MFDS Radiology and Radiography 2 — Questions and Answers
Question 1: On a dental panoramic tomograph (DPT/OPG), which anatomical structures are commonly misidentified as pathology?
- The mental foramen, incisive canal, maxillary sinus floor, and the ghost image of the opposite mandibular angle (Correct answer)
- Only the zygomatic arch can be misidentified
- No normal structures are ever confused with pathology
- The tonsils and adenoids are the only structures commonly misidentified
Correct answer: The mental foramen, incisive canal, maxillary sinus floor, and the ghost image of the opposite mandibular angle
Common normal structures misidentified as pathology on DPT include: the mental foramen (mimics periapical radiolucency of premolars), incisive canal (midline radiolucency), maxillary sinus (radiolucency over premolar/molar apices), nasal fossa (radiolucency over incisor apices), ghost images (contralateral structures projected to the opposite side), the submandibular fossa (radiolucency in the posterior mandible), and the external oblique ridge. Understanding the normal radiographic anatomy of the DPT is essential to avoid misdiagnosis.
Question 2: What is the bisecting angle technique for periapical radiography, and when might it be preferred over the paralleling technique?
- It involves bisecting the tooth in half for measurement purposes
- The X-ray beam is directed perpendicular to the bisector of the angle formed between the long axis of the tooth and the film/sensor plane; it may be preferred when palatal/floor of mouth anatomy prevents parallel placement of the sensor (Correct answer)
- It is a technique used only for panoramic radiography
- The film is placed at 45 degrees to the tooth and exposed from behind the patient
Correct answer: The X-ray beam is directed perpendicular to the bisector of the angle formed between the long axis of the tooth and the film/sensor plane; it may be preferred when palatal/floor of mouth anatomy prevents parallel placement of the sensor
In the bisecting angle technique, the film/sensor is placed as close to the tooth as possible (not necessarily parallel), and the X-ray beam is directed perpendicular to an imaginary line that bisects the angle between the long axis of the tooth and the plane of the sensor. This minimises image elongation or foreshortening. It may be preferred over the paralleling technique when anatomical limitations (shallow palate, torus, floor of mouth, small mouth) prevent parallel placement of the sensor behind the teeth. However, it is less geometrically accurate than the paralleling technique.
Question 3: A patient requires a radiographic assessment of the TMJ (temporomandibular joint). Which imaging modality provides the best visualisation of the articular disc and soft tissue components?
- Periapical radiograph
- MRI (magnetic resonance imaging) (Correct answer)
- Dental panoramic tomograph (DPT)
- Lateral cephalometric radiograph
Correct answer: MRI (magnetic resonance imaging)
MRI is the gold standard for imaging the TMJ soft tissues, particularly the articular disc (meniscus). It provides excellent soft tissue contrast without ionising radiation, allowing assessment of disc position (normal, anterior displacement with or without reduction), disc morphology, joint effusion, and bone marrow changes. Sagittal and coronal sequences with open and closed mouth views demonstrate disc dynamics. CBCT/CT provides superior bone detail for osseous pathology (e.g., condylar erosion, osteophytes), while MRI is preferred for disc and soft tissue assessment.
Question 4: What quality assurance measures should be routinely performed on dental X-ray equipment in the UK?
- No quality assurance is required for dental X-ray equipment
- Regular testing of tube output, timer accuracy, beam alignment, film processing (if applicable), image quality using test objects, and maintaining QA records as required by IRR17 and IRMER 2017 (Correct answer)
- Only annual servicing by the manufacturer is required
- Quality checks are only needed when the equipment is new
Correct answer: Regular testing of tube output, timer accuracy, beam alignment, film processing (if applicable), image quality using test objects, and maintaining QA records as required by IRR17 and IRMER 2017
UK regulations (IRR17 and IRMER 2017) require a written quality assurance programme including: acceptance testing of new equipment, regular testing of tube output consistency and kV accuracy, timer accuracy, beam alignment and collimation, film processor monitoring (sensitometric strips if using film), digital sensor/phosphor plate checks, image quality assessment using test objects, reject/retake analysis to identify systematic problems, and record keeping of all QA activities. Critical examination must be performed before first use, and routine QA testing should be carried out at intervals recommended by the radiation protection adviser.
Question 5: What is the typical effective dose of a dental panoramic tomograph (DPT/OPG) and how does it compare to other dental radiographic examinations?
- A DPT gives a dose of approximately 50 mSv, much higher than a chest X-ray
- A DPT gives an effective dose of approximately 5-25 microsieverts, equivalent to approximately 2-8 periapical radiographs, and much less than a medical CT scan (Correct answer)
- A DPT gives zero radiation dose as it uses ultrasound
- A DPT gives approximately 1 millisievert, equivalent to a full mouth series
Correct answer: A DPT gives an effective dose of approximately 5-25 microsieverts, equivalent to approximately 2-8 periapical radiographs, and much less than a medical CT scan
A DPT delivers an effective dose of approximately 5-25 μSv (microsieverts), depending on the equipment and patient size. This is equivalent to approximately 2-8 intraoral periapical radiographs (each approximately 1-8 μSv) or approximately 1-3 days of natural background radiation. For comparison, a small-volume CBCT scan delivers approximately 20-200 μSv, a full medical CT head approximately 2,000 μSv, and annual UK background radiation approximately 2,700 μSv. These comparisons help in justifying exposures under IRMER and explaining risk to patients.
Question 6: What are the radiographic features of a dentigerous cyst on a dental panoramic tomograph?
- A diffuse, ill-defined radiopacity surrounding an erupted tooth
- A well-defined, unilocular radiolucency with a corticated margin, attached to the cemento-enamel junction of an unerupted tooth and surrounding its crown (Correct answer)
- A multilocular radiolucency with a soap-bubble appearance not associated with any tooth
- A radiopaque mass replacing the normal tooth structure
Correct answer: A well-defined, unilocular radiolucency with a corticated margin, attached to the cemento-enamel junction of an unerupted tooth and surrounding its crown
A dentigerous cyst appears radiographically as a well-defined, unilocular radiolucency with a corticated (sclerotic) border, attached to the cemento-enamel junction of an unerupted tooth, enveloping its crown. The pericoronal radiolucency must exceed 2.5-3 mm to be considered pathological (normal follicular space is up to 2-2.5 mm). It may displace the tooth or cause resorption of adjacent teeth. The most commonly affected teeth are lower third molars, upper canines, and upper third molars.
MFDS Radiology and Radiography 2
On a dental panoramic tomograph (DPT/OPG), which anatomical structures are commonly misidentified as pathology?