Imaging geometry Flashcards
What is Frankforts plane?
Line from lower orbital margin to top of tragus of the ear. (Parallel to the hard palate).
Used for OPG and lateral Cephalometry
Upper positioning line?
Line from top of tragus of the ear to alla of nose
Used for all paralleling techniques
Lower positioning line?
Top of tragus of the ear to canthus of the mouth (parallel to lower occlusal plane).
Median sagittal plane?
Line directly down the middle of the face to divide the face into 2 equal parts.
Why is it important to always use the paralleling technique?
- Give accurate image
- Technique is reproducible
- Fewer need for repeats due to errors
- More practical when using rectangular collimeter
Describe the technique to carry out bitewings
- Patient seated erect
- Median sagittal plane vertically postioned
- Upper positioning line horizontally positioned
- Blip mesially @ biting surface
- Film/ phosphor plate positioned palatally or lingually
- Central short axis for posterior bitewings and central long axis for anterior bitewings coincident with central point of the image
- Lower edge of the film is 3mm below the bite surface
- Central long axis coincident with occlusal surface and posterior border will include last interproximal space
Why should the cone is placed in contact with the ring of the holder?
Ensures that the central ray is perpendicular to the centre of the film.
What are the conditions needed to produce an accurate image?
- X-ray photons emitted from infinitely small point source
- Object parallel to imaging recording plate
- Recording plate and object should be closed to eachother
- Focus should be as far away from the object as possible
- Focus should be as far away from the recording plate as possible
- Central ray of x-ray beam should be directed to the centre of the object and recording plate
- The central ray should be perpendicular to the image recording plate.
What happens when FOD is increased
Image expands but less sharp as unable to create x-rays from infinitely small source.
What happens when the OFD is long?
theoretically increase magnification and increases unsharpness due to decreased divergence.
What happens when the central ray is perpendicular to film but not object?
Foreshortening
What happens when the central ray is perpendicular to the object but not the film?
Elongation
Why is it important that the beam is 20cm from the patients face?
if more = film underexposed
if less = won’t get magnfication and sharpness
Why do we get blurring?
Increase OFD means x-rays coming from infinitely small point source but a relatively large area so x-rays at the edge of focus will pass at the same of object making images of the same part in a different place. Thus blurring gets bigger.
Why does the film and object need to be parallel?
distorts scale (foreshortening and elongation) Image will still be accurate as long as they are parallel !
When can bisecting angle be used?
Anatomical difficulties: shallow palate/ tongue tied
Recent trauma/ surgery
What effect does incorrect vertical angle have on the bitewing image?
With paralleling technique the Image will still be accurate as central ray is at the centre (perpendicular) of the object & plate
Posteriors= cusps are separated and occlusal surface is
visible
Anteriors = won’t change the appearance of biting surfaces as they’re a flat plane
What effect does an incorrect central ray have on the image?
some areas won’t be exposed
What effect does incorrect lateral (horizontal) angle have on the image?
Overlap thus cannot see interproximal spaces.
How would you ensure that the film is parallel?
By utilising the full length of the bite block
How can the vertical angle be assessed?
checking the distance between the bite surface and edge of the film is 3mm. If more or less the angle is incorrect.
How do you assess density?
Directly exposed areas are black and there is enough detail to see bony detail or maxilla and mandible.
can see apical areas in PA and bony crests in BW
How do you assess the contrast ?
Differentiate between different density of areas. E.g. pulp cavity, dentine and enamel are seperate entities.
How do you check for sharpness (level of fine detail)?
Sufficient detail for adequate diagnosis. Check bony trabecular pattern, lamina dura, canal for periodontal ligament
If the hard palate appears as double line on an OPG, what does this mean?
Frankforts plane was not positioned horizontally.
If 3-3 are not as sharp as the premolars on an OPG what does this mean?
The teeth were not in the focal trough in the 2/3 region
What happens if the maxiallary teeth overlap the mandibular teeth in an OPG?
The patient was not biting edge to edge on the biting block.
How do you check if the median sagittal plane is correct and not tilted?
Measure condyles should be equal distances from the top of the film on the left and right.
If the condyles are not shown on the OPG how else can you assess the correct orientation of the median sagittal plane?
Draw a line between the extreme edges of the hard palate this line should be parallel to a line drawn across the centre of the plate.
If a patient was looking to the left what side would the ramus of the mandible look to be larger?
Left - the side the patient looks towards is the side that would appear larger thus the median sagittal plane is rotated.
If an OPG showed that the palate as a double line what was wrong with the positioning?
Frankforts plane was not horizontal.
In an OPG what has likely to have happened when the palate is a double line on one side and single on the other?
Rotation or tilting of the head
In an OPG what is indicated when there is air (translucency) below the palate?
The tongue was not in the roof of the mouth.
