Interaction of radiation with matter

Question 1

3 things that can happen to beam in patient

what does each contriobute to?

Answer 1

1. transmitted - image
2. Scattered - staff dose & background on image
3. absorbed - patient dose

Question 2

Define Intensity

Units

How do you get to energy?

Answer 2

Energy/Area/time

Jm^-2s^-1

Energy = sum of (photon * energy)

Question 3

Define attenuation

How is radiation attenuated in a material?

equation

Answer 3

How photons are removed from beam

Exponerntial

N=N_o e^-ux

Question 4

Define HVL

Answer 4

Half Value Layer - thickness of material that reduces the incident intensity to hald

N/No = 0.5, read off graph for the thickness at which this happens.

Question 5

Why do real beams depart from exponential at low depths?

Answer 5

• Beams are a apsectrum of energies, low energies are preferentially attenuated
• scatter from irradiated volume

Question 6

How are HVL and u related?

Answer 6

HVL = ln(2)/u

More penetrating beam =

• higher HVL
• lower u

Question 7

What happends to u with:

increasing energy

increasing density

Answer 7

increasing energy - decrease in u, radiation is more penetrating

increasing density - u increases, more molecults/vol

Question 8

Defiine Mass Attenuation Coefficent

Units

Why do we bother?

Answer 8

u/P

N = No e-(u/P)(Px)

cm²g^-1

Density dependece of u is annoying, especially when substance has several density states

Question 9

What are u and u/P proportional to?

Answer 9

the probability that an x-ray photon will undergo some kind of interaction with the material.

Question 10

Difference between attenuation and apsorbtion

Why are they not the same?

Answer 10

Attenuation is removal of energy from the beam

Absorbtion is gain of energy in the material

Not the same because the photons removed from the beam are not always depositied at that point, they have secondary range, go on to interact further etc.

Question 11

Absorbed Dose

Definition & EQn

Unit

Answer 11

D = delta E/ delta m

Energy absorbed per unit mass

Gy = 1J/Kg

Question 12

KERMA

Definition & EQn

Unit

Why is it not equal to dose in air?

Answer 12

Kinetic energy released from the beam per unit mass

Energy transferred from the beam to material’s electrons at the specified point

Gy

Kerma not equal to dose in air because energy removed from beam is not the same as the enrgy absorbed in that masss:

• secondary electron energy re-radiated as bremsstrahlung photons & escape
• finite electron range
  *

Question 13

When is air kerma approx equal to dose in air?

Answer 13

diagnostic energy range

• negligable bremsstrahlung
• electron range very short

Question 14

What is exposure?

Answer 14

The amount of radiation which produces, in air, ions equal to 1C/Kg

X = delta Q / delta m

Only apples to ionisation in air

Question 15

How do you go from dose-in-air to dose in any other substance?

Answer 15

Absorbed dose (energy absorbed per unit mass) depends on the mass energy absorption coefficent

Dm/Dair = (u/p)_m/(u/p)_air

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