X-ray interaction with Matter Flashcards
1
Q
What range of photon energies is present in an x-ray beam?
A
- continuous range up to 70keV
- spikes at 59 and 67keV
2
Q
What are the three ways in which x-ray photons can interact with matter?
A
- transmission
- pass through matter unaltered
- absorption
- stopped by matter
- energy deposited into tissue
- photon ceases to exist
- scatter
- changes direction
- photon deflected by tissue
- partial deposition of energy into tissues
- photon continues in new direction
3
Q
What are the specific attenuation interactions?
A
- photoelectric effect
- complete absorption
- Compton effect
- partial absorption and scatter
4
Q
What is the photoelectric effect?
A
- photon in x-ray beam interacts with inner cell electrons
- results in absorption of the photon
- creation of photoelectron
- occurs when energy of incoming photon is equal to or greater than the binding energy of the inner cell electron
- photoelectric effect predominates with low energy photons
- photon energy overcomes binding energy resulting in inner shell electron being ejected as photoelectron
- any excess photon energy becomes kinetic energy
- photoelectron can ionise adjacent tissues
- vacancy in inner shell is filled by cascade of outer shell electrons
- produces light photons and heat
- absorption prevents x-ray photons from reaching the receptor
- lighter area on radiographic image
5
Q
What is the Compton effect?
A
- photon in x-ray beam interacts with outer shell electron in subject
- partial absorption and scattering of photon
- creation of recoil electron
- occurs when energy of incoming photon is much greater than the binding energy of the electron
predominates with higher energy photons and outer shell electron - some photon energy transferred to electron to overcome binding energy and provide kinetic energy
- electron ejected and called a recoil electron
- recoil electron can damage adjacent tissues
- photon loses anergy and scatters
- can undergo photoelectric effect
- can undergo further Compton effect interactions
6
Q
What is the direction of scatter?
A
- photons can be deflected in any directions but are influenced by the energy of the incoming photon
- higher energy photons are deflected more forward
- forward scatter - lower energy photons are deflected more backwards
- back scatter
- higher energy photons are deflected more forward
- the majority of scatter produced by an X-ray tube is forward
6
Q
What is the direction of scatter?
A
- photons can be deflected in any directions but are influenced by the energy of the incoming photon
- higher energy photons are deflected more forward
- forward scatter - lower energy photons are deflected more backwards
- back scatter
- higher energy photons are deflected more forward
- the majority of scatter produced by an X-ray tube is forward
7
Q
What is the effect of scatter on radiographic images?
A
- photons scattered backwards, sideways or very obliquely forward will not reach the receptor
- do not affect image
- photons scattered slightly obliquely forward may still reach the receptor
- will interact with the wrong area
- causes darkening of the image in the wring place
- results in fogging of the image
- reduces image contrast and quality
8
Q
How can scatter be reduced?
A
- collimation
- reduced surface area irradiated
- reduced number of scattered photons produced in tissue
- reduced scattered photons produced in the tissue
- reduced scattered photons interacting with the receptor
- reduced loss on contrast on radiographic image
- reduced patient radiation dose
9
Q
What is the impact of the photoelectric effect on the radiation dose?
A
- deposition of all x-ray photon energy into tissue
- increased patient dose
- necessary for image formation
10
Q
What is the impact of the Compton effect on the radiation dose?
A
- deposition of some x-ray photon energy in tissue
- increased patient dose but scattered photons do not contribute
- not useful to image
- may increase dose to operators
- due to back scatter
11
Q
Provide patient doses of common dental imaging
A
- periodical/bitewing
- 4uSv
- maxillary occlusal
- 8uSv - panoramic
- 20uSv
12
Q
How does lowering voltage of the x-ray unit affect patient dose and contrast between tissues?
A
- increased contract between tissues with different atomic numbers
- increased radiation dose absorbed by patient
13
Q
How does increasing voltage of the x-ray unit affect patient dose and contrast between tissues?
A
- decreased radiation dose absorbed by patient
- decreased contrast between tissues with different atomic numbers
14
Q
Briefly outline continuous and characteristic radiation interactions
A
- occur in X-ray tube
- electrons interacting with tungsten atoms
- production of x-ray photons
