Attenuation Applications Flashcards
Is compton scatter more likely to be created at higher or lower kVp?
Lower kVp is higher probability of creating compton scatter
Is Photoelectric absorption more likely to be created at higher or lower kVp?
More probability at lower kVp
What occurs more at lower energy, photoelectric effect or compton scatter?
Photoelectric effect
When does compton scatter and PE decrease?
As kVp increase
What occurs more at higher kVp’s, compton scatter or PE?
Compton scatter
Once a photon reaches 20 kEV, what is likely to happen to the photon
More likely to be absorbed than scatter
At what kVp is a photon more likely to scatter in the bone?
40
At what kVp can you see differences in the image due to differential absorption?
80kVp
What is most likely to happen to the photons once they get above 80 kVp?
Why?
You see differencds in images due to scatter. Decrease in subject contrast because it only shows differencesin density, not atomic number
How do you find the differences in atomic number and density?
What do you need to remeber when figuring out the math equation?
Divide the smaller number by the larger number and REMEBER THE PROPORTION EQUATION
Will you see more or less subject contrast with low energy and the PE?
More subject contrast
Will you see more or less subject contrast with high energy and compton scatter?
Less subject contrast
At low energy, are you more likely to see PE or CS? What about high energy:?
Image formation at low energy: PE
Image formation at high energy: CS
What tissues are present in chest imaging?
Air, soft tissue, bone
What tissues are present in mammography?
- Soft tissue
- Fat
- Glands
- Tumours
Very similar to soft tissue and close in density
What has higher subject contrast naturally, chest imaging, or mammography? What about lower subject contrast?
Chest imaging: Higher natural subject contrast
Mammography: Lower subject contrast
When imaging the chest, what are we trying to achieve with subject contrast?
How and why?
-Trying to reduce subject contrast with high energy to decrease subject since it naturally has high subject contrast
When preforming a mammogram, what are we trying to achieve with subject contrast?
How and why?
Trying to increase subject contrast with low energy too see more absorption and to highlight the differences in atomic number while still seeeing differences in density.
What is the range of kVps (average) that shows the a chest image?
110-130 kVp for more SC
What is the kVp that allows us to see the breasts? Why?
30 kVp for more PE and more charectoristic photons
Does K-edge absorption happen in humans?
No, only happens with materals of high atomic number
What is k-edge absorption
A sharp decrease in absorption once you reach the k-edge (the k-shells binding energy in a material)
When is L-shell absorption most likely to happen?
-Greatest probability with very low E photons (12 kEV)
What are the two types of k-edge absorption?
L shell
K shell
When is absorption most likely to take place with k and L shell when refereing to the k-edge? When does probability drop off?
-Greatest probability of absorption happening if the energy is just above the binding energy of tungsten
-Zero probability if the enrgy is less than the binding energy of tungsten
What things in x ray display k-edge absorption?
- Heavy element filters
- Contrast studies
- Image receptors
- Lead sheilding
What do k-edge filters in the tube do?
They are made of a high atomic number, and only let through a narrow range of photons in the beam
What will heavy element filters absorb?
Will absorb low E photons-any E just above the k-edge (the binding E of te k shell)
What type of photons will heavy element filters allow through?
-Lets through photons below BE of the material of the filter
-Will let through photons way above BE
-charecterisitc radiation that is made from itself since the target is also made of the same material
What is the purpose of contrast studies? How does it work?
To better visualize anatomy-show structures and function by absorbing radiation with thier high atomic #
What are two examples of things you can see with contrast studies?
-Vessels
-GI tract
What are two types of contrast? What is thier binding E?
- Iodine: BE=33kEV
- Barium: BE=37 kEV
What are some examples of what IR could be made of?
- Gadolinium oxysulfate
- Cescium iodide
- Selenium
What are the benefits of having a faster IR?
Generally, what is the k-edge
-Need less mAs
-All have a k-edge of what is closer to our energy in our beam, so more absorption takes place and a decrease in dose occurs
What type of IRs do we use now days and what is their K-edge?
Use rare earth detectors-k-edge is around 30
Why does lead provide scatter protection?
-Has a high atomic number and a k-edge that matches our beam, so theres more absorption
What are some examples of scatter protection?
- Lead sheilding
- Lead walls/glass
- Grids
