1. Barium & Iodine Imaging Physics
Categorical Course
Barium & Iodine Imaging Physics
George David
Associate Professor
Medical College of Georgia
Department of Radiology

2. **
Photoelectric Effect
photon interacts with bound (inner-shell) electron
electron liberated from atom (ionization)
photon disappears
Photon in
Electron out -

3. Photoelectric Effect Exiting electron kinetic energy
****
Photoelectric Effect
Exiting electron kinetic energy
incident energy - electron’s 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

4. 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

5. Photoelectric Effect Interaction much more likely for
low energy photons
high atomic number elements
Z=7.4 for soft tissue
P.E. ~ Z3 1
P.E. ~
energy3
Atomic # doubles:
Interaction 8X more likely
Energy doubles:
Interaction 8X less likely

6. Photoelectric Effect Incoming Photon Energy Threshold
> 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 > electron binding energy

7. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 15
Which shells are candidates for photoelectric interactions?
Photon in

8. Photoelectric Threshold
Binding Energies
K: 100
L: 50
M: 20
Photon energy: 15 NO NO NO
Which shells are candidates for photoelectric interactions?
Photon in

9. 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 NO
Which shells are candidates for photoelectric interactions?
Photon in

11. 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

12. 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 NO
Which shells are candidates for photoelectric interactions?
Photon in

14. 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
YES
Which shells are candidates for photoelectric interactions?

16. Photoelectric Threshold1
P.E. ~
energy3
Photoelectric interactions decrease with increasing photon energy BUT …

17. 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
Photon energy: 49 NO
YES
Photon energy: 51
YES
Binding Energies
K: 50
L: 25

18. Photoelectric Threshold
***
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

19. Photoelectric Threshold
causes step increases in interaction probability as photon energy exceeds shell binding energies
Photon Energy
Interaction
Probability
L-edge
K-edge

20. 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

21. K-Edge Barium Iodine Atomic #: 56 K-edge: 36.45 keV Atomic #: 53

22. Iodine Absorbtion: k-edge 33.17 keV
Mass Attenuation Coefficient (cm2/gm) 25
13.7 28
10.2 31
7.27
33.1
6.62
33.2
36.4 35
31.6 41
21.4

23. 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

24. Characteristic Radiation
only iodine & barium in diagnostic radiology have characteristic radiation which can reach film-screen