Extraoral radiographic Techniques l Oral radiology MCQs

 

Extraoral radiographic Techniques ( Lateral Oblique View, Skull Radiography, Cephalometrics l Oral radiology MCQs for dental students 

Important points to remember in extraoral radiographic technique

1. The cranium encloses and protects the brain and is made up of eight bones, two of which, the parietal and temporal, are paired. The technical requirements for extraoral imaging include a detector, a grid and equipment capable of a long source-to-film distance.
2. Caudal difference: It is important to realise that there is an average caudal difference of approximately 7° to 10° between the OML and FH line.
3. Detector used: The most common detectors used in extraoral radiography are film based, however digital detectors, which can be either direct (charge couple device) or indirect.
4. Size of cassette: The most common size of extraoral rigid cassette used in extraoral radiography is 8 × 10 in. (20.3 × 25.4 cm); however, 24 × 30 cm and 18 × 24 cm cassettes can be used.
5. Grid: A grid comprises a meshwork of narrow lead foil strips arranged in a parallel pattern divided by an aluminium separation material.
6. The ALARA (as low as reasonably achievable) principle is universally accepted as governing the use of ionizing radiation in diagnostic imaging.
7. The use of a collimator, particularly in cephalometric radiography, restricts the area of the head exposed to radiation.
8. Effective dose: Effective dose ranges for film-based anteroposterior, posteroanterior and lateral skull radiography are reported to range from 8.8–25.4 mSv, to 8.2–27.3 mSv and 8.4–22.7 mSv, respectively. Evaluation of the image quality should include considerations of image density, contrast and sharpness and image accuracy.
9. PA views: Posteroanterior projections include the true PA, Waters’, modified Waters’, Caldwell and reverse Towne’s projections.
10. Occipitofrontal projection: Acceptable image criteria for occipitofrontal projection are that the petrous temporal bone is below the orbital rim on both sides (20° occipitofrontal).
11. Water projection: The Waters’ projection, also referred to as the ‘sinus view’, is the best posteroanterior projection to view the maxillary sinuses. Acceptable image criteria for water’s projection are that the petrous temporal bone is symmetrical and below the posterior recess of the maxillary sinus, and that they are parallel to inferior edge of the image.
12. Caldwell projection: The Caldwell projection is referred to as the ‘eye view’ as it demonstrates the margins and walls of the orbits better than any other view.
13. Submentovertex view: The submentovertex allows visualization of the base of the cranium including the occipital bone, the sphenoid and ethmoid sinuses, petrous ridge and mastoid sinuses of the temporal bone.
14. Lateral oblique view is one of the most enterprising and satisfactory techniques of having an extraoral view of the jaw.
15. Size of cassette used for lateral oblique view is 5 × 7 in.
16. The central rays of beam are directed at –15° to –20° vertical angulations. The beam should be kept perpendicular to cassette.
17. Exposure parameters can vary, but it is customary to use 65 kVp, 10 mA and ¼ s for medium speed screen and film.
18. For a patient whose intraoral view cannot be taken, it can be a good choice.
19. Gardener view is the lateral oblique view of the right and left side of the jaws in the same radiograph.
20. Broadbent and Brodie first demonstrated the use of the radiographic cephalometer. The source-to-object distance varies depending on whether a European (2 m) or an American (152.4 cm or 5 ft.) convention is used.
21. Three standardised radiographic projections are used in cephalometrics including the lateral, PA and axial radiographs.
22. Other than orthodontic use the lateral cephalometric image can also be used for the assessment of fractures involving the maxilla or cervical spine, the maxillary sinuses, localization of foreign bodies and anomalies involving sella turcica enlargement, such as pituitary adenomas or craniopharyngiomas.
23. The larger the distance between the cephalometric points, the lesser will be the error in angular measurements.
24. In cephalogram soft-tissue angles are less reliable than hard tissue angles.
25. FH plane best represents the natural orientation of the skull.
26. PTV plane is a vertical reference plane and it is constructed by drawing a line perpendicular to the Frankfort plane at the posterior margin of the pterygopalatine fossa.
27. Downs was the first to comprehensively describe the nature of the facial skeletal pattern of normal occlusion as well as apprise the relationship of the dentition and alveolar process to the facial skeleton.
28. The Tweed triangle is formed by the derived FH plane, the mandibular plane and the long axis of the lower incisor.
29. Bjork analysis was the first analysis to take into account the influence of cranial base on facial structure.
30. Harvold analysis is a skeletal analysis intended to demonstrate the changes in the maxilla and the mandible resulting from the usage of ‘functional appliance’.
31. The Ricketts bioprogressive analysis is divided into six fields (designated I–VI): denture, skeletal, denture base to skeletal, aesthetic problems, cranial and internal structures.
32. The Wits appraisal is a comparison of the apical dental base and was proposed as an adjunctive diagnostic aid to determine the anteroposterior disharmony independent of S and Na.
33. Tanner et al developed a technique that is currently recommended for use by most of the medical radiologists known as the TW2 and most recently TW3 method.
34. By far the most clinically applicable and orthodontically relevant method for skeletal maturity assessment is that developed by Fishman.