Inrta-Oral techniques paralleling Flashcards
What are the conditions for ideal projection geometry? (4)
- Image receptor and object in contact and parallel
- Parallel beam of x-rays (coming towards the patient and image receptor)
- x-ray beam perpendicular to object plane and image receptor
- Image size identical to object size (this would happen if all of the above happen)
- But does this happen?
Can the conditions for ideal projection geometry happen?
- No, this doesn’t and can’t happen
What are the problems with projection geometry? (4)
- Image receptor and object NOT in contact (this is because the tooth is supported by bone so can contact some of the tooth but not all)
- Beam of x-rays NOT parallel (because x-ray beam is divergent beam)
- 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
Why can the image receptor and tooth NOT be fully in contact?
- Because the tooth is supported by bone so can contact some of the tooth but not all
Why are x-ray beams not parallel?
- Because the beam is a divergent beam so cannot be parallel
In which type of x-ray imaging is there no magnification?
- Cone beam CT
There are 2 solutions to the problems with ideal projection geometry. What are these solutions?
- The paralleling technique
- Bisecting angle technique
What happens in the paralleling technique?
- Image receptor and object parallel but not touching
What happens in the bisecting angle technique?
- The image receptor and the object are partially in contact (usually in contact at one end), and not parallel to each other
Explain the paralleling technique?
- Image receptor now some distance away from the tooth but plane is parallel to the long axis of the tooth
- Only the central ray is truly at right angles (perpendicular) to the long axis of the tooth and the image receptor (at a glance the other rays look like they are parallel but they are slightly divergent)
What does fsd mean?
- focus to skin distance - measured on a machine
Look at divergent x-ray beam slide
Too hard to put into a flashcard
How can you reduce the magnification in an x-ray image?
- Use a long x-ray focus- skin distance (fsd) to reduce magnification
- At least 20 cm
- n.b. beam aiming device of film holder should alwasy be close to, but not actually touching the patient
How do long fsd allow you to reduce magnification?
- Means that the x-rays on the outer part of the beam are not quite so divergent so you get less magnification (which is good)
- Fot this technique you should use an fsd of at least 20cm - this is part of the regulations relating to the kind of x-ray tube we are using because we always use a 60KV or possibly 70KV x-ray tube - they require long fsd (20cm)
To measure focus-skin distance, where do you measure from ?
- Measure from mark on outside of tube head (mark that is directly over where the x-ray source is) to the patient end of the spacer cone
Rectangular collimation should be combined with beam-aiming devices and film holders, Why is this? (3)
- Dose reduction
- Improved quality
- Fewer rejects
What are the separate parts of a film holder? (4)
- Bite block
- Beam aiming device
- Rod
- Image receptor support
Are all components of film holders reusable?
- Yes as they are autoclaved
What type of radiograph are blue film holders used for?
- For anterior periapicals
What type of radiograph are yellow film holders used for?
- Posterior periapicals
What type of radiograph are red film holders used for?
- Bitewings
What happens if film holder assembly is correct?
- Get a good result
What happens if film holder assembly is incorrect?
- ‘coning off’
How can you tell if you have assembled the film holder correctly?
- When you put them together you should look through the ring and see the support for the image receptor right in the middle and if you don’t see that then something is wrong
What is ‘coning off’?
- Where the x-rays go though the lower part of the image receptor but they are not going to reach the upper part
- You have an image in part of the x-ray image and a completely white bit in another part
What colour are endodontic film holders?
- Green
What are the components of an endodontic film holder? (5)
- Image receptor support (receptor vertical or horizontal)
- Bite block
- Beam-aiming device
- Connecting rod - no colour
- PLUS basket to support instrument heads or gutta percha points
Are endodontic film holders autoclavable?
Yes
Why do endodontic film holder have a basket?
- Supports instrument head or gutta percha points
- Going to take radiographs when you have files in the root canals so patient can’t bite tooth to tooth, they have to have something that keeps the teeth apart and protects the instruments which go into the basket
What is collimation?
- Collimation is about controlling the shape and the size of the x-ray beam
Why do we use rectangular collimation?
- Because we are using rectangular image receptors it is logical to use a rectangular shaped x-ray beam but also have a rectangle in exactly the same orientation as the image receptor
What is the regulation for maximum beam diameter of circle on the collimator?
- 60mm at patient end of spacer cone
- If have an x-ray machine where that dimension is greater it is illegal
Why is there lead in the spacer cone?
- as lead is very good at absorbing x-rays and so everywhere except where we have aluminium, x-rays x=cannot get out the x-ray tube head
What are the dimensions of a rectangular collimator?
