Resident Physics Lectures

1
Resident Physics Lectures

• Christensen, Chapter 4
• Basic Interactions Between X-Rays and Matter

George David Associate Professor Medical College
of Georgia Department of Radiology
2
Basic Interactions

• Coherent Scattering
• Pair Production
• Photodisintegration
• Photoelectric Effect
• Compton Scattering

3
Photon Phate

• absorbed
• completely removed from beam
• ceases to exist
• scattered
• change in direction
• no useful information carried
• source of noise
• Nothing
• Photon passes unmolested

X
4
Noise

• covers valid information with distracting or
  obscuring garbage

5
Image Noise

• covers valid information with distracting or
  obscuring garbage

Caution! Image Noise
6
Image Noise Example
Caution! Image Noise
7
Coherent Scattering

• Also called
• unmodified scattering
• classical scattering
• Types
• Thomson
• photon interacts with single electron
• Rayleigh
• photon interacts with all electrons of an atom

8
Coherent Scattering

• Change in direction
• No change in
• energy
• frequency
• wavelength
• No ionization
• Contributes to scatter as film fog
• Less than 5 of interactions
• insignificant effect on image quality compared to
  other interactions

9
Pair Production Process

• high energy photon interacts with nucleus
• photon disappears
• electron positron (positive electron) created
• energy in excess of 1.02 MeV given to
  electron/positron pair askinetic energy.

-
-







-
-
10
Positron Phate

• Positron undergoes ANNIHILATION REACTION
• Two 0.511 MeV photons created
• Photons emerge in exactly opposite directions

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Pair Production

• Threshold energy for occurrence
• 1.02 MeV
• energy equivalent of rest mass of 2 electrons
• Threshold is above diagnostic energies
• does not occur in diagnostic radiology

12
Photodisintegration

• photon causes ejection of part of atomic nucleus
• ejected particle may be
• neutron
• proton
• alpha
• particle cluster

-






?
-
-
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Photodisintegration

• Threshold photon energy for occurrence
• nuclear binding energy
• typically 7-15 MeV
• Threshold is above diagnostic energies
• does not occur in diagnostic radiology

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Photoelectric Effect

• photon interacts with bound (inner-shell)
  electron
• electron liberated from atom (ionization)
• photon disappears

Electron out
Photon in
-
15
PHOTOELECTRIC EFFECT
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Photoelectric Effect

• Exiting electron kinetic energy
• incident energy - electrons binding energy
• electrons in higher energy shells cascade down to
  fill energy void of inner shell
• characteristic radiation

M to L
Electron out
Photon in
-
L to K
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Photoelectric Interaction Probability

• inversely proportional to cube of photon energy
• low energy event
• proportional to cube of atomic number
• more likely with inner (higher) shells
• tightly bound electrons

1 P.E. -----------
energy3
P.E. Z3
18
Photoelectric Effect

• Interaction much more likely for
• low energy photons
• high atomic number elements

1 P.E. -----------
energy3
P.E. Z3
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Photoelectric Effect

• Photon Energy Threshold
• gt binding energy of orbital electron
• binding energy depends on
• atomic number
• higher for increasing atomic number
• shell
• lower for higher (outer) shells
• most likely to occur when photon energy
  electron binding energy are nearly the same

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Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 15
Which shells are candidates for photoelectric
interactions?
Photon in
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Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 15
NO
NO
Which shells are candidates for photoelectric
interactions?
NO
Photon in
22
Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 25
Which shells are candidates for photoelectric
interactions?
Photon in
23
Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 25
YES
NO
Which shells are candidates for photoelectric
interactions?
NO
Photon in
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Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 25
1 P.E. -----------
energy3
A
Which photon has a greater probability for
photoelectric interactions with the m shell?
Photon in
B
Photon energy 22
25
Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 55
Which shells are candidates for photoelectric
interactions?
Photon in
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Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 55
YES
YES
Which shells are candidates for photoelectric
interactions?
NO
Photon in
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Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 105
Which shells are candidates for photoelectric
interactions?
Photon in
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Photoelectric Threshold

• Binding Energies
• K 100
• L 50
• M 20

Photon energy 105
YES
YES
Which shells are candidates for photoelectric
interactions?
YES
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Photoelectric Threshold
1 P.E. -----------
energy3

• Photoelectric interactions decrease with
  increasing photon energyBUT

30
Photoelectric Threshold

• When photon energies just reaches binding energy
  of next (inner) shell, photoelectric interaction
  now possible with that shell
• shell offers new candidate target electrons

L-shell interactions possible
Interaction Probability
L-shell binding energy
K-shell interactions possible
K-shell binding energy
Photon Energy
31
Photoelectric Threshold

• causes step increases in interaction probability
  as photon energy exceeds shell binding energies

L-edge
K-edge
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Characteristic Radiation

• Occurs any time inner shell electron removed
• energy states
• orbital electrons seek lowest possible energy
  state
• innermost shells

M to L
L to K
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Characteristic Radiation

• electrons from higher states fall (cascade) until
  lowest shells are full
• characteristic x-rays released whenever electron
  falls to lower energy state

M to L
characteristic x-rays
L to K
34
Characteristic Radiation

• only iodine barium in diagnostic radiology have
  characteristic radiation which can reach
  film-screen

35
Photoelectric Effect

• Why is this important?
• photoelectric interactions provide subject
  contrast
• variation in x-ray absorption for various
  substances
• photoelectric effect does not contribute to
  scatter
• photoelectric interactions deposit most beam
  energy that ends up in tissue
• always use highest kVp technique consistent with
  imaging contrast requirements

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Compton Scattering

• Source of virtually all scattered radiation
• Process
• incident photon (relatively high energy)
  interacts with free (loosely bound) electron
• some energy transferred to recoil electron
• electron liberated from atom (ionization)
• emerging photon has
• less energy than incident
• new direction

-
Electron out (recoil electron)
Photon out
Photon in
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Compton Scattering

• What is a free electron?
• low binding energy
• outer shells for high Z materials
• all shells for low Z materials

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Compton Scattering

• Incident photon energy split between electron
  emerging photon
• Fraction of energy carried by emerging photon
  depends on
• incident photon energy
• angle of deflection
• similar principle to billiard ball collision

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Compton Scattering Angle of Deflection

• higher incident energy less photon deflection
• high energy (1MeV) photons primarily scatter
  forward
• diagnostic energy photons scatter fairly
  uniformly
• forward backward
• at diagnostic energy photons lose very little
  energy during Compton Scattering
• higher deflection less energy retained
• photons having small deflections retain most
  incident incident energy

Electron out (recoil electron)
-
deflection angle
Photon in
Photon out
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Compton Scattering Angle of Deflection

• Photons having small deflections retain most
  incident incident energy
• Photons will scatter many times, losing a little
  energy each time.

41
Compton Scattering

• Formula
• D l 0.024 (1-cos Q)
• where
• D l change in wavelength (A) for photon
• angle of photon deflection (0-180 degrees)

0o results in no change in wavelength 180o
results in maximum change in wavelength
recoil electron
-
Angle Q
Photon in
Photon out
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Compton Scattering Probability of Occurrence

• independent of atomic number (except for
  hydrogen)
• Proportional to electron density (electrons/gram)
• fairly equal for all elements except hydrogen (
  double)

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Compton Scattering Probability of Occurrence

• decreases with increasing photon energy
• decrease much less pronounced than for
  photoelectric effect

Interaction Probability
Compton
Photoelectric
Photon Energy
44
Photon Interaction Probabilities
100
Pair Production
Photoelectric
Z protons
COMPTON
10
0.01 0.1 1.0
10 100
E energy (MeV)