Extra Oral Views - Lateral Cephalometric Flashcards
principles of extra-oral radiography
- X-ray source outside the patient
- image receptor outside the patient
- digital CCD/CMOS and storage phosphor options
- previously indirect X-ray film used with intensifying screens
- patient positioning critical
- X-ray beam and image receptor related to patient
reference lines and planes
mid-sagittal plane (MSP) (green)
inter-orbital/pupillary line (green)
orbito-meatal line (OM line) = Radiographic Baseline (RBL): outer canthus of eye to centre of EAM (yellow)
Frankfort Plane: superior border EAM to lowest point of infra-orbital rim
- 10º difference between RBL and FP
4 different maxillofacial views
- postero-anterior
- antero-posterior
- orbitomeatal (line) and occipitomental (view)
- submentovertex
PA
postero-anterior (beam direction)
AP
antero-posterior
OM
orbitomeatal (line) and occipitomental (view)
back of head to chin
SMV
submentovertex (view)
underneath chin to top of head
2 types of lateral radiography
true
oblique
true lateral radiography
film and MSP are parallel and X-ray beam is perpendicular to both
Oblique lateral radiograohy
film and MSP are not parallel
X-ray beam is not perpendiculat to either, but oblique to both
less used
3 lateral views
- Lateral cephalometric radiograph (Lateral ceph.)
- Lateral Oblique (mandible) (LOJ) only one side of pt
- Bimolar (both sides on one receptor)
rods position in lateral cephalometric view
rods in EAM
rods position in PA cephalometric views
rods in EAM still
but pt facing image receptor - turn 90o
cephalometric radiography
- Standardised and reproducible form of skull/facial bones radiography
- Used extensively in orthodontics
- lateral and PA projections
- Orthognathic surgery – replaced by CBCT*
- (Implants, but largely replaced by cone beam CT)*
lateral cephalometric radiography
True lateral view of facial bones, base of skull and upper cervical spine.
Also shows paranasal sinuses and nasopharyngeal soft tissues
- Whole mandible, base of skull – how the relate
- Soft tissue – nose, lips, chin
- Scale
- Upper cervical spine
- Sinus – maxillary , sphenoid and ethmoid
- Pharyngeal soft tissue – posterior border and upper surface of tongue, soft palate
guidelines for orthodontic radiographs
BOS Orthodontic Radiographs Guidelines, 2008 & 2015
updated in 4th re CBCT indications
ISBN 1 899297 09 X
Ref. to CBCT in 2008, Full section in 2015
pdf available on-line:
cephs for who?
(4)
- patients with skeletal vertical or antero-posterior discrepancy – judged clinically prior
- requiring fixed or functional appliance therapy, for labio-lingual movement of incisors
- requiring orthognathic surgery in addition to orthodontics (if doing CBCT, don’t do both)
Flow charts pp 20 and 21 (pre-treatment) – unchanged in 2015 since previous edition (10-18years or above and below)
2 indications for lateral cephalometry
Orthognathic Surgery
- Pre-op assessment and post-op review
- Neurovasxular canals
- How bones moved - success
Implant planning - historically
- Anterior mandible - cross sectional image – implant should go through symphysis but actually crosses in – causing bother as alignment wrong
Both of these often superseded if cone beam CT available
cephalometric analysis
- often traced or digitised
- reference lines and planes
- direct digital techniques now available
equipment for ceph
- direct or indirect digital film in casette
- distances
- nasion marker
- magnification scale
- automatic facial contour in direct digital machines (soft tissue)
- heigh and width adjustment
- thyroid collar
direct or indirect digital film in cassette
for lat cephs
Cephalostat (free-standing or attached to panoramic machine)
- ear rods
- CCD/CMOS sensor or cassette holder (phosphor plate or intensifying screens)
- Historic - anti-scatter grid - but higher dose to patient – don’t need increased detail so not justified - not at GDH&S
distances
for lat ceph
Source to patient’s MSP - 152.4cm (5 feet) in traditional equipment
Image receptor to MSP – manufacturer dependent, fixed or adjustable
- in GDH&S fixed
distance from source to pt MSP in lat ceph
152.4cm (5 feet) in traditional equipment
image receptor to MSP distance in lat cep
manufacturer dependent
can be fixed or adjustable
in GDH&S it is fixed
effect of anode-object distance on magnification
Short anode-object distance
- Double images – larger is closer
Long anode-object distance
- Desired – reduce magnification and less different between right and left side
- long reduction in object magnification*
- short get considerable magnification*
collimation on lat ceph
height and depth of field of view/depth (adjust where triangle is) or adjustable, by programme or visual
- shine light through to see where it is on pt
- stainless steel – not able to get photons through*
collimation impact on effective dose in µSv
- 6 – with collimator
- 0 – without collimator
* 50% substantial (smaller field of view = less dose)
colliamtion on current digital cephs in GDH&S
17cm (reduced height) covers enough for most
- Larger head may need a greater height
May need to increase depth e.g. prognathic mandible
can expose whole face of skeleton and cranium – not needed mostly
nasion marker in lat cephs
Perspex,
brings to rest on pt – keep still, scale to get magnification factor in that plane
Automatic Facial Contour in direct digital machines
for soft tissue
OR aluminium wedge filter - ideally at tube head (before going through pts better than after)
how can collimation be adjusted in lat cephs
heigh and width adjustment
OR triangular lead collimator in older machines
position for lateral cephs
- Select for Lateral Ceph, and press button to line up X-ray tube head and cephalostat with receptor
- Hinge nasion rest up and sideways
- Thyroid collar on
- Prevent exposure to radiation that is not needed
- Frankfort plane horizontal – use light
- MSP vertical and parallel to cassette (head straight)
- MSP correct distance from cassette if adjustable
- Teeth together – in centric occlusion or as requested
- Ear rods in EAM – move symmetrically
- Nasion support in place
thyroid collar
prevent exposure to radiation that is not needed
esp in growing pts
programme selection for lat cephs
according to patient’s size and clinical requirement, OR move triangular collimation
adjust field of view
how to capture soft tissues in lat cephs
Automatic exposure adjustment (facial contour), OR aluminium soft tissue filter, preferably pre-patient
why have fixed distances
subsequent images will always be able to be directly comparable
oblique lateral radiography
film and MSP not parallel
- X-ray beam not perpendicular to either MSP or film*
extra-oral view of jaws - right and left sides taken separately
uses dental or E/O X-ray set
limited use now due to panoramic radiography
indications for oblique lateral radiography
Generally same as for panoramic radiography, but particularly when:
- panoramic not available or possible e.g. handicapped patient, not able to still - quicker
