Attenuation and Attenuation Applications Flashcards
What is the term that defines the weakening of the primary beam following penetration through a given thickness?
Attenuation
When is there the greatest chance of the photoelectric effect occuring?
When the E of the photon is just above the binding E of the material
Can also occur when the energy of the photon is equal to the binding energy of the electron
When is there NO chance that the photoelectron effect will take place?
When the energy of the photon is below the binding energy of the material
What is the probability that a given x-ray photon will undergo CS dependant on?
The energy of the incident ray
At higher energies, what type of attenuation is more likely to take place?
At higher energies, compton scatter is more likely to occur than PE
However, probability goes down for both as the E goes up
At above 80 kVp, what type of attenuation is likely to show differences in the image?
Compton scatter
How much of the energy is absorbed by the electron from the incident photon in PE effect?
All of it-the photon ceases to exist
Describe the interaction that happens during the photoelectric effect:
- Photon ionizes atom and a K shell electron in the body is ejected, called a photoelectron
- All of the energy is absorbed-none left over from photon
- An outershell electron drops down to vacancy and secondary charecteristic radiation is produced
What is the energy of the secondary charecteristic radation and what happens to it?
The energy is very low, therefore, it is absorbed by the body and never escapes
What happens to the photoelectron after being ejected from the inner shell?
They interact with other atoms within the tissue and never penetrate outside of the body
All of the energy of the photon goes to the photoelectron
What effect does photoelectrons have on the final radiographic image?
None, they cannot reach the image receptor
If the energy of a photon is greater than the binding energy of an electron, what happens to the remaining energy in the PE. Effect?
The remaining energy is given to the photoelectron
How is the energy divided with compton scatter?
-An amount equal to binding E ejects e- from orbit
-Small amount of E given to the speed of the ejected e-
-Remainder is reemitted as compton scatter (photon)
Describe the interaction that takes place during compton scatter:
- Photon ionizes atom (ussually outershell)
- During ionization, it knocks out e- called a compton e-
- The photon gives random amount of kE to compton e-
- The x ray photon changes direction and has less energy due to how much it gave up
What particle carries most of the energy after a compton scatter interaction?
The scattered photon
How much energy does the scattered photon retain after a compton interaction?
2/3 of its initial energy
Are lower or higher energy photons more likely to hit the image receptor?
Higher energy
What type of attenuation contributes most to personnel?
Compton scatter
Generally, in which direction are the scattered photons with higher energy deflected?
Deflected less from the original direction(straight)
What effect does compton scatter have on the final image?
-Reduces image contrast
-Provides NO useful information
With more energy lost during a compton scatter interaction, is the photon more likely to scatter?
Yes
What is the angle of deflection of a photon in compton scattering dependant on?
Dependant on how much E of the photon is lost
If a photon in CS retains 76% of its E, at what angle does it travel?
At a 45 degree angle
If a photon in CS retains 84% of its E, at what angle does it travel?
At a 90 degree angle
If a photon in CS retains 92% of its E, at what angle does it travel?
Travels at a 135 degree angle
If a photon in CS retains 68% of its E, at what angle does it travel?
Travels at 180 degrees (back scatter)
If a photon in CS retains 100% of its E, at what angle does it travel?
Straight (0 degrees)
What is inherent subject contrast in the image generated by?
Generated by the differrence between areas where more x rays are attenuated and areas where fewer x rays are attenuated
As kVp is reduced, does differential attenuation increase or decrease? What is the downside?
-Differential absorption increases
-Downside=dose increases
Define mass density:
The quantity of matter per unit volume; how tightly atoms of a substance are packed together
Is CS and PE effect more likely to happen at higher or lower energies?
Lower energies
As the number of x-rays being transmitted increases, does image contrast increase or decrease?
Image contrast decreases
What is the k-edge?
The binding energy of the k shell electron of an atom
When is absorption most likely to take place with k-edge elements?
When the E is just above the BE
What does attenuation coefficants tell us?
How much attenuation (scatter and absorption) takes place per unit thickness.
Are attenuator coefficants linear or exponential?
Linear because it is talking about the thickness of the material
What does total linear attenuation depend on in relation to the attenuator?
Depends on the probability of interactions between the photons and the material
What does the probability of x ray interactions with an attenuator depend on?
- KVp
- Atomic number
- Denisty
What is total linear attenuation represented by?
u (mew)
What does TMAC stand for?
The total mass attenuation coefficient
What interactions is TMAC a result of?
Scattering AND absorption
What does PEMAC stand for?
The photoelectric mass attenuation coefficent
When does u increase an decrease?
Increase: When attenuation is higher
Decrease: When energy increases
