2. 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

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

4. 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 *

5. Image Noise Example Caution! Image Noise

6. Basic Interactions Coherent Scattering Compton ScatteringCompton Scattering Photoelectric EffectPhotoelectric Effect Pair Production Photodisintegration

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

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

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

10. Basic Interactions Coherent Scattering D Compton Scattering DCompton Scattering D Photoelectric Effect DPhotoelectric Effect D Pair Production T Photodisintegration T D=Diagnostic radiology T=Treatment radiology

11. Coherent scattering Pair productio n Compton scattering Photoelec tric effect photon interacts with all electrons of an atom Photon interacts With nucleus Photon interacts With outer electrons Photon interacts With inner electrons Source highmediumlowPhoton energy TDDD OR T

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

13. 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

14. Pair Production Process high energy photon interacts with nucleus photon disappears electronpositronelectron & positron (positive electron) created energy in excess of 1.02 MeV given to electron/positron pair as kinetic energy. - - - + + ~ ~ + ~ - + ***

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

16. 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 - - - + + ~ ~ + ~ - +

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

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

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

20. PHOTOELECTRIC EFFECT

21. 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 Electron out Photon in M to L L to K - ****

22. 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. ~ ----------- energy 3 P.E. ~ Z 3

23. Photoelectric Effect Interaction much more likely for  low energy photons  high atomic number elements 1 P.E. ~ ----------- energy 3 P.E. ~ Z 3

24. 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

25. Photoelectric Threshold Binding Energies  K: 100  L: 50  M: 20 Photon in Photon energy: 22 Which photon has a greater probability for photoelectric interactions with the m shell? Photon energy: 25 A B 1 P.E. ~ ----------- energy 3

26. Photoelectric Threshold Photoelectric interactions decrease with increasing photon energy BUT … 1 P.E. ~ ----------- energy 3

27. 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 Photon Energy Interaction Probability K-shell interactions possible L-shell interactions possible L-shell binding energy ** K-shell binding energy

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

29. 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 **

30. 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 L to K characteristic x-rays **

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

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

33. 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 in Photon out - ***

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

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

36. 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)

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

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

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

40. Beam Characteristics Quantity  number of photons in beam ~ ~~ ~ ~ 1, 2, 3,...

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

42. Beam Characteristics Intensity  weighted product of number and energy of photons  depends on »quantity »quality 324 mR ~ ~~ ~ ~ ~ ~ ~

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

44. Attenuation Coefficient Parameter indicating fraction of radiation attenuated by a given absorber thickness Attenuation Coefficient is function of  absorber  photon energy

45. Linear Attenuation Coef. Why called linear?  distance expressed in linear dimension “x” Formula N = N o e -  x where N o = number of incident photons N = number of transmitted photons e = base of natural logarithm (2.718…)  = linear attenuation coefficient (1/cm); property of energy material x = absorber thickness (cm) NoNo N x

46. Linear Attenuation Coef. 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 = N o e -  x Larger Coefficient = More Attenuation

47. 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

48. 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 Higher Energy Lower Energy

49. N = N o e -  x HVL =.693 /  absorber thickness that reduces beam intensity by exactly half Units of thickness value of “x” which makes N equal to N o / 2 Half Value Layer (HVL)

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

51. 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

52. Polychromatic Attenuation Yields curved line on semi-log graph  line straightens with increasing attenuation  slope approaches that of monochromatic beam at the peak energy mean energy increases with attenuation  beam hardening Attenuator Thickness 1.1.01.001 Fraction Transmitted Monochromatic Polychromatic

53. Photoelectric vs. Compton Fractional contribution of each determined by  photon energy  atomic number of absorber Equation  =  coherent +  PE +  Compton Small

54. Photoelectric vs. Compton As photon energy increases  Both PE & Compton decrease  PE decreases faster »Fraction of  that is Compton increases »Fraction of  that is PE decreases  =  coherent +  PE +  Compton Photon Energy Interaction Probability Compton Photoelectric

55. Photoelectric vs. Compton As atomic # increases  Fraction of  that is PE increases  Fraction of  that is Compton decreases  =  coherent +  PE +  Compton

56. Interaction Probability Photoelectric Compton Pair Production Photon Energy Atomic Number of Absorber PE dominates for very low energies

57. Interaction Probability Photoelectric Compton Pair Production Photon Energy Atomic Number of Absorber For lower atomic numbers –Compton dominates for high energies

58. Interaction Probability Photoelectric Compton Pair Production Photon Energy Atomic Number of Absorber For high atomic # absorbers –PE dominates throughout diagnostic energy range

59. Attenuation & Density Attenuation proportional to density  difference in tissue densities accounts for much of optical density difference seen radiographs # of Compton interactions depends on electrons / unit path  which depends on »electrons per gram »density

60. Relationships Density generally increases with atomic #  different states = different density »ice, water, steam no relationship between density and electrons per gram atomic # vs. electrons / gram  hydrogen ~ 2X electrons / gram as most other substances  as atomic # increases, electrons / gram decreases slightly

61. Applications As photon energy increases  subject (and image) contrast decreases  differential absorption decreases »at 20 keV bone’s linear attenuation coefficient 6 X water’s »at 100 keV bone’s linear attenuation coefficient 1.4 X water’s

