Occlusal radiography and the bisecting angle technique Flashcards
Where is the image receptor in occlusal radiographs?
- Image receptor in occlusal plane
What are the dimensions of the image receptor used for occlusal radiographs?
- Image receptor 7x5cm (smaller in small mouths)
What is an oblique occlusal radiograph similar to?
- Similar to a large periapical
- Because it is a bigger end receptor there are more teeth on it than a periapical but in terms of the way it looks it is very similar
What kind of sensors are sued for occlusal radiographs?
- Film or digital sensors (PSP - photostimulable phosphor) available
What sensors are used in the dental hospital for occlusal radiographs?
- Phosphor plates
What is a cross-sectional occlusal radiograph?
- Called this because the view of the teeth you get is as if the x-ray beam is going right up through the long axis so have a cross sectional view
On which jaw would you take a cross-sectional occlusal radiograph?
Only do this on the lower jaw
Are PSP’s direct or indirect digital sensors?
Indirect
Why do we not use direct digital sensors for occlusal radiographs?
-They would be very expensive to make and would not be a market for that
In which types of radiographs do we use phosphor plates? (3)
- Occlusals
- Periapicals
- Bitewings
- Need to be very careful in how we handle them (phosphor plates)
What are the indications for taking an oblique occlusal radiograph? (4)
- Periapical type assessment where periapicals are not possible (e.g. if patient has trismus)
- Pathology too large to be seen on a single periapical (would likely use Cone Beam CT now though)
- Trauma - fractures to teeth and alveolus
- Localisation using parallax (+ panoramic, other occlusal or periapical) (useful when we are trying to work out where structures that we can’t actually see are in relation to each other or a structure that we can see)
What is the ideal projection geometry for a radiograph? (4)
- Image receptor and object in contact and parallel
- Parallel beam of x-rays
- X-ray beam perpendicular to object plane and image receptor
- Image size identical to object size
This does not happen
- Cannot have image receptor and object completely in contact and parallel
- Don’t have a parallel beam of x-rays
- We can decide how to direct our x-ray beam, what it is perpendicular to but we never get an image size that is identical to the object because of that divergent beam of x-rays (always get some magnification)
What are the problems with ideal projection geometry? (4)
- Image receptor and object NOT in contact
- Beam of x-rays NOT parallel
- X-ray beam central ray MAY OR MAY NOT BE perpendicular to object plane and image receptor
- Image size NOT identical to object size DUE to MAGNIFICATION - DIVERGENT BEAM
One solution to projection geometry problems is the BISECTING ANGLE TECHNIQUE. Explain the properties of this technique? (4)
- Image receptor and object partly in contact but not parallel
- Image receptor and object close together at crowns, but apart at apices
- Still use long x-ray focus-skin distance as requires for machines operating at 60kV and over (@ least 20cm)
- Can be done without image receptor holders
Can the bisecting angle technique be done without image receptor holders?
- Yes
In the bisecting angle technique, how far apart is the image receptor and object?
- Touching at one end but as you go further to the apex they get further apart
- How far apart they are is dependent on which part of the mouth you are looking at
- Place where they are least far apart is the lower molar region
In the bisecting angle technique the image receptor and tooth touch at the crown and are apart at the apex. If the x-ray beam is at 90 degrees to the long axis of the tooth what will the image be like?
- Elongated image
In the bisecting angle technique the image receptor and tooth touch at the crown and are apart at the apex. If the x-ray beam is at 90 degrees to the plane of the image receptor what will the image be like?
- Short image (foreshortened)
When we talk about the x-ray beam. What are we actually thinking about?
- We are thinking about the imaginary central ray, think of 1 ray and how we are directing it in relation to either the tooth or the image receptor
Because of the law of equal triangles, if we actually bisect the angle formed by the long axis of the tooth and the plane of the image receptor and draw a line in which bisects that and we direct our x-ray beam at right angles to that, apart from the inherent magnification we always get we will get an accurate image
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What position will a patient be in when taking a periapical or bitewing radiograph?
- Sitting up (but can do it lying down)
What position will a patient be in when taking an occlusal radiograph?
