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Radiology > X-ray Interation With Matter > Flashcards

X-ray Interation With Matter Flashcards

1
Q

How do photons travel?

A

travel in a straight line at the speed of light until they interact with something

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2
Q

How can photons in a diagnostic x-ray beam interact with matter?

A

in three ways
• Transmission (i.e. passes through matter unaltered)
• Absorption (i.e. stopped by the matter)
• Scatter (i.e. changes direction)

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3
Q

What happens to an absorbed photon?

A

• Energy fully deposited into tissue
• Photon ceases to exist

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4
Q

What happens to a scattered + absorbed photon?

A

• Photon deflected by tissue
• Partial deposition of energy into tissue
• Photon continues in new direction
• To be transmitted, absorbed or scattered again

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5
Q

In what scenarios does attenuation occur?

A

Scattered
Absorbed

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6
Q

What is the x-ray beam intensity?

A

Quantity of photon energy passing through a cross-sectional area of the beam per unit
time

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7
Q

What is the intensity proportional to?

A

• Energy of X-ray beam affected by potential difference across X-ray tube (kV)
• ↑ kV → ↑ average photon energy & ↑ maximum photon energy
• Number of photons in X-ray beam primarily affected by current in filament (mA)
• ↑ mA → ↑ number of photons
• Note: increasing potential difference will also increase number of photons since an electron with
more energy can undergo more interactions at the target

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8
Q

What does minimal attenuation look like on a radiograph?

A

Black

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9
Q

What does complete attenuation appear on a radiograph?

A

White

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10
Q

How can you predict x-ray photon interactions?

A

Unable to predict outcome of a single photon, but can estimate proportion of
interactions in an X-ray beam (which consists of millions of photons)
Predictions based on physical properties of matter being exposed
• e.g. thick lead → essentially all attenuation
• e.g. piece of paper → essentially all transmission
• e.g. enamel → mostly attenuation
• e.g. cheek → mostly transmission

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11
Q

What is the photoelectric effect?

A

Photon in X-ray beam interacts with inner shell electron in subject, resulting in absorption of the photon & creation of a photoelectron

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12
Q

How does the photoelectric effect occur?

A

Occurs when energy of incoming photon is equal to, or just greater than, binding energy of inner shell electron
• Therefore photoelectric effect predominates with lower energy photons (since
human tissues have relatively low binding energies)

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13
Q

What does the photoelectric effect result in?

A

Photon energy overcomes binding energy, resulting in inner shell electron
being ejected (now called a “photoelectron”)
• Any excess photon energy becomes kinetic energy of photoelectron
• Photoelectron can ionise (& potentially damage) adjacent tissues
Vacancy in inner shell is filled by cascade of outer shell electrons
• Produces light photons &/or heat

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14
Q

What does the photoelectric effect appear on a radiograph?

A

Absorption by the photoelectric effect prevents X-ray photons reaching the receptor → leads to lighter area on radiographic image

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15
Q

What is the probability of the photoelectric effect occurring?

A

• Proportional to atomic number cubed (Z3)
• Inversely proportional to photon energy cubed (1 / E3)
• Proportional to physical density of material (ρ)
p x Z3/ E3

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16
Q

What does increasing kV do to the beam?

A

• Increasing kV → less attenuation of the X-ray beam

17
Q

What is the photoelectric effect proportional to?

A

Photoelectric effect proportional to Z3

18
Q

What do small steps in z result in?

A

• Small steps in Z result in large jumps in absorption
• Results in good contrast between different tissues on radiographic image

19
Q

What is the Compton effect?

A

• Photon in X-ray beam interacts with outer shell electron in subject, resulting in partial
absorption & scattering of the photon & creation of a recoil electron

20
Q

When does the Compton effect occur?

A

Occurs when energy of incoming photon is much greater than binding energy
of electron
• Therefore Compton effect predominates with higher energy photons & outer shell
electrons (which are loosely bound)

21
Q

What is a recoil electron and what can it do?

A

Some photon energy transferred to electron to overcome binding energy &
provide kinetic energy
• Electron ejected & called a “recoil electron”
• Recoil electron can ionise (& potentially damage) adjacent tissues

22
Q

What happens to the photon in the Compton effect?

A

Photon loses energy & changes direction (i.e. is scattered)
• Can undergo Photoelectric effect & further Compton effect interactions

23
Q

What is the direction of scattered photons?

A

Scattered photons can be deflected in any direction 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 backward → “back scatter”

24
Q

What direction is the majority of scatter from an x-ray beam at 70kV?

A

Forward

25
Q

Why does the controlled area need to completely surround the patient

A

Scattered

26
Q

Why does the controlled area need to completely surround the patient

A

Scatter

27
Q

What is the effect of photons scattered backwards, sideways or very obliquely forwards?

A

Will not reach the receptor and do not affect image

28
Q

What is the effect of photons scattered slightly obliquely forwards?

A

Photons scattered slightly obliquely forwards may still reach the receptor but
will interact with the wrong area
• Causes darkening of the image in the wrong place
• Results in “fogging” of image → reduces image contrast/quality

29
Q

What is the probability of Compton effect occurring dependent on

A

• Independent of Z
• Weakly proportional to photon energy
• Proportional to density of material

30
Q

What will increasing the photon energy do to the Compton effect?

A

increasing photon energy has minimal effect on the likelihood of the Compton effect, but higher energy photons are more likely to scatter forwards,
reach the receptor, & degrade the radiographic image

31
Q

How can scatter be reduced?

A

Collimation

32
Q

What does collimation result in?

A
  1. ↓ surface area irradiated
  2. ↓ volume of irradiated tissue
  3. ↓ number of scattered photons produced in the tissue
  4. ↓ scattered photons interacting with receptor
  5. ↓ loss of contrast on radiographic image
33
Q

What impact does the photoelectric effect have on radiation dose?

A

• Deposition of all X-ray photon energy into tissue
• ↑ patient dose but is necessary for image formation

34
Q

What impact does the Compton effect have on radiation dose?

A

• Deposition of some X-ray photon energy in tissue
• ↑ patient dose but scattered photons do not contribute usefully to image
• May ↑ dose to operators (from back scatter)

35
Q

What is the effect of lowering kV on x-ray unit?

A

Lower X-ray tube potential difference (kV) >
Overall lower energy photons produced >
↑ photoelectric effect interactions >
↑ contrast between tissues with different Z >
BUT
↑ dose absorbed by patient

36
Q

What is the effect of raising kV on x-ray unit?

A

Higher X-ray tube potential difference (kV) >
Overall higher energy photons produced >
↓ photoelectric effect interactions (+ ↑ forward scatter) >
↓ dose absorbed by patient >
BUT
↓ contrast between tissues with different Z

37
Q

What does deciding on kV compromise?

A

Decision is a compromise between image quality
& patient radiation dose
UK guidance advises a suitable range of 60-70kV
for intraoral X-ray units
• Units typically give the option of either 60kV or 70kV
• Glasgow Dental Hospital uses 70kV

38
Q

Where do continuous/characteristic radiation interactions occur, what is the interaction between and what does it lead to?

A

• Occur in X-ray tube (at target)
• Electrons interacting with tungsten atoms
• Lead to production of X-ray photons

39
Q

Where do photoelectric/ Compton effects occur, what is the interaction between and what does it lead to?

A

• Occur in patient/receptor/shielding
• X-ray photons interacting with atoms
• Lead to attenuation of X-ray beam

Radiology (15 decks)

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