X-ray interaction with Matter Flashcards
Summary of beam leaving dental x-ray unit
- Focused stream of x-ray photons going in same direction
- Photons are diverging but near parallel
- Ideally collimated to shape of receptor e.g. rectangular cross-section
- There is a range of photon energies present
- Lower energy non diagnostic and photons removed by filtration
- If x-ray unit set up to operating voltage 70kV, then beam consists of continuous range of energies up to 70kV with characteristics spikes around 59 and 67keV
- Photons in x-ray beam travel in straight line at speed of light until they interact with something
What are three different ways X-ray photons can interact with matter?
- Transmission
- Absorption
- Scatter
What is transmission?
- Photon Passes through matter unaltered
What is absorption?
- Energy full deposited into tissue
- Photon ceases to exist
What is Scatter?
- Interact with matter and Changes direction as its deflected by tissue
What happens when photons are scattered and absorbed in a x-ray beam?
- Photon deflected by tissue
-Partial deposition of energy into tissue - Photon continues in new direction (to be transmitted, absorbed or scattered again)
What is attenuation?
- Reduction in intensity of x-ray beam
- Can occur by absorption or scattering of x-ray photons when interacting with matter
What is x-ray beam intensity?
- Quantity of photon energy passing through a cross-sectional area of the beam per unit of time
What is x-ray beam intensity proportional to?
- Proportional to energy and number of photons
How is energy affected in x-ray tube?
- Energy of x-ray beam affected by potential difference across x-ray tube (kV)
- Increase kV = increase average photon energy and increase maximum photon energy
How is number of photons affected in x-ray tube?
- Primarily affected by current in filament (mA)
- Increase mA = increase number of photons
- Increasing potential difference also increase number of photons because an electron with more energy can undergo more interactions at target
What does attenuation lead to?
- Indirectly leads to radiographic image
- Different tissues/materials have varying degress of attenuation
- This determine how many photons reach receptor
What do different levels of attenuation give colour wise?
- Minimal attenuation = black
- Partial attenuation = grey
- Complete attenuation = white
How can we predict x-ray photon interactions?
- Unable to predict outcome of single photon
- Can estimate proportion of interactions in x-ray beam (which consists of millions of photons)
What predictions can we make based on physical properties of matter being exposed?
- Thick lead = Essentially all attenuation
- Piece of paper = Essentially all transmission
- Enamel = Mostly attenuated (mostly white)
- Cheek = Mostly transmission (radiopaque - black)
What are the two specific attenuation interactions?
- Photoelectric effect (Complete absorption)
- Compton effect (Partial absorption and scatter)
What is the photoelectric effect?
- Photon in x-ray beam interacts with inner shell electron in subject
- Results in absorption of photon and creation of photoelectron
- Electron from outer shell drops down to inner shell
- Extra energy from different binding energies is released as light (not as high energy as x-ray)
- Drop downs occur until all inner shells filled
Important points to remember for photoelectric effect? When does it occur?
- Occurs when energy of incoming photon is equal to or just greater than binding energy of inner shell electron
- So photoelectric effect predominates with lower energy photons
- Because human tissues have relatively low binding energies
What does photon overcome during photoelectric effect?
- Photon energy overcomes binding energy
- Results in inner shell electron being ejected (now called photoelectron)
- Any excess photon energy becomes KE of photoelectron
- Photoelectron can ionise and potentially damage adjacent tissues
What is a by-product of photoelectric effect?
- Vacancy in inner shell filled by cascade of outer shell electrons
- Produces light photons (not visible to naked eye) and or heat
What leads to lighter area on radiographic image during photoelectric effect?
- Absorption by photoelectric effect prevents x-ray photons reaching receptor
- Leads to lighter area on radiographic image
What is the probability of photoelectric effect occurring?
- Proportional to atomic number cubed (Z^3)
- Inversely proportional to photon energy cubed (1/E^3)
- Proportional to physical density of material (p)
What is the photoelectric effect equation?
