Interaction of Radiation with Matter

Question 1

3 ways x-rays interact and what they contribute to

Answer 1

Transmitted (form image)
Scattered (staff doses)
Absorbed (patient dose)

Question 2

Attentuation

Answer 2

How photons are removed from the beam.

Question 3

HVL

Answer 3

Half value layer - thickness with which incident intensity falls to half.

Question 4

Why do real beams show departures from expo. shape at low depths?

Answer 4

Spectrum of energies (low energies preferentially attenuated)
Scatter from irradiated volume (not narrow beam)

Question 5

Properties of HVL and mu (linear attenuation coeff.) with increasing energy

Answer 5

In diagnostic range, increasing energy means decrease in mu (increase in HVL).

Question 6

Properties of mu with increasing density

Answer 6

mu increases - more molecules per unit volume.

Question 7

HVL diagnostic range for water(~soft tissue) and lead

Answer 7

Water 30mm
Lead 0.1mm

Question 8

Absorbed dose

Answer 8

D = deltaE/delta m
energy absorbed in mass delta m. Unit gray, 1Gy = 1J per kg

Question 9

KERMA

Answer 9

KE released per unit mass - energy transferred from x-ray beam to electrons at a given point. To good approx. air kerma = dose to air.

Question 10

Photoelectric effect

Answer 10

X-ray ejects k shell electron, outer shell electron moves down, emits characteristic photon - difference between k and l shells.

Question 11

Dominant process at low energies

Answer 11

Photoelectric effect, but need threshold energy (binding energy).

Question 12

Soft tissue energy differences

Answer 12

Low Z - low energy differences between shells - characteristic radiation absorbed locally.

Question 13

High Z metals energy difference

Answer 13

High energy difference - radiation can escape.

Question 14

Auger Electron

Answer 14

Outer shell electron emitted instead of photon in Photoelectric effect.

Question 15

Fluorescent yield

Answer 15

w_k = no. of k x-ray photons/ no. of k shell vacancies
= 1, no Auger electrons
=0 all Auger electrons

Z<30, wk<0.5, mostly auger
Z>65, wk>0.9, mostly characteristic x-ray emission

Question 16

Probability Photoelectric effect

Answer 16

Scales with Z^3, 1/E^3

Question 17

Compton Scattering

Answer 17

X-ray interacts with ‘free electron’, photon scatters and energetic electron released. Inelastic - energy loss. Change depends on initial energy and angle.

Question 18

Higher energy Compton scattering - electron energy

Answer 18

More energy taken by electron, up to ~50%.

Question 19

Probability Compton Scattering

Answer 19

Diagnostic range (10-100keV) - constant
>100keV - 1/E
Independent of Z
Depends on electron density - more electrons more likely to occur. Electron density in Hydrogen 1 - scatters a lot.

Question 20

Disadvantages of scattered radiation

Answer 20

Detrimental to image quality (low energies may not escape patient to be detected, higher energies less likely to be scattered but may more easily escape patient)
Scattered radiation gives dose to staff
Reduces contrast

Question 21

Backscattered radiation

Answer 21

Coming back out of patient.
Backscatter factor = dose at P with scatter/ dose at P without scatter (P is top of patient).
~1.2-1.3.
This is why undercouch is preferred.

Question 22

Elastic scattering (Raleigh)

Answer 22

Whole atom absorbs recoil, ‘bound’ electrons vibrate at frequency of photons, re-radiate energy at same frequency in forward direction. <10% of interactions, of little importance.

Question 23

Probability of elastic scattering

Answer 23

scales with Z^2
1/E^2

Question 24

K-edge

Answer 24

K-edge continuity, rises when there is energy to overcome binding energy. Match energy of k-edge to peak of x-ray spectrum, maximise absorption in detector.

Question 25

Pair production

Answer 25

Incident high energy photon produces electron positron pair. Need 1.02 MeV because mc^2 is 511keV for each. Not going to happen in diagnostic radiology.

Question 26

Probability of pair production

Answer 26

scales with Z
scales with (E-1.02)MeV