- 50mm length
- 40mm width
What is the % reduction in area of the rectangular collimator compared to the circular collimator?
- 30% reduction
- Circle area = 2828 sq. mm
- Rectangle area = 2000 sq. mm
What size of film holder would yo use for a vertical anterior periapical?
- Size 0
What size of film holder would yo use for a horizontal posterior periapical?
- Size 2
How would you position the film holder for a periapical?
- Position against teeth of interest, parallel to the line of arch and long axis of tooth
Why do you have to handle the sensors very carefully?
- As the sensor itsle fis really easy to damage the surface and so you will get a part in the image where you don’t get an image
Explain the process of preparing for taking a periapical? ( 5)
- Bit block against teeth
- Cotton roll on opposite side to stabilise
- Beam aiming device to patient, close to but not touching
- Rectangular collimator: correct orientation to match image receptor, and spacer cone close to beam-aiming device
- Align tube head - check from 2 directions
Why would you place a cotton wool roll in the mouth when pre paring for a periapical radiograph?
- If you have teeth biting hard plastic biting teeth you often don’t have a stable solid object so often use cotton wool rolls
- Use them always between the bite block and the tooth on the opposite side
- Ask the patient to close up gently but firmly
When aligning the tube head what 2 directions do you want to look from to ensure it is correct?
- Need to check that it is lined up to the connecting rod both looking as if we were standing here looking at it from the side but also wanting to look up underneath because your x-ray tube head has got angulation in 2 different directions so need to check that it is correct
Give a summary of the paralleling technique? (5)
- Image receptor and object parallel but not in contact
- Image receptor and object some distance apart - potential for undesirable magnification
- Use long x-ray focus - skin (fsd) distance to reduce magnification - at least 20cm
- Requires the use of film holders
- Stabilise with cotton roll between bite block and teeth in arch opposite to that being x-rayed (if you put between the bite block and the teeth that you are x-raying you will risk not getting the apex of the tooth on your image)
What are you looking to see in a bitewing radiograph? (4)
- Side teeth (premolars and molars)
- Symmetry of upper and lower teeth
- Minimal overlap of adjacent teeth
- Inter-dental bone
How many bitewing radiographs would you take for a patient ?
- One or two per side
What size of film holder would you use for an adult patient when taking a bitewing?
- Size 2
What size of film holder would you use for a small child patient when taking a bitewing?
- Size 0
How would you position a film holder for a bitewing radiograph?
- Assemble correctly
- Position against lower teeth, parallel to line of arch
- Front edge of film packet mesial to canine/premolar contact
You would normally do horizontal bitewings where the long axis of the image receptor is horizontal but it is possible to do a vertical bitewing where the long axis is superior-inferior. When would you do this?
- Something you would do if you had a patient with substantial loss of supporting bone due to periodontal disease
- But this would be very specifically requested
Where do you want the bite block to be positioned when taking a bitewing?
- Want it with the bite block against the occlusal surfaces of the lower teeth looking along there to make sure that the surface of your image receptor is parallel to the line of the teeth because when the patient bites you won’t be able to see it
Where should the image receptor be for a bitewing radiograph?
- The front of the image receptor is just on front of the first premolar
When checking your alignment from 2 directions for a bitewing, what directions should you be looking in?
- From on front and look downwards to check in the other direction
Explain the process of positioning for a bitewing after the patient has bitten together? (4)
- Beam-aiming device to patient
- Rectangular collimator orientation correct
- Spacer cone to beam aiming device
- Align tube head - check from 2 directions
What are the critical points that need to occur for a bitewing radiograph? (5)
- Image receptor parallel to line of arch (true for all periapicals as well)
- Central ray at 90 degrees
- Vertical angle controlled by film holder
- No film holder is possible
- Vertical angle + 5-10 degrees
You need to be able to determine the vertical angle. What is this?
- This is the angle of the central ray relative to the occlusal plane
What is the vertical angle related to?
- Related to the curve of monsoon
What is the curve of Spee?
- Anero-posterior curve
- Curves up posteriorly
- Produces a happy smile
- Useful to help us tell which teeth are which
What is the curve of monsoon?
- Bucco-lingual curve
- One of the occlusal plane curves
- Influences the x-ray technique e.g. bitewings and panoramic radiography (panoramic vertical angle is negative to occlusal plane: minus 8 degrees)
How can you tell which way to put a bitewing radiograph when analysing it?
- If can see bone between the roots of molars then know yo u are looking at a mesial and distal root and therefore must be a lower molar - most reliable way of knowing which teeth are upper and lower
