection geometry
- Paralleling technique: receptor parallel to the long axis and central ray perpendicular; least distortion, preferred whenever the patient tolerates it.
- Bisecting technique: central ray perpendicular to the bisector of the receptor and tooth axis; too little vertical angulation elongates, too much foreshortens.
- Panoramic imaging uses 70-90 kVp and a curved focal trough; magnification is roughly 20-30% and is not uniform, so it is unsuitable for measuring or for caries diagnosis.
- Bitewings are for proximal caries and crestal bone; periapicals for apices.
Localisation and artefacts
- SLOB: an object moving in the same direction as the tube shift is lingual; opposite means buccal.
- Ghost images in panoramics appear on the opposite side, higher and blurrier; the hard palate, opposite ramus and cervical spine are common sources.
- CBCT: voxel size governs spatial resolution; metal produces beam hardening and streak artefact; grey values are not reliable Hounsfield units; keep the field of view as small as the task allows.
- MRI for soft tissue extent; contrast CT for cortical bone erosion.
Worked example
An impacted maxillary canine appears mesial to the lateral incisor root on a periapical. A second periapical taken with the tube shifted distally still shows the canine mesial. It moved in the same direction as the tube, so it lies palatal. A palatal canine needs a different surgical approach than a labial one, so the localisation changes the operation, not just the report.
Traps
Reversing elongation and foreshortening; assuming panoramic magnification is constant; calling a ghost image a real lesion; forgetting that CBCT dose depends mainly on field of view and exposure settings; treating idiopathic osteosclerosis or condensing osteitis as disease requiring surgery.