12 Extra Oral Radiography Flashcards
what are the extra oral views of the mandible?
- Postero-anterior mandible
- Lateral oblique of the mandible
- DPT
What are the extra oral views/images relating to the maxilla and the cranium?
- Lateral cephalogram (skull)
- Occipito-mental views (0, 10, and 30 degrees of the face)
- Sailography
what does the word oblique refer to in radiology?
the x-ray beam at an oblique angle to film/ object
- not perpendicular or parallel
What does PA stand for in extra oral radiographs
Postero-anterior view
- X-ray beam is going from Posterior to anterior
What does AP stand for in extra oral radiographs
Antero-posterior
- X-ray beam is going from anterior to posterior
What does OM stand for in extra oral radiographs
occipito-mental
If the x-ray beam is entering the patient at the occiput and exiting between the mental foramen, what view is this?
OM - occipito-mental
what are the 3 main positioning land marks for extra oral radiographs?
- radiographic base-line
- frankfort plane (anthropological base line)
- maxillary occlusal plane
What is the main positioning land marl in extra oral radiographs?
radiographic base line
what does the radiographic base line mark?
represents the base of the skull
- line from the outer canthes of the eye to thee external auditory meatus
what positioning landmark is used in DPTs
the Frankfort plane
where is the frankfort plane?
inferior orbital margin to the upper border of the EAM (external auditory meatus)
where is the maxillary occlusal plane?
from the ala of the nose to the tragus of the ear
what positioning landmark is used a lot in cone beam CT
maxillary occlusal plane
what equipment do you need for extra-oral radiography?
- Skull unit/ cephalometric unit, with high intensity, highly penetrating beams
- Image receptor, which will either be a cassette with intensifying screens and film, or a digital system cassette containing a phosphor plate
- Anti-scatter grid
what is Compton scattering?
when weaker x-ray photons are deviated off track as they do not posses enough radiation to pass all the way through to the receptor and make a useful interection
is there more or less scattering in skull radiography?
more, as the field of x-ray required to cover the entire skull is much larger
what is the result if scatter radiation is left unchecked?
the scatter radiation will add to background fog and produce a greyer image lacking in contrast
what is used to combat background fog caused by photon scattering?
an anti-scatter grid
what makes up an anti-scatter grid?
alternating strips of lead and plastic
what is the purpose of an anti-scatter grid?
to stop photons scattered in patients from reaching the receptor and consequently degrading the film
sometimes you can see the lead strips from the anti-scatter grid on your radiograph, is this more likely to be seen on a fixed or moving anti-scatter grid?
fixed because on a moving grid the grid oscillates very quickly from side to side which prevents these lines from appearing, as the lead strips are constantly moving
what are the 3 things to consider when positioning a patient for extra-oral radiographs
- the position of the patient relative to the film
- the position of the x-ray beam relative to the patient
- the angle of the x-ray beam relative to the patient
for a lateral extra-oral, what angle should the inter-orbital line and the median saggital plane to the film?
Infra-orbital line at 90 degrees to the film
Median sagittal plane is parallel to the film
what is the main indication to request a PA mandible?
fractures
it will show any medio-lateral displacement of the mandible at the fracture site
apart from fractures, what are other indications for a PA mandible view?
cysts and malignancy - causing medio-lateral expansion or bone destruction
What does a PA view of the mandible give good and limited visualiation of?
good visualisation - posterior body and ramus
limited visualisation - head/neck of condyle and midline tends to be obscured by the spine
what are PA mandible usually requested in conjunction with?
a DPT
Why do we prefer to take a mandible film PA?
- It reduced magnification of the facial structures on the receptor (you always want the object as close to the receptor as possible)
- It also reduced radiation dose to the eyes (lense of the eye is quite radiosensitive so you do not want high energy x-ray photons straight into the patients eye)
how do you want to position your patient for a PA mandible?
patient faces the film
nose and forhead touch the film holder - this bringds the radiographic baseline 90 degrees to the film
the central x-ray should be in the midline of the patient at the height of the mid ramus, which is roughly just below the occiput
how do we field the midline of the patient both from the front and the back?
front
midline - usually between the eyes
back
midline - external occipital protuberance
true or false, the tip of the nose is the midline land mark?
false - tip of the nose usually deviates to a side
in what plane should you oblique the angle of the x-ray for the mandible?
vertical plane to see the full one side
why cant you take lateral radiographs of the mandible
you would take an image of both sides on top of each other and the superimposition would make it almost impossible to make sense of anything
what are the main indications for a lateral oblique mandible view?
- fractures of body, ramus and condyle
- pathology (e.g. cysts)
- assessments of wisdom teeth (although would always request a DPT in the first instance as it is much better quality)
- dental assessments in special needs patients
- caries in children (who cannot tolerate bitewings)
what are the 3 technique to take a lateral oblique mandible?
