Interaction of Radiation with Matter Flashcards
3 ways x-rays interact and what they contribute to
Transmitted (form image)
Scattered (staff doses)
Absorbed (patient dose)
Attentuation
How photons are removed from the beam.
HVL
Half value layer - thickness with which incident intensity falls to half.
Why do real beams show departures from expo. shape at low depths?
Spectrum of energies (low energies preferentially attenuated)
Scatter from irradiated volume (not narrow beam)
Properties of HVL and mu (linear attenuation coeff.) with increasing energy
In diagnostic range, increasing energy means decrease in mu (increase in HVL).
Properties of mu with increasing density
mu increases - more molecules per unit volume.
HVL diagnostic range for water(~soft tissue) and lead
Water 30mm
Lead 0.1mm
Absorbed dose
D = deltaE/delta m
energy absorbed in mass delta m. Unit gray, 1Gy = 1J per kg
KERMA
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.
Photoelectric effect
X-ray ejects k shell electron, outer shell electron moves down, emits characteristic photon - difference between k and l shells.
Dominant process at low energies
Photoelectric effect, but need threshold energy (binding energy).
Soft tissue energy differences
Low Z - low energy differences between shells - characteristic radiation absorbed locally.
High Z metals energy difference
High energy difference - radiation can escape.
Auger Electron
Outer shell electron emitted instead of photon in Photoelectric effect.
Fluorescent yield
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
Probability Photoelectric effect
Scales with Z^3, 1/E^3
Compton Scattering
X-ray interacts with ‘free electron’, photon scatters and energetic electron released. Inelastic - energy loss. Change depends on initial energy and angle.
Higher energy Compton scattering - electron energy
More energy taken by electron, up to ~50%.
Probability Compton Scattering
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.
Disadvantages of scattered radiation
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
Backscattered radiation
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.
Elastic scattering (Raleigh)
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.
Probability of elastic scattering
scales with Z^2
1/E^2
K-edge
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.
Pair production
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.
Probability of pair production
scales with Z
scales with (E-1.02)MeV
