X-ray Interaction with Mattre Flashcards
Photons in a diagnostic X-ray beam can interact with matter in three ways, what are they?
Transmission = passes through matter unaltered
Absorption = stopped by the matter
Scatter = changes direction
What occurs when a photon is absorbed?
- energy fully deposited into tissue
- photon ceases to exist
what occurs when photons are scattered?
- photon deflected by tissue
- partial deposition of energy into tissue
- photon continues in a new direction
What is X-ray beam intensity?
Quantity of photon energy passing through a cross-sectional area of the beam per unit time
What is X-ray beam intensity proportional to?
Energy & number of photons
What is the energy of the X-ray beam affected by?
The potential difference across the X-ray tube
increased kV —> increased average photon energy & increased maximum photon energy
What is the number of photons in the X-ray beam affected by?
The current in the filament
increased mA —> increased number of photons
How does minimal attenuation present on a radiograph?
black
How does partial attenuation present on a radiograph?
grey
How does complete attenuation present on a radiograph?
white
What is the photoelectric effect?
Photon in X-ray beam interacts with inner shell electron in subject, resulting in absorption of the photon & creation of a photoelectron
When does the photoelectric effect occur?
when energy of incoming photon is equal to or just greater than binding energy of inner shell electron
The photoelectric effect predominates with …
Lower energy photons
What occurs in the photoelectric effect?
incoming photon energy overcomes binding energy, resulting in the inner shell electron being ejected
- photoeletron can ionise adjacent tissues
What occurs in the Compton effect?
photon in x-ray beam interatcs with outer shell electron in subject, resulting in partial absorption and scattering of the photon & creation of a recoil electron
What does the Compton effect predominately affect?
higher energy photons & outer shell electrons
What type of photos deflect forwards? (‘forward scatter’)
higher energy photons
What type of photons deflect backwards? (‘back scatter’)
lower energy photons
What is the function of collimation?
- lower surface area irradiated
- lower volume of irridiated tissue
- lower number of scattered photons produced in the tissue
- lower scattered photons interacting with receptor
- lower loss of contrast on radiographic image
What are the positive effects of lowering the kV on an X-ray unit?
lower x-ray tube potential difference —> overall lower energy photons produced —> higher photoelectric effect interactions —> higher contrast between tissues with different Z (atomic number)
What is the negative effect of lowering the kV on an X-ray unit?
- decreased image quality which leads to…
- higher dose absorbed by patient as x-ray beam will need to be on for longer to achieve same exposure level
What should the kV be for an intra-oral X-ray unit based on UK guidance?
60-70kV