- Will definitely be sitting up
What is the correct vertical angle for the bisecting angle technique?
- X-ray beam at 90 degrees to line bisecting angle formed by long axis of tooth and plane of image receptor - correct sue to identical triangles
By hoe many mm is the image receptor positioned beyond the tooth edge for an occlusal radiograph?
- Image receptor positioned with 2-3mm beyond tooth edge
How is the vertical angle judged for an occlusal radiograph using the bisecting angle technique?
- Judged by eye
The vertical angle may need to be adjusted to adapt to incisor angulation. If the tooth is proclined. How will we adjust the angle?
Will increase the angle
The vertical angle may need to be adjusted to adapt to incisor angulation. If the tooth is retroclined (a bit more upright). How will we adjust the angle?
- We will decrease the angle
What do we want the head position to be like when taking an occlusal radiograph? (3)
- Oblique occlusals require occlusal plane (of jaw being examined) to be horizontal
- Altered by degree of mouth opening
- Required external soft tissue points to assess if correct
For oblique occlusals how do you want the maxilla to be positioned?
- Ala-tragus line horizontal (parallel to the floor)
- This is for a seater upright patient
What is the ala-tragus line?
- From fleshy bit on the side of the nose to the little cartilaginous bit at the front of your external auditory meatus
- Need to see this when positioning the head for an oblique occlusal so might need to ask patient to move their hair if you can’t see this
For oblique occlusals how do you want the mandible to be positioned?
- Corner of moth-tragus line horizontal (parallel to the floor)
- This is for a seated upright patient
- To get mandible horizontal need to slightly tip head back, need to support the head in the position you put the patient in
What do you want the head position to be like for maxillary occlusals?
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What do you want the head position to be like for mandibular occlusals?
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For maxillary occlusals what do you want the upper teeth to be like?
- Upper teeth stable
For mandibular oblique occlusals what do you want the teeth to be like?
- Teeth apart due to image receptor
For mandibular oblique occlusals do you want the head to be tilted?
- Yes want it to be tilted back
What are storage phosphor plates?
- Multi use sensors
What is used to protect phosphor plates when using them?
- Protected by plastic cover
- Protected from tooth marks by cardboard (single use) or plastic (multi-use)
How should you insert a phosphor plate into the plastic cover?
- Put the sensor into plastic cover so writing on the sensor can be seen on the clear side - means side of plastic cover that is black will have the phosphor side of the sensor against it - this is what is going to receive the x-rays
What must the black side of the cover on a phosphor plate face?
- Must face the x-ray source
There is a dot on all of the phosphor sensors. Where should this dot be placed?
- The dot is going to end up showing on the image and you should slide this in so that the little dot that is in one corner goes in first so that it is visible
What is the upper left image?
Outer cover
What is the top right image?
Hinged cardboard
What is the bottom left image?
Active surface
What is the image on the bottom, in the middle?
Surface with writing with little dot
What is the bottom right image
- Piece of plastic
How do we protect the phosphor plate sensor? (4)
- Check correctly positioned in plastic cover
- Place inside hinged card protector
- Check dark surface of cover facing up for maxillary teeth, down for mandibular teeth
- Position in mouth, hinged end first
Where should the dark surface of a phosphor plate sensor be facing for maxillary teeth?
- Should be facing up
Where should the dark surface of a phosphor plate sensor be facing for mandibular teeth?
- Should be facing down
How can we ensure that the sensor is kept still and in the correct position? (4)
- PAtient to bite gently to hold protected sensor still
- Remind them to keep biting gently
- Watch them to ensure no chewing action
- Plastic protectors can be used as an alternative to card
When you line up an x-ray tube head for occlusals, bitewings and periapicals, want to check it from 2 directions. What are these?
- Want to check it from the side (where you will be measuring vertical angulation)
- Also want to check it from the front (so if doing anterior occlusal - should be symmetrically in the middle of the nose, if doing one round to the side it would be further round|)
What should the horizontal angle when taking an radiograph be?
90 degrees to line of arch to avoid overlaps
What is the maxillary centring point for a periapical radiograph?
- On ala-tragus line
What is the mandibular centring point for a periapical radiograph?