62. Applications At low x-ray energies  attenuation differences between bone & soft tissue primarily caused by photoelectric effect »related to atomic number & density Photo- electric Compton Pair Production

63. Applications At high x-ray energies  attenuation differences between bone & soft tissue primarily because of Compton scatter »related entirely to density Photo- electric Compton Pair Production

64. Applications Difference between water & fat only visible at low energies  effective atomic # of water slightly higher »yields photoelectric difference  electrons / cm almost equal »No Compton difference  Photoelectric dominates at low energy

65. Scatter Radiation NO Socially Redeeming Qualities  no useful information on image  detracts from film quality  exposes personnel, public represents 50-90% of photons exiting patient

66. Abdominal Photons ~1% of incident photons on adult abdomen reach film fate of the other 99%  mostly scatter »most do not reach film  absorption

67. Scatter Factors Factors affecting scatter  field size  thickness of body part  kVp Factors affecting scatter  field size  thickness of body part  kVp An increase in any of above increases scatter.

68. Scatter & Field Size Reducing field size causes significant reduction in scatter radiation II Tube X-Ray Tube II Tube X-Ray Tube

69. Field Size & Scatter Field Size & thickness determine volume of irradiated tissue Scatter increase with increasing field size  initially large increase in scatter with increasing field size  saturation reached (at ~ 12 X 12 inch field) »further field size increase does not increase scatter reaching film »scatter shielded within patient

70. Thickness & Scatter Increasing patient thickness leads to increased scatter but saturation point reached  scatter photons produced far from film  shielded within body

71. kVp & Scatter kVp has less effect on scatter than than  field size  thickness Increasing kVp  increases scatter  more photons scatter in forward direction

72. Scatter Management Reduce scatter by minimizing  field size »within limits of exam  thickness »mammography compression  kVp »but low kVp increases patient dose »in practice we maximize kVp

73. Scatter Control Techniques: Grid directional filter for photons Increases patient dose

74. Scatter Control Techniques: Air Gap Gap intentionally left between patient & image receptor Natural result of magnification radiography Grid not used (covered in detail in chapter 8) Patient Grid Film Cassette Air Gap

75. Resident Physics Lectures Christensen, Chapter 8Grids George David Associate Professor Department of Radiology Medical College of Georgia

76. Purpose Directional filter for photons Ideal grid  passes all primary photons »photons coming from focal spot  blocks all secondary photons »photons not coming from focal spot Film Patient “Good” photon “Bad” photon Grid X Focal Spot

77. Grid Construction Lead ~.05“ thick upright strips (foil) Interspace  material between lead strips  maintains lead orientation  materials »fiber »aluminum »wood Lead Interspace

78. Grid Ratio Ratio of interspace height to width h w Grid ratio = h / w Lead Interspace

79. Grid Ratio Expressed as X:1 Typical values 8:1 to 12:1 for general work 3:1 to 5:1 for mammography Grid function generally improves with higher ratios h w Grid ratio = h / w

80. Lines per Inch # lead strips per inch grid width Typical: 103 25.4 Lines per inch = ------------ W + w w = thickness of interspace (mm) W = thickness of lead strips (mm) w W

81. Grid Structure

82. Grid Patterns Orientation of lead strips as seen from above Types  Linear  Cross hatched »2 stacked linear grids »ratio is sum of ratios of two linear grids »very sensitive to positioning & tilting »Rare; only found in specials

83. Grid Styles Parallel Focused

84. Parallel Grid lead strips parallel useful only for  small field sizes  large source to image distances

85. Focused Grid Slightly angled lead strips convergence lineStrip lines converge to a point in space called convergence line Focal distance  distance from convergence line to grid plane Focal range  working distance range »width depends on grid ratio »smaller ratio has greater range Focal range Focal distance

86. Grid Cassette Grid built into cassette front Sometimes used for portables  formerly used in mammography low grid ratios focused

87. Ideal Grid passes all primary radiation  Reality: lead strips block some primary Lead Interspace

88. Ideal Grid block all scattered radiation  Reality: lead strips permit some scatter to get through to film Lead Interspace

89. Grid Performance Measurements Primary Transmission (Tp) Bucky Factor (B) contrast improvement factor (K)

90. Resident Physics Lectures Christensen, Chapter 6Filters George David Associate Professor Department of Radiology Medical College of Georgia

91. Energy Spectrum X-ray beams from tubes  Polychromatic »Brehmstrahlung »Characteristic  spectrum of energies from 0 – kVp set on generator average beam energy 1/3 to 1/2 of peak (kVp) kVp (as set on generator)

92. Unfiltered Beams most energy deposited in first few centimeters of tissue  lowest energy photons selectively removed energy of low energy photons  contributes to dose  does not contribute to image »photons don’t reach film Patient film

93. Ideal Filtration absorption characteristics  absorbs all low energy radiation  absorbs no high energy radiation high atomic number desirable  increases photoelectric absorption of low energy photons

94. Filter’s Function shape beam’s energy spectrum selectively attenuate low energy photons  less low energy radiation incident on patient  energy deposited in filter, not in patient Filter Film

95. Filtration Locations x-ray tube and housing  inherent filtration metal sheets placed in beam path  placed between tube and collimator or in collimator  Usually aluminum  added filtration collimator mirror* table (for under-table tube fluoro) * not mentioned in book Filter Tabletop X-Rays Tabletop Light Lamp