2.2 - Paralleling Technique Flashcards
What is the ideal projection geometry in radiology?
Image receptor and object parallel
parallel beam of x-rays
x-ray beam perpendicular to object plane and image receptor (tooth and image receptor)
image size identical to object size
What are the issues with ideal projection geometry?
Image receptor and object are not in contact - tooth is supported by bone so the image receptor can only contact some tooth
Beams of x-rays are not parallel BUT INSTEAD DIVERGENT and as a result image size won’t be identical
X-ray beam central ray may or may not be perpendicular to object plane and image receptor
image and object size are not the same due to magnification caused by divergent beam
Why are the object and image size different?
Due to magnification caused by the divergent beam
What type of x-ray is there no magnification in?
CBCT
What are the solutions to projection geometry problems
- paralleling technique
2. bisecting angle technique
What is the paralleling technique?
this is where we must have image receptor and object parallel but not touching
What technique is the best for taking PAs?
paralleling technique as it shows most accurate rep of tooth
For the tooth and image receptor to be parallel what must they have to be positioned like?
Good distance apart
Describe the x-ray beam in paralleling technique?
This is when the x-ray beam comes in and only the central beam is perpendicular to long axis of tooth and image receptor - outer beams are diverging
What happens if x-ray beam diverges and we can’t see all f tooth?
Then we need to retake x-ray which increases pt dose due to poor technique
What is the issue with paralleling technique?
In order to have the image receptor and object parallel resulting in the x-ray central beam being perpendicular to long axis of tooth and image receptor they must be some distance apart which creates a potential for undesirable magnification
How do we make up for undesirable magnification caused by paralleling technique?
we use a long skin to focus distance to reduce this magnification
What is focus to skin distance?
The 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.
What must the FSD be for paralleling technique?
at least 20cm
What does a long focus to skin distance her with?
It means x-rays on outer part of beam are less divergent resulting in less magnification - MUST BE AT LEAST 20CM as its a regulation
How do we measure FSD?
From the mark outside of the tube head (usually a red dot which is directly over x-ray source) and measure to pt end of spacer cone to check its at least 20cm
What is a rectangular collimator?
This is used to decreased amount of radiation dosage a pt is exposed to during an intra oral radiograph
it reduces scatter radiation by focusing beam into same rectangular shape as the sensor or film being used
What can rectangular collimators also do?
They also increase image quality as reduced scatter
What do rectangular collimators and beam aiming devices and film holders combined allow for?
reduced dose
improved quality
fewer reject images
inc benefit to pts as dont need several x-rays
Where must rectangular collimators be?
On all new equipment and retro fitted to old equipment
What does the collimator at the back of X-ray tube have?
Lead to absorb x-rays
aluminium
What is the maximum diameter of circle at pt end of spacer cone?
60mm
What colour is a red film holder used for?
BWs
What colour is blue film holder red for?
Anteriors
