X-ray interaction with Matter Flashcards
1
Q
Summary of beam leaving dental x-ray unit
A
- 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
2
Q
What are three different ways X-ray photons can interact with matter?
A
- Transmission
- Absorption
- Scatter
3
Q
What is transmission?
A
- Photon Passes through matter unaltered
4
Q
What is absorption?
A
- Energy full deposited into tissue
- Photon ceases to exist
5
Q
What is Scatter?
A
- Interact with matter and Changes direction as its deflected by tissue
6
Q
What happens when photons are scattered and absorbed in a x-ray beam?
A
- Photon deflected by tissue
-Partial deposition of energy into tissue - Photon continues in new direction (to be transmitted, absorbed or scattered again)
7
Q
What is attenuation?
A
- Reduction in intensity of x-ray beam
- Can occur by absorption or scattering of x-ray photons when interacting with matter
8
Q
What is x-ray beam intensity?
A
- Quantity of photon energy passing through a cross-sectional area of the beam per unit of time
9
Q
What is x-ray beam intensity proportional to?
A
- Proportional to energy and number of photons
10
Q
How is energy affected in x-ray tube?
A
- 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
11
Q
How is number of photons affected in x-ray tube?
A
- 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
12
Q
What does attenuation lead to?
A
- Indirectly leads to radiographic image
- Different tissues/materials have varying degress of attenuation
- This determine how many photons reach receptor
13
Q
What do different levels of attenuation give colour wise?
A
- Minimal attenuation = black
- Partial attenuation = grey
- Complete attenuation = white
14
Q
How can we predict x-ray photon interactions?
A
- Unable to predict outcome of single photon
- Can estimate proportion of interactions in x-ray beam (which consists of millions of photons)
15
Q
What predictions can we make based on physical properties of matter being exposed?
A
- Thick lead = Essentially all attenuation
- Piece of paper = Essentially all transmission
- Enamel = Mostly attenuated (mostly white)
- Cheek = Mostly transmission (radiopaque - black)
16
Q
What are the two specific attenuation interactions?
A
- Photoelectric effect (Complete absorption)
- Compton effect (Partial absorption and scatter)
17
Q
What is the photoelectric effect?
A
- 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
18
Q
Important points to remember for photoelectric effect? When does it occur?
A
- 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
19
Q
What does photon overcome during photoelectric effect?
A
- 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
20
Q
What is a by-product of photoelectric effect?
A
- Vacancy in inner shell filled by cascade of outer shell electrons
- Produces light photons (not visible to naked eye) and or heat
21
Q
What leads to lighter area on radiographic image during photoelectric effect?
A
- Absorption by photoelectric effect prevents x-ray photons reaching receptor
- Leads to lighter area on radiographic image
22
Q
What is the probability of photoelectric effect occurring?
A
- Proportional to atomic number cubed (Z^3)
- Inversely proportional to photon energy cubed (1/E^3)
- Proportional to physical density of material (p)
23
Q
What is the photoelectric effect equation?
A
Approx = (p x Z^3)/ E^3
24
Q
What does increasing KV do?
A
- Less attenuation of x-ray beam
25
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
26
Why are gold and lead so good at shielding?
- High atomic numbers
- High photoelectric effect
- Z^3 very high numbers
27
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
28
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)
29
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
30
What is the majority of scatter from an x-ray beam produced by x-ray tube operating at 70kV?
- Forward scatter
31
Why does the controlled area need to completely surround the patient?
- Scatter
32
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
33
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
34
What is the likelihood of the Compton effect occuring?
- Independent of Z
- Weakly proportional to photon energy
- Proportional to density of material
35
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
36
What are the 5 collimation steps to reduce scatter?
1. Decrease SA irradiated
2. Decrease volume of irradiated tissue
3. Decrease number of scattered photons produced in tissue
4. Decrease scattered photons interacting with receptor
5. Decrease loss of contrast on radiographic image
**Also reduces patient radiation dose and amount of radiation being released into surrounding**
37
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
38
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)
39
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
40
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
41
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
42
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
