Basic Interactions Between X-Rays and Matter

1. Resident Physics Lectures (year 1) • Christensen, Chapter 4 Basic Interactions Between X-Rays and Matter George David Associate Professor Medical College of Georgia Department of Radiology

2. Basic X-Ray Interactions • Pair Production • Photoelectric Effect • Compton Scattering

3. *** Pair Production Process • high energy photon interacts with nucleus • photon disappears • electron & positron (positive electron) created - - + ~ + ~ + ~ + - -

4. *** Pair Production Process • energy in excess of 1.02 MeV given to electron/positron pair askinetic energy. • 0.51 MeV is energy equivalent of • Electron • Positron - - + ~ + ~ + ~ + - -

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

6. Pair Production • Threshold energy for occurrence: 1.02 MeV • energy equivalent of rest mass of electron-positron pair • Threshold above diagnostic energies • does not occur in diagnostic radiology • Principle of PET scanning

7. * Photon Phates • Nothing • Photon passes unmolested • Absorbed • completely removed from beam • ceases to exist • Scattered • change in direction • no useful information carried • source of noise X

8. Primary vs Scatter Focal Spot • Primary = Good photon • Scatter = Bad photon “Good” photon Patient “Bad” photon X Receptor

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

10. PHOTOELECTRIC EFFECT

11. **** 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

12. 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

13. Photoelectric Effect • Interaction much more likely for • low energy photons • high atomic number elements 1 P.E. ~ ----------- energy3 P.E. ~ Z3

15. Photoelectric Effect • Photon Energy Threshold • > 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 energies nearly the same

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

17. 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

18. Photoelectric Threshold • Binding Energies • K: 100 • L: 50 • M: 20 Photon energy: 25 Which shells are candidates for photoelectric interactions? Photon in

19. 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

20. 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

21. Photoelectric Threshold • Binding Energies • K: 100 • L: 50 • M: 20 Photon energy: 55 Which shells are candidates for photoelectric interactions? Photon in

22. 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

23. Photoelectric Threshold • Binding Energies • K: 100 • L: 50 • M: 20 Photon energy: 105 Which shells are candidates for photoelectric interactions? Photon in

24. Photoelectric Threshold • Binding Energies • K: 100 • L: 50 • M: 20 Photon energy: 105 YES YES Which shells are candidates for photoelectric interactions? YES

25. Photoelectric Threshold 1 P.E. ~ ----------- energy3 • Photoelectric interactions decrease with increasing photon energyBUT …

26. ** 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

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

28. ** 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

29. ** 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

30. 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 • Use highest kVp technique consistent with imaging contrast requirements

31. *** 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

32. - Electron out (recoil electron) Photon in Photon out Compton Scattering • What is a “free” electron? • low binding energy • outer shells for high Z materials • all shells for low Z materials

33. - Electron out (recoil electron) Photon in Photon out 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

34. - - - - 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.

35. 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)

36. Compton Scattering Probability of Occurrence • decreases with increasing photon energy • decrease less pronounced than for photoelectric effect Interaction Probability Compton Photoelectric Photon Energy

37. Photon Interaction Probabilities 100 Pair Production Photoelectric Z protons COMPTON 10 0.01 0.1 1.0 10 100 E energy (MeV)