Interactions with matter

Question 1

What is Attenuation?

Answer 1

Reduction of intensity of x-ray before hitting the detector

Question 2

What is Absorption?

Answer 2

Process where energy is converted from wave of light to kinetic energy, to move electron out of orbit

Question 3

What is Scattering?

Answer 3

Process where original path of incident photon is changed

Question 4

Describe the basic concept of image forming absorption

Answer 4

X-ray photons all have different energies
Lower photon energy = more likely to be absorbed by body
Bone has more ‘stopping power’

Question 5

Do we want x-rays to be absorbed?

Answer 5

Image forming absorption of photons is the only radiation absorption we want
We must minimise and optimise radiation used

Question 6

What are the 2 examples of absorption?

Answer 6

Photoelectric absorption
Pair Production

Question 7

What are the 2 examples of scattering?

Answer 7

Compton scattering
Elastic scattering

Question 8

Describe Photoelectric Absorption

Answer 8

-Incident photon interacts with atom
-K shell electron struck
-All energy is absorbed by struck electron
-Becomes photoelectron, leaves orbit
-L shell electron fills vacancy
-Transition creates low energy secondary photon, absorbed by patient

Question 9

Describe Pair production

Answer 9

-Incident photon strikes nucleus
-Photon transformed, energy transferred to electron and positron
-Electron quickly absorbed into other atom shell
-As positron rests, combines with near electron, neutralise each other
-Convert back to 2 photons, travel away from each other

Question 10

Describe Compton scattering

Answer 10

• Incident photon strikes outer shell electron
• Half of energy absorbed by struck electron, ejected at 45 degrees
• Original photon scattered opposite direction

Question 11

Describe Elastic scattering

Answer 11

-Incident photon travels towards atom
-Doesn’t have required energy to eject electrons
-Incident photon scattered