Resident Physics Lectures

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

2. Photons atoms interations What happen when photons interact with human tissue?

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. Image Noise Example Caution! Image Noise

5. Basic Interactions • Coherent Scattering • Compton Scattering • Photoelectric Effect • Pair Production • Photodisintegration

6. What is important in this lecture How the interaction happen? When happen? The interaction affect on the image quality?

7. Interaction depends on - + ~ + ~ + ~ - Photon energy e.v Atom atomic number Z -

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

9. Basic Interactions • Coherent Scattering D • Compton Scattering D • Photoelectric Effect D • Pair Production T • Photodisintegration T • D=Diagnostic radiology • T=Treatment radiology

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

12. 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

13. 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. - - + ~ + ~ + ~ + - -

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

15. - - + ~ + ~ + ~ + - - 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

16. Photodisintegration * • photon causes ejection of part of atomic nucleus • ejected particle may be • neutron • proton • alpha • particle cluster - + ~ + ~ + ~ ? - -

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

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

19. PHOTOELECTRIC EFFECT

20. 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

21. 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

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

23. 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 energy are nearly the same

24. Photoelectric Threshold Photon energy: 25 • Binding Energies • K: 100 • L: 50 • M: 20 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 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. Characteristic Radiation • only iodine & barium in diagnostic radiology have characteristic radiation which can reach film-screen

31. 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

32. 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

33. - 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

34. - 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

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 much 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)

38. Resident Physics Lectures • Christensen, Chapter 5 Attenuation George David Associate Professor Medical College of Georgia Department of Radiology

39. Beam Characteristics • Quantity • number of photons in beam 1, 2, 3, ... ~ ~ ~ ~ ~

40. ~ ~ ~ ~ ~ ~ ~ Beam Characteristics • Quality • energy distribution of photons in beam 1 @ 27 keV, 2 @ 32 keV, 2 at 39 keV, ... ~

41. Beam Characteristics • Intensity • weighted product of number and energy of photons • depends on • quantity • quality 324 mR ~ ~ ~ ~ ~ ~ ~ ~

42. Beam Intensity • Can be measured in terms of # of ions created in air by beam • Valid for monochromatic or for polychromatic beam 324 mR - ~ +

43. Attenuation Coefficient • Parameter indicating fraction of radiation attenuated by a given absorber thickness • Attenuation Coefficient is function of • absorber • photon energy Monochromatic radiation beam

44. Linear Attenuation Coef. • Why called linear? • distance expressed in linear dimension “x” • Formula N = No e -mx where No = number of incident photons N = number of transmitted photons e = base of natural logarithm (2.718…) m = linear attenuation coefficient (1/cm); property of energy material x = absorber thickness (cm) • No • N x Monochromatic radiation beam

45. Linear Attenuation Coef. Larger Coefficient = More Attenuation • Units: 1 / cm ( or 1 / distance) • Properties • reciprocal of absorber thickness that reduces beam intensity by e (~2.718…) • ~63% reduction • 37% of original intensity remaining • as photon beam energy increases • penetration increases / attenuation decreases • attenuating distance increases • linear attenuation coefficient decreases • Note: Same equation as used for radioactive decay • N = No e - m x Monochromatic radiation beam

46. Polychromatic Radiation • X-Ray beam contains spectrum of photon energies • highest energy = peak kilovoltage applied to tube • mean energy 1/3 - 1/2 of peak • depends on filtration

47. Higher Energy Lower Energy X-Ray Beam Attenuation • reduction in beam intensity by • absorption (photoelectric) • deflection (scattering) • Attenuation alters beam • quantity • quality • higher fraction of low energy photons removed • Beam Hardening

48. Half Value Layer (HVL) N = No e -mx • absorber thickness that reduces beam intensity by exactly half • Units of thickness • value of “x” which makes N equal to No / 2 • HVL = .693 / m

49. Half Value Layer (HVL) • Indication of beam quality • Valid concept for all beam types • Mono-energetic • Poly-energetic • Higher HVL means • more penetrating beam • lower attenuation coefficient

50. Factors Affecting Attenuation • Energy of radiation / beam quality • higher energy • more penetration • less attenuation • Matter • density • atomic number • electrons per gram • higher density, atomic number, or electrons per gram increases attenuation