Interaction of Radiation and Matter

Question 1

What three (general) processes occur when an x-ray beam interacts with a patient, and what do they cause?

Answer 1

Transmission - Forms image
Scattering - Staff Dose
Absorption - Patient Dose

Question 2

State and explain the equation for Exposure (X).

Answer 2

X = dQ/dm
X - The amount of radiation that produces, in air, ion of either sign equal to 1C/kg
dQ - The sum of all the charges (of all ions of one sign) produced when all electrons produced in a volume (dV) are stopped in air.
dm - The mass of air in volume dV

Question 3

State and explain the equation for Absorbed Dose (D).

Answer 3

Energy absorbed per unit mass; Gray (Gy)

Question 4

Explain how to go from Exposure to Dose in Air.

Answer 4

X = Q/m
X/e = No. Ion pairs produced
W - average energy to produce 1 ion pair.
E = W(X/e)

Question 5

What two assumptions are needed to equate KERMA to absorbed dose-in-air?

Answer 5

• Negligible Bremsstrahlung

- Short electron ranges

Question 6

How is dose in air converted to dose in another medium?

Answer 6

D(m)/D(a) = u/p/u/p
u - mass attenuation coefficient for the medium
p - density of the medium

Question 7

Explain the photoelectric effect.

Answer 7

X-Ray photon transfers all energy to inner (k) shell electron
Electron liberated with energy of photon - binding energy
Vacancy in k-shell filled by outer electron
Characteristic X-ray emitted (may be absorbed locally in low z material)
Dominant at lower energies.

Question 8

How does the photoelectric effect change with increasing atomic number?

Answer 8

Proportional to Z^3

Question 9

How does the photoelectric effect change with increasing energy?

Answer 9

Proportional to 1/E^3

Question 10

What is the name of an outer shell electron emitted instead of the photon through the photoelectric effect?

Answer 10

Auger Electron

Question 11

What is the equation for fluorescent yield (probability of no Auger Electrons)?

Answer 11

w = no of k x-ray photons/no of k shell vacancies

Question 12

Explain compton scattering.

Answer 12

Photon incident on “free” electron
Photon scatters at reduced energy
Electron recoils from collision
Change in photon energy depends on initial energy and angle of scatter.

Question 13

What is the change in wavelength from compton scatter?

Answer 13

Δλ = (h/mc)*(1-cosθ)

Question 14

What is the change in energy from compton scatter?

Answer 14

ΔE = E(0)*[α(1-cosθ)/(1 + α(1-cosθ))]
α = E(0) (in MeV)/0.511

Question 15

How does compton scatter change with increasing energy?

Answer 15

Constant (below 100keV)

Proportional to 1/E (above 100keV)

Question 16

How does compton scatter change with increasing Z

Answer 16

Independent of Z

Question 17

Explain elastic scattering.

Answer 17

Whole atom absorbs recoil
Bound electrons vibrate at frequency of photons
Electrons re-radiate energy at same frequency as photon
Scattering in forward radiation
Less than 10% of interaction - of little practical importance
Proportional to (Z^2)/E

Question 18

Explain pair production.

Answer 18

High energy (>1.02MeV) photon produces an electron-positron pair.
Positron annihilates with a local free electron.
Produces two photons (0.511MeV) in opposite directions.
Proportional to E-1.02 and Z

Question 19

Explain how k edge matching is used in X-Ray imaging

Answer 19

Match detector/contrast material with k-edge at peak of x-ray spectrum.
Maximises absorption, (k-shell electrons can now be emitted) for fewer photons.
Lower dose to patient for same image quality.

Question 20

Define the half value layer of a material

Answer 20

Thickness of a material at which the intensity of radiation is reduced by a half

Question 21

What are the 2 reasons for not observing true exponential attenuation of a clinical x-ray?

Answer 21

1. Spectrum of energies (low energies mainly attenuated)
2. Scatter from irradiated volume

There is a spectrum of energies, each with its own attenuation factor; and there is broad not narrow beam geometry, so a detector will measure some scattered radiation

Question 22

What is KERMA?

Answer 22

Kinetic energy released per unit mass

Question 23

What is ment by attenuation and absorption of radiation?

Answer 23

Attenuation: removal of radiation from the beam
Absorption: taking up of energy from the beam by irradiatied material

Question 24

What is the definition of mass attenuation coefficient? And its units?

Answer 24

linear coefficient/density

cm^2g^-1

Question 25

What is the definition of intensity of a radiation beam, and its unit?

Answer 25

Energy per unit mass, Sv

Dose multiplied by beam cross-sectional area, Gy cm2

Energy per unit mass, J Kg-1

Question 26

State two factors that will cause an increase in attenuation coefficient for a given material

Answer 26

Decreasing energy; Increasing density

Question 27

What two criteria are required for air kerma to be approximately equal to dose to air?

Answer 27

Negligible bremsstrahlung production; very short electron ranges