X-ray Interaction with Mattre

Question 1

Photons in a diagnostic X-ray beam can interact with matter in three ways, what are they?

Answer 1

Transmission = passes through matter unaltered
Absorption = stopped by the matter
Scatter = changes direction

Question 2

What occurs when a photon is absorbed?

Answer 2

• energy fully deposited into tissue
• photon ceases to exist

Question 3

what occurs when photons are scattered?

Answer 3

• photon deflected by tissue
• partial deposition of energy into tissue
• photon continues in a new direction

Question 4

What is X-ray beam intensity?

Answer 4

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

Question 5

What is X-ray beam intensity proportional to?

Answer 5

Energy & number of photons

Question 6

What is the energy of the X-ray beam affected by?

Answer 6

The potential difference across the X-ray tube

increased kV —> increased average photon energy & increased maximum photon energy

Question 7

What is the number of photons in the X-ray beam affected by?

Answer 7

The current in the filament

increased mA —> increased number of photons

Question 8

How does minimal attenuation present on a radiograph?

Answer 8

black

Question 9

How does partial attenuation present on a radiograph?

Answer 9

grey

Question 10

How does complete attenuation present on a radiograph?

Answer 10

white

Question 11

What is the photoelectric effect?

Answer 11

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

Question 12

When does the photoelectric effect occur?

Answer 12

when energy of incoming photon is equal to or just greater than binding energy of inner shell electron

Question 13

The photoelectric effect predominates with …

Answer 13

Lower energy photons

Question 14

What occurs in the photoelectric effect?

Answer 14

incoming photon energy overcomes binding energy, resulting in the inner shell electron being ejected
- photoeletron can ionise adjacent tissues

Question 15

What occurs in the Compton effect?

Answer 15

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

Question 16

What does the Compton effect predominately affect?

Answer 16

higher energy photons & outer shell electrons

Question 17

What type of photos deflect forwards? (‘forward scatter’)

Answer 17

higher energy photons

Question 18

What type of photons deflect backwards? (‘back scatter’)

Answer 18

lower energy photons

Question 19

What is the function of collimation?

Answer 19

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

Question 20

What are the positive effects of lowering the kV on an X-ray unit?

Answer 20

lower x-ray tube potential difference —> overall lower energy photons produced —> higher photoelectric effect interactions —> higher contrast between tissues with different Z (atomic number)

Question 21

What is the negative effect of lowering the kV on an X-ray unit?

Answer 21

• 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

Question 22

What should the kV be for an intra-oral X-ray unit based on UK guidance?

Answer 22

60-70kV