3. IONISING RADIATION: X-RAY IMAGING

Question 1

1. How do some images reveal tissues?

Answer 1

• they do this using both Photoelectric absorption and
  the Compton Scattering Effects
• this is due to the different densities in tissues

Question 2

2. What contributes to the contrast in this X-Ray image?

Answer 2

THE MOST DOMINANT ROLE:
- is done by Photoelectric Absorption
- the contrast is affected by the chemical composition of
the materials
- it is also affected by the photon energy

THE LESS DOMINANT ROLES ARE PLAYED BY:
- the differences in density
- the differences in thickness

Question 3

3. Which is the least important method of absorbtion?

Answer 3

• Pair Production

Question 4

4. What kind of photons undergo Pair Production?

Answer 4

• very high energy photons

Question 5

5. What happens in Pair Production if the photon has sufficient energy?

Answer 5

• the photon can be absorbed by an atomic nucleus in
  the absorber
• this results in the production of an electron and a
  positron

Question 6

6. What is Pair Production a case of?

Answer 6

• energy being converted into mass

Question 7

7. What is needed to produce the pair of particles in Pair Production?

Answer 7

• the mass of the electron and the positron has to be
  such that 1.02 MeV is needed to produce the pair of
  particles

Question 8

8. What happens if the incident photon has more than 1.02 MeV of energy?

Answer 8

• the excess energy will increase the velocity of the
  electron and the positron

Question 9

9. How long does a Positron live?

Answer 9

• not very long

Question 10

10. Why does a Positron not live very long?

Answer 10

IF IT MEETS AN ELECTRON IT CAN COMBINE WITH:
- it will produce two photons of 0.51 MeV
- these photons are called Annihilation Radiation

Question 11

11. What are Positron Emitters the basis of?

Answer 11

• they are the basis of the technique of Positron
  Emission Tomography (PET)

Question 12

12. How does PET Technology work?

Answer 12

• it uses the synchronous detection of two 0.51 MeV
  gamma photons to localise an emitter

Question 13

13. When do we use Pair Production?

Answer 13

• it is the dominant effect for Photon Energies above
  5 MeV

Question 14

14. Does this diagram make sense?

Answer 14

• yes

Question 15

15. What do Absorption processes do to the intensity of a beam of photons?

Answer 15

• they decrease it

Question 16

16. How else can we decrease the intensity of a beam of photons?

Answer 16

1. the intensity of the radiation falls off as you move
   away from the source
   - this source is known as the Point Source
2. the intensity of the radiation falls off in inverse
   proportions to the distance squared
   - this happens if the radiation of a source can spread
   in all directions
   - EG: a doubled distance = reduced exposure by a
   factor of 4

Question 17

17. Does this diagram make sense?

Answer 17

• yes