Approx = (p x Z^3)/ E^3
What does increasing KV do?
- Less attenuation of x-ray beam
What does it mean for photoelectric effect to be proportional to Z^3?
- Small steps in Z (atomic number) result in large jumps in absorption
- Results in good contrast between different tissues on radiographic image
- e.g. Soft tissue is 7 so 7^3 = 343
- Bone is 12 so 12^3 = 1728
- Calcium is 20 so 20^3 = 8000
- Gold is 79 s0 79^3 = 493,039
- Large differences so good contrast
Why are gold and lead so good at shielding?
- High atomic numbers
- High photoelectric effect
- Z^3 very high numbers
What is the summary of the Compton effect?
- Photon in x-ray beam interacts with outer shell electrons in subject
- Some photon energy transferred to electron to overcome binding energy and provide KE
- This electron then ejected and called recoil electron
- Recoil electron can ionise and potentially damage adjacent tissues
- Photon loses energy and changes direction (scattered)
- Can undergo photoelectric effect and further Compton effect interactions
When does the Compton effect occur?
- Occurs when energy of incoming photon much greater than binding energy of electron
- Compton effect predominates with higher energy photons and outer shell electrons (which are loosely bound)
What is the direction of scatter during the Compton effect?
- Scattered photons can be deflected in any direction but are influenced by energy of incoming photon
- Higher energy photons deflected more forward called Forward scatter
- Lower energy photons deflected more backward called Back scatter
What is the majority of scatter from an x-ray beam produced by x-ray tube operating at 70kV?
- Forward scatter
Why does the controlled area need to completely surround the patient?
- Scatter
What happens when photons are scattered backwards, sideways or very obliquely forwards in regard to radiographic image?
- Will not reach the receptor
- Do not affect image
What happens when photons scattered slightly obliquely forward in regard to radiographic image?
- May still reach receptor but interact with wrong area
- Causes darkening of image in wrong place
- Results in fogging of the image and reduces image contrast/quality
What is the likelihood of the Compton effect occuring?
- Independent of Z
- Weakly proportional to photon energy
- Proportional to density of material
How does increasing photon energy effect likelihood of Compton effect?
- Has minimal effect on Compton effect
- But higher energy photons more likely to scatter forwards, reach receptor and degrade radiographic image
What are the 5 collimation steps to reduce scatter?
- Decrease SA irradiated
- Decrease volume of irradiated tissue
- Decrease number of scattered photons produced in tissue
- Decrease scattered photons interacting with receptor
- Decrease loss of contrast on radiographic image
Also reduces patient radiation dose and amount of radiation being released into surrounding
What is impact of Photoelectric effect on radiation dose?
- Deposition of all x-ray photon energy into tissue
- Increase patient dose
- But its necessary for image formation
What is the impact of Compton effect on radiation dose?
- Deposition of some x-ray photon energy in tissue
- Increase patient dose but scattered photons don’t contribute usefully to image
- May increase dose to operators (from back scatter)
What is the effect of lowering kV on x-ray unit?
- Lower x-ray tube potential difference (kV)
- Overall lower energy photons produced
- Increase photoelectric effect interactions
- Increase contrast between tissues with different Z which is good
- Increase dose absorbed by patient which is not good
What is the effect of raising kV on x-ray unit?
- Higher x-ray tube potential difference (kV)
- Overall higher energy photons produced
- Decrease photoelectric effect interactions
- Increase forward scatter
- Decrease dose absorbed by patient which is good
- Decrease contrast between tissues with different Z which is bad
How do you decide on potential difference kV?
- Decision is compromise between image quality and patient radiation dose
- UK guidance advises suitable range of 60-70kV for intraoral x-ray units
- GDH uses 70kV
Important notes for Photoelectric effect and Compton effect
- Occur in patient/receptor/shielding
- X-ray photons interacting with atoms
- Lead to attenuation of x-ray beam
