12 - Extra Oral Radiography Techniques

Question 1

extra-oral views: images relating to the mandible?

Answer 1

• postero-anterior mandible
• lateral oblique of mandible
• DPT

Question 2

extra-oral views: images relating to the maxilla and cranium?

Answer 2

• lateral cephalogram (skull)

- occipito-mental views

Question 3

describe the following positioning landmarks:

• radiographic base line
• frankfort plane
• maxillary occlusal plane

Answer 3

• line from outer canthus of eye to EAM
• infraorbital border to upper border of EAM
• ala of nose to tragus of ear

Question 4

scatter radiation:

• when x-rays pass through tissues, some are scattered - what is this effect called?
• in skull radiography, what increases the scatter produced?
• more scatter contributes to?
• how does this affect the resulting film?

Answer 4

• compton scattering
• increase in field of beam produces more scatter
• background fog
• greyer film, lacking contrast

Question 5

anti-scatter grid:

• what does it do?
• how does it affect the film?
• what is it made of

Answer 5

• stop photons scattered in patient from reaching the film
• avoids degrading image and reducing contrast
• narrow strips of lead alternating with plastic

Question 6

postero-anterior mandible radiograph - indications?

Answer 6

• fractures (shows medio-lateral displacement)
• cysts and malignancy causing medio-lateral expansion or bone destruction
• good visualization of posterior body & ramus
• limited visualization of head/neck of condyle
• midline can be obscured by spine

Question 7

postero-anterior mandible radiograph - why is it used?

Answer 7

• reduced magnification of facial structures

- reduced dose to eyes

Question 8

lateral oblique jaw radiography - indications?

Answer 8

• fracture of body, ramus and condyle
• pathology e.g. cysts
• assessment of wisdom teeth
• dental assessment in special needs patients
• caries in children (those who cannot tolerate bitewings)

Question 9

lateral oblique jaw radiography - vertical angulation technique disadvantages?

Answer 9

• angulation can cause vertical distortion of the teeth

- maxillary teeth are not always shown clearly

Question 10

lateral oblique jaw radiography - techniques?

Answer 10

1. isocentric positioning using skull unit
2. dental tube with vertical angulation
3. dental tube with horizontal angulation

Question 11

anatomical features of lateral oblique mandible radiography?

Answer 11

• coronoid process
• condylar head
• condylar neck
• cervical spine
• ramus of mandible
• angle of mandible
• hyoid
• body of mandible
• mental foramen

Question 12

cephalometric radiography: indications?

Answer 12

• orthodontic assessment

- pre-orthognathic surgery

Question 13

lateral ceph: positional requirements?

Answer 13

• mid saggital plane parallel to film/receptor
• frankfort plane horizontal
• center of xray beam aimed at EAM
• teeth in occlusion

Question 14

cephalometric radiography:

• how is magnification kept to a minimum?
• why should ceph films have a magnification rod?

Answer 14

• by having a log focus to patient distance and a short patient to film distance
• this allows for the calculation of any magnification that has occurred

Question 15

aluminium filter:

• placed where?
• designed for?
• this allows what?

Answer 15

• positioned over anterior part of the face
• designed to attenuate beam in the anterior facial region
• allows visualization of bone and soft tissue on one film, despite relatively high exposure factors needed for facial bones

Question 16

occipito-mental views - indications?

Answer 16

• viewing facial bone fractures: zygoma, Le Fort and orbital blowout
• pathology: limited use in sinus disease. only used occasionally
• different angulations can be used to view different areas of complex anatomy

Question 17

occipito-mental views: how does increasing angulation change the image?

Answer 17

• projects dense bones of the skull base away from facial structures
• improves view of zygomatic arch
• gives different perspectives: may be useful for evaluation of bony displacement