1cm above the lower border of the mandible
What is the maxillary centring point for an oblique occlusal radiograph?
1cm above the ala-tragus line
What is the mandibular centring point for an oblique occlusal radiograph?
- Through the lower border of the mandible
What is a centring point?
- Where your imaginary central ray is going to go in
What is the horizontal angle?
- Angle that the beam is coming in, in relation to teeth - this is straight forward: which ever group of teeth you are looking at it should always be at 90 degrees to the line of the arch
Why are the centring points slightly different for periapicals and oblique occlusals?
- The difference is because of the different size of sensor
- Essentially 1cm further away from the centring point if doing occlusals compared to periapicals
Where does the ala-tragus line start at?
- Starts at the ala
We don’t do many oblique occlusals, however if we were to do one where in the mouth would it be?
- Likely to be an upper anterior (standard occlusal)
- Angle used for that is 60 degrees
As we go further back in the mouth we tend to drop the vertical angle by about 5 degrees every time. Why is this?
This is because the teeth become more and more upright as you go further back to molars
What is the vertical angle for the upper anterior (standard) region for oblique occlusals?
60 degrees
What is the vertical angle for the upper occlusal centred on canine region for oblique occlusals?
55 degrees
What is the vertical angle for the upper occlusal centres on premolar region for oblique occlusals?
50 degrees
What is the vertical angle for the upper occlusal centred on molar region for oblique occlusals?
45 degrees
What is the vertical angle for the lower anterior occlusal region for oblique occlusals?
40 degrees to the occlusal plane
What is the vertical angle for the lower occlusal centred laterally region for oblique occlusals?
35 degrees to the occlusal plane
What is the standardised head position when taking oblique occlusals? (2)
- Mid-sagittal plane perpendicular to the floor (e.g. patients head must be straight)
- Occlusal plane correct for arch
The orientation of the image receptor is dependent on the size of the mouth and patient tolerance. What is usually the most comfortable orientation?
- Long axis front to back is usually the most comfortable
What size of image receptor would you use for a true/cross-sectional occlusal radiography?
- Occlusal size image receptor or periapical size
What view do you want to plan when taking a true-cross-sectional occlusal radiography?
- Plan view when beam is through long axis of tooth
what is the only jaw we can do true-cross-sectional occlusals on?
- Only do for lower jaw
Why cant we do true-cross-sectional occlusals on the upper jaw?
- Can’t do on the upper jaw as can’t get x-ray source close to the patient and have to go through many more structures which causes higher radiation and poorer imaging so just don’t do it
What are the indications to take a mandibular true occlusal radiograph? (5)
- Detection of submandibular duct calculi
- Assessment of bucco-lingual position of unerupted teeth - unless advanced imaging indicated (cone beam CT)
- Evaluation of pathological bucco-lingual expansion
- Horizontal displacement of fractures
- cone beam CT may be used nowadays if available
What is concentric growth of submandibular duct calculi?
- Central bit is much more opaque so has been mineralised for longer
- Starts in middle and keeps developing more and more layers around it
What is a submandibular duct calculi that conforms to the duct ?
- Conforming to duct shape
- Anterior bit is more mineralised as it is more opaque
What is the name for 2 teeth that are unerupted and almost touching?
- Kissing teeth
Where should the image receptor be for mandibular true occlusals?
- Image receptor transverse in occlusal plane OR lengthwise over region of interest
What position should the patients head be in when taking a mandibular true occlusal?
- Head tipped as far back as is comfortable
Where should the x-ray beam be directed for a mandibular true occlusal?
- X-ray beam directed at 90 degrees to image receptor in midline OR through region of interest
Why, when taking mandibular true occlusals do we want the patients head tipped as far back as is comfortable?
- Problem is that we want x-ray tube to be at the patients chest
- Only thing we can do is move the patients head so we do that by asking the patient to tip their head back as far as possible but make sure they are still comfortable
- image receptor will be at different angles depending on how far back you have gone but hopefully you’ll be able to have the x-ray tube just on front of their chest and where you want it to be
- Tricky if looking at posterior mandible as cannot go straight back, have to also turn the head