- isocentric positioning using a skull unit (queerying a fracture or pathology in the mandible)
- dental tube in vertical angulation (visual dental pathology)
3 dental tube in horizontal angulation (visualise dental pathology)
how would you carry out a lateral oblique mandible using the isocentric skull unit?
the patient would be supine
the skull unit has been turned so that it is horizontal to the floor
then angled 25 degrees so the x-ray tube moves towards the patients shoulders
the patient then turns the top of his head to the receptor
how would you carry out a lateral oblique mandible using a dental tube in the vertical tube angulation?
the patient holds the receptor against and parallel to the area under investigation
the tube head is positioned beneath the lower border of the mandible that is NOT under examination
aim the tube slightly upwards, towards the teeth under investigation
what are the disadvantages of the lateral oblique mandible using a tube head in the vertical angulation?
angulation can cause vertical distortion of the teeth
maxillary teeth are not always shown clearly
how do you take a lateral oblique using a tube head in the horizontal angulation
patient holds the cassette against and parallel to the area under examination
tubehead is positioned aiming along the occlusal plane just below the ear towards the maxillary and mandibular teeth that are to be examined
this aims through the radiographic key hole
where is the radiographic key hole
triangular space between the back of the ramus and the cervical spine
what are the disadvantages of the lateral oblique mandible using a tube head in the horizontal angulation
x-ray beam may not pass directly between the contact points of the teeth therefore causing them to be overlapped on the film
what is cephalometric radoigraphy used for in orthodontics?
to assess the relationship of the teeth to the jaws and the jaws to the rest of the facial skeleton
gives a standardised and reproducible form of skull radiograph
what are the indications for cephalometric radiographs
orthodontic assessment and pre-orthagnathic surgery
what are cephalometric radiographs taken using
cephalostat unit
- usually attached to a DPT unit
during a cephalometric radiograph, why is the patient positioned with ear rods?
the patient is positioned with ear rods to give a standardised positioning, this is important because the image must be reproducible
to take a lateral ceph, how should the patient be positioned relative to the cephalostat
to position a patient for a lateral ceph, the mid saggital plane should be parallel to the receptor. The frankfurt plane should be horizontal and the centre of the beam aimed at the external auditory meatus. The patients teeth must be in occlusion
what radiograph do you give the patient lead protection?
lateral cephalogram
the exposure factors required and consequent dose are greater
what kind of lead protection do you give for a lateral ceph?
a thyroid sheath
the thyroid is one of the more radiosensitive anatomical structures and should be protected
how do you achieve minimum magnification?
to keep magnification to a minimum you want a long source to patient distance and a short patient to receptor distance
what do clinicians use in lateral cephs to calculate any magnification of the anatomy in the final image?
placing a magnification rod/ ruler om the image
doing this is necessary for surgical planning
how long roughly should the distance between the object and unit be in a lateral ceph
1.5 - 2 metres
It is also important to see soft tissues on a lateral ceph. The exposure required to penetrate a lateral face/skull is so great in normal circumstances the beam would not be attenuated at all by soft tissues (therefore not visible). How is this over come?
this is over come by using an aluminium filter, positioned over the anterior part of the face
This helps to attenuate (slightly absorb) the beam is the anterior facial region
This allows visualisation of bone and soft tissue on the one film
how is a patient positioned for a PA ceph
the patient is positioned with the rods in their ears, with the radiographic baseline parallel to the floor
the x-ray beam is aimed in the middle at the level of the external auditory meatus
what is the main indication for x-raying facial bones (occipitomental views)
following trauma to rule out fracture
the most common fractures are of the zygoma, le fort fractures and orbital blow out fractures
To see all the bones of the face different angulations of the x-ray beam are required, what are the standard views of the face?
OM0
OM10
OM30
How do you position a patient for occipito-mental views (facial bone x-ray)
the patient faces the receptor with their nose and chn touching the holder - this means the radiographic baseline is at 45 degrees to the film
aim the centre of the beam to the midline of the patient at the level of the base of the nose - OM0degrees
the standard views are taken with a 10 or 30 degree downward angle - the beam facing the patients feet
what are the standard views for facial bone x-rays
OM10 and OM30
what does increasing the angulation of the beam in OM/ facial bone x-ray’s do?
Increasing the angulation of the beam projects the dense bones of the skull base down away from the facial structures.
It improves the view of the zygomatic arch (especially in OM30)
when views skull radiographs, what is it important to be aware of (that could be mistaken for fractures)
sutures
what does a * on a radiograph if in the max fax department indicate?
a fracture
Why should cone beam computed tomography (CBCT) not be considered as a first form of imaging?
The dose to the patient is much greater
