Categorical Course

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Categorical Course

• Barium Iodine Imaging Physics

George David Associate Professor Medical College
of Georgia Department of 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
-
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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
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Photoelectric Effect

• Interaction much more likely for
• low energy photons
• high atomic number elements
• Z7.4 for soft tissue

P.E. Z3
1 P.E. -----------
energy3
Atomic doubles Interaction 8X more likely
Energy doubles Interaction 8X less likely
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Photoelectric Effect

• Incoming Photon Energy Threshold
• gt binding energy of orbital electron
• binding energy of particular electron depends on
• atomic number
• higher for increasing atomic number
• shell
• lower for higher (outer) shells
• most likely to occur when photon energy close to
  but gt electron binding energy

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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
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Photoelectric Threshold

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

Photon energy 25
Which shells are candidates for photoelectric
interactions?
Photon in
10
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
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Photoelectric Threshold

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

Photon energy 55
Which shells are candidates for photoelectric
interactions?
Photon in
13
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
15
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

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Photoelectric Threshold

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

• Binding Energies
• K 50
• L 25

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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
M-shell interactions possible
Photon Energy
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Photoelectric Threshold

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

L-edge
K-edge
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Notes

• Barium iodine used to provide contrast
• Contrast greatest when agent most efficiently
  absorbs x-rays
• Maximum contrast obtained when photon energy of
  beam close to but slightly higher than absorber
  k-edge

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K-Edge

• Barium
• Atomic 56
• K-edge 36.45 keV
• Iodine
• Atomic 53
• k-edge 33.17

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Iodine Absorbtion k-edge 33.17 keV
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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

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Characteristic Radiation

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