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