Intra-oral techniques and Paralleling Flashcards
If the ideal projection geometry is met what will this result in
the image size will be identical to the object size
What is the ideal projection geometry
Image receptor and object in contact and parallel
Parallel beams of X-rays coming towards the patient and the image receptor
X ray beam is perpendicular to object plane and image receptor plan
What are problems in projection geometry
image receptor and object not in contact
the beams of X-rays are NOT parallel
X -ray beam central ray may nor may not be perpendicular to the object plane and image receptor
Image size is not identical to object size
Why can the image receptor and object not fully be in contact
because the tooth is supported by bone so you cannot contact all of it
Why are beams of X-rays not parallel
X-ray beams are divergent beams so the rays will not be parallel
Why is the image size not identical to object size
due to magnification caused by the divergent beam
What type of X-ray does not have magnification
cone beam ct
What are two solutions to the problems in projection geometry
paralleling technique
bisecting angle technique
What is the paralleling technique
the image receptor and object are parallel but they are not touching
What is the bisecting angle technique
image receptor and object partially in contact but not parallel to each other
Are all beams parallel in the paralleling technique
no only the central ray is perpendicular to the long axis of the tooth, the outer rays are not quite perpendicular
Why does magnification happen
Due to the image receptor and the object being some distance apart, there is potential for undesirable magnification
What is the fsd
distance between the focal spot of the x-ray tube (where x-rays are produced) marked with a red dot on the intra-oral x-ray head to the skin surface of the patient
Wha can be done to reduce magnification
Use long X-ray focus skin distance
What is the minimum length for X-ray focus-skin distance
20cm
What does the long X-ray focus skin distance allow for
more parallel, non-diverging X ray beam as the outer part of the beam are not quite as divergent leading to less magnification
Where should the beam aiming device of the film holder be placed
close to but not actually touching the patient
How do you measure the fsd
you measure the mark on the outside of the tube head to the patient end of spacer cone
What should rectangular collimation be combined with
beam-aiming devices and film holders
What is the benefit of film holders, BAD and RC
Dose reduction
Improved quality
Fewer rejects
What are the different colors of film holders
red blue and yellow
What part of the film holder is color coded
bite block, beam aiming device and rod
What is the blue FH
anterior periapicals
What is the red FH
bitewings
What is the yellow FH
posterior periapicals
What is the image receptor support for
holding and supporting the image receptor
What is the bite block for
what the patients bite on
How are the film holders cleaned
autoclaved
How can you tell if you have assembled the film holder correctly
○ If you look through the circle you should see the support for the image receptor right in the middle, if you don’t then something is wrong
What does incorrect film holder assembly result in
coning off
What do endodontic film holders consist of
○ Image receptor support (receptor vertical or horizontal) ○ Bite block ○ Beam-aiming device ○ Connecting rod - no colour Autoclavable
What does endodontic film holders also have
○ Basket to support instrument heads or gutta percha points so that the patient does not bite the tooth with the files in it, the basket ensures the teeth are kept apart and protect the heads of the instruments
What is collimation
controlling the shape and size of x ray beam
Why does the beam aiming device have rectangular marks
so that it can be lined up easily with the x ray device
What machines should RC be on
should be provided on new equipment and retrofitted to existing equipment
What is the collimator made of
lead
What are the shapes of collimator
circular or rectangular
What is the maximum diameter of a circular collimator
60mm
What is the silver bit of the collimator that lets x ray out
aluminium
Why does rectangular collimator emit less radiation
Reduction in surface area
but is more technically difficult as need to ensure everything is lined out
What are the different sizes of image receptors for periapicals
Size 0 vertical anteriors image receptor (blue holder) - smallest, use that with long axis vertical for anterior teeth
Size 2 horizontal posteriors image receptor (yellow holders) - bigger, use that for long axis going horizontally
What is the procedure for a periapical
Select the film holder and correct image receptor
Assemble correctly - be careful with sensors
Position patient at right height, open wide
Position against teeth of interest, parallel to line fo arch and long axis of tooth
Where should beam aiming device be when putting film holder into mouth
at free end
How do you position film holder into mouth
○ Bite block against the teeth
○ Cotton roll between bite block and teeth on opposite side to stabilise
○ Beam aiming device to patient (was previously on the free end); 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, for a periapical check from underneath and from infront
Summarise the paralleling technique
- Image receptor and object parallel but not in contact for a paralleling periapical
- 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 use of film holders, cannot do it without them
Stabilise with cotton roll between bite block and teeth in arch opposite to that being x-rayed, if you put it on the wrong side then there is risk of not getting the apex of the tooth in the image
What is the ideal bitewing
- Of the side teeth (premolars and molars)
- Symmetry of upper and lower teeth (film holder should guarantee this)
- Minimal overlap of adjacent teeth
- Want to see inter-dental bone
One or two per side
Describe procedure for taking bitewing
Select film holder (red holder) and image receptor (long axis horizontal for both)
Assemble correctly
Position against lower teeth, parallel to line of arch, tongue out the way
Front edge of film packet mesial to canine/premolar contact
What are the different sizes of image receptor
size 2 (adults) and size 0 (children)
Describe how the film holder is positioned in the mouth for bitewings
○ Patient to bite together
○ Beam-aiming device to patient, again make sure it is close but not touching
○ Rectangular collimator orientation correct
○ Spacer cone to beam-aiming device
Align tube head - check from 2 directions. In front and from below
What are the critical points for bitewings
- Image receptor is parallel to the line of arch
- Central ray at 90 degrees to arch
Vertical angle controlled by the film angle
If no film holder then vertical angle needs to be +5-10
Why do you need to change vertical angle for bitewing if no film holder
takes into account curve of Monson
What is curve of spee
anterior-posterior
○ Curves up posteriorly
○ Produces a happy smile
○ Useful to us when we are trying to see what way the radiograph is
What is curve of monson
buccal-lingual
Influences X-ray technique e.g bitewings and panoramic radiography (Panoramic vertical angle is negative to occlusal plane (-8)
Curve of Monson influences the position of the film holder which determines the vertical angle
How can you determine orientation of x-ray
curve of spee
look at the molars, you can’t see how many roots there are in the upper one, only in the lower and this is due to the mesial distal roots of the lower so this is the most reliable way of checking the orientation of the X-rays
