X-Ray Interaction with Matter

Question 1

How can Xray photons transvering tissues pass by? (4)

Answer 1

1. Pass through unaltered
2. Change direction with no energy loss (scatter)
3. Change direction losing energy (scatter + absorption)
4. Be stopped, depositing all energy within the tissue (absorption)

Question 2

Define attenuation

Answer 2

• Reduction in no. of photons (Xrays) within the beam

- Affects number of photons reaching image receptor

Question 3

What does attenuation occur as a result of?

Answer 3

Absorption + scatter

Question 4

How does photon absorption show on image?

1. Black
2. Grey
3. White

Answer 4

BLACK
all photons reach film

GREY
partial attenuation

WHITE
complete attenuation

Question 5

Whats an atomic number

Answer 5

Number of protons

Question 6

How to calculate atomic mass?

Answer 6

Protons + neutrons

Question 7

In what order do electrons orbit the nucleus?

Answer 7

1. K
2. L
3. M

Question 8

Compare K shells to M shells

Answer 8

K has highest binding energy (requires more energy to eject electron from that shell)

Outer shells have lower binding energy

Question 9

What are the principle interactions of diagnostic Xrays in tissue (2)

Answer 9

1. Photoelectric effect - Absorption

2. Compton effect - scatter + absorption

Question 10

What are the effects of photoelectric absorption?

Answer 10

1. Complete absorption of photon, preventing any interaction with active component of image receptor
2. Image appears white if all photons involved
3. Grey if some photons not invovled

Question 11

What is the compton effect? (4)

Answer 11

1. Xray photon interacts with loosely bound outer shell electron
2. Photon energy considerably greater than e- binding energy
3. e’ is ejected taking some of photon energy as kinectic energy
4. Atom is then positively charged

Question 12

What happens to the excess energy in original photon?

Answer 12

Following collision, the photon has a lower energy

Its now called a scatter photon

It undergoes a change of direction

Question 13

What affects the direction of scatter photons?

Answer 13

High energy = forward direction

Backward direction = low energy

Question 14

When is the absorption of photons more likely?

Answer 14

1. If the object transvered has a high atomic number
2. If the object transversed is thicker
3. If the photon energy is lower