X-ray Interation With Matter

Question 1

How do photons travel?

Answer 1

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

Question 2

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

Answer 2

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

Question 3

What happens to an absorbed photon?

Answer 3

• Energy fully deposited into tissue
• Photon ceases to exist

Question 4

What happens to a scattered + absorbed photon?

Answer 4

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

Question 5

In what scenarios does attenuation occur?

Answer 5

Scattered
Absorbed

Question 6

What is the x-ray beam intensity?

Answer 6

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

Question 7

What does minimal attenuation look like on a radiograph?

Answer 7

Black

Question 8

What does complete attenuation appear on a radiograph?

Answer 8

White

Question 9

How can you predict x-ray photon interactions?

Answer 9

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

Question 10

What is the photoelectric effect?

Answer 10

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

Question 11

How does the photoelectric effect occur?

Answer 11

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)

Question 12

What does the photoelectric effect result in?

Answer 12

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

Question 13

What does the photoelectric effect appear on a radiograph?

Answer 13

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

Question 14

What does increasing kV do to the beam?

Answer 14

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

Question 15

What is the photoelectric effect proportional to?

Answer 15

Photoelectric effect proportional to Z3 (atomic number cubed)

Question 16

What do small steps in z result in?

Answer 16

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

Question 17

What is the Compton effect?

Answer 17

• 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

Question 18

When does the Compton effect occur?

Answer 18

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)

Question 19

What is a recoil electron and what can it do?

Answer 19

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

Question 20

What happens to the photon in the Compton effect?

Answer 20

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

Question 21

What is the direction of scattered photons?

Answer 21

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”

Question 22

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

Answer 22

Forward

Question 23

Why does the controlled area need to completely surround the patient

Answer 23

Scatter

Question 24

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

Answer 24

Will not reach the receptor and do not affect image

Question 25

What is the effect of photons scattered slightly obliquely forwards?

Answer 25

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

Question 26

What is the probability of Compton effect occurring dependent on

Answer 26

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

Question 27

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

Answer 27

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

Question 28

How can scatter be reduced?

Answer 28

Collimation

Question 29

What does collimation result in?

Answer 29

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

Question 30

What impact does the photoelectric effect have on radiation dose?

Answer 30

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

Question 31

What impact does the Compton effect have on radiation dose?

Answer 31

• 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)

Question 32

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

Answer 32

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

Question 33

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

Answer 33

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

Question 34

What does deciding on kV compromise?

Answer 34

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

Question 35

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

Answer 35

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

Question 36

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

Answer 36

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