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L05 Interaction Presentation Transcript

  • 1. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY L 5: Interaction of radiation with matter IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 2. Topics
    • Introduction to the atomic basic structure
    • Quantities and units
    • Bremsstrahlung production
    • Characteristic X Rays
    • Primary and secondary ionization
    • Photo-electric effect and Compton scattering
    • Beam attenuation and half value thickness
    • Principle of radiological image formation
  • 3. Overview
    • To become familiar with the basic knowledge in radiation physics and image formation process.
  • 4. Part 5: Interaction of radiation with matter Topic 1: Introduction to the atomic basic structure IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 5. Electromagnetic spectrum 10 4 10 3 10 2 10 1 3 eV 0.001 0.01 0.1 1 10 0.12 keV 100 1.5 Angström keV X and  rays UV IR light E  4000 8000 IR : infrared, UV = ultraviolet
  • 6. The atomic structure
    • The nuclear structure
      • protons and neutrons = nucleons
      • Z protons with a positive electric charge
        • (1.6 10-19 C)
      • neutrons with no charge (neutral)
      • number of nucleons = mass number A
    • The extranuclear structure
      • Z electrons (light particles with electric charge)
        • equal to proton charge but negative
    • The atom is normally electrically neutral
  • 7. Part 5: Interaction of radiation with matter Topic 2: Quantities and units IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 8. Basic units in physics (SI system)
    • Time: 1 second [s]
    • Length: 1 meter [m]
    • Mass: 1 kilogram [kg]
    • Energy: 1 joule [J]
    • Electric charge: 1 coulomb [C]
    • Other quantities and units
    • Power: 1 watt [W] (1 J/s)
    • 1 mAs = 0.001 C
  • 9. Quantities and units
    • electron-volt [eV]: 1.603 10-19 J
    • 1 keV = 103 eV
    • 1 MeV = 106 eV
    • 1 electric charge: 1.6 10-19 C
    • mass of proton: 1.672 10-27 kg
  • 10. Atom characteristics
    • A, Z and associated quantities
    • Hydrogen A = 1 Z = 1 E K = 13.6 eV
    • Carbon A = 12 Z = 6 E K = 283 eV
    • Phosphor A = 31 Z = 15 E K = 2.1 keV
    • Tungsten A = 183 Z = 74 E K = 69.5 keV
    • Uranium A = 238 Z = 92 E K = 115.6 keV
  • 11. Part 5: Interaction of radiation with matter Topic 3: Bremsstrahlung production IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 12. Electron-nucleus interaction (I)
    • Bremsstrahlung :
      • radiative energy loss (E) by electrons slowing down on passage through a material
      •  is the deceleration of the incident electron by the nuclear Coulomb field
      •  radiation energy (E) (photon) is emitted.
  • 13. Electrons strike the nucleus N N n(E)  E E 1 E 2 E 3 n 1 n 3 n 2 E 1 E 2 E 3 n 1 E 1 n 2 E 2 n 3 E 3  E E max Bremsstrahlung spectrum
  • 14. Electron-nucleus interaction (II)
    • With materials of high atomic number
      • the energy loss is higher
    • The energy loss by Bremsstrahlung
      • > 99% of kinetic E loss as heat production, it increases with increasing electron energy
    • X Rays are dominantly produced by Bremsstrahlung
  • 15. Bremsstrahlung continuous spectrum
    • Energy (E) of Bremsstrahlung photons may take any value between “zero” and the maximum kinetic energy of incident electrons
    • Number of photons as a function of E is proportional to 1/E
    • Thick target  continuous linear spectrum
  • 16. Bremsstrahlung spectra dN/dE (spectral density) dN/dE From a “ thin ” target E E 0 E E 0 E 0 = energy of electrons, E = energy of emitted photons From a “ thick ” target
  • 17. X Ray spectrum energy
    • Maximum energy of Bremsstrahlung photons
      • kinetic energy of incident electrons
    • In X Ray spectrum of radiology installations:
      • Max (energy) = Energy at X Ray tube peak voltage
    Bremsstrahlung  E keV 50 100 150 200 Bremsstrahlung after filtration keV
  • 18. Ionization and associated energy transfers
    • Example: electrons in water
    • ionization energy: 16 eV (for a water molecule
    • other energy transfers associated to ionization
      • excitations (each requires only a few eV)
      • thermal transfers (at even lower energy)
    • W = 32 eV is the average loss per ionization
      • it is characteristic of the medium
      • independent of incident particle and of its energy
  • 19. Part 5: Interaction of radiation with matter Topic 4: Characteristic X Rays IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 20. Spectral distribution of characteristic X Rays (I)
    • Starts with ejection of e- mainly from k shell (also possible for L, M,…) by ionization
    • e- from L or M shell fall into the vacancy created in the k shell
    • Energy difference is emitted as photons
    • A sequence of successive electron transitions between energy levels
    • Energy of emitted photons is characteristic of the atom
  • 21. Spectral distribution of characteristic X Rays (II) L K M N O P Energy (eV) 6 5 4 3 2 0 - 20 - 70 - 590 - 2800 - 11000 - 69510 0 10 20 30 40 50 60 70 80 100 80 60 40 20 L  L  L  K  1 K  2 K  2 K  1 (keV)
  • 22. Part 5: Interaction of radiation with matter Topic 5: Primary and secondary ionization IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 23. Stopping power
    • Loss of energy along track through both collisions and Bremsstrahlung
    • The linear stopping power of the medium
    • S =  E /  x [MeV.cm -1 ]
      •  E : energy loss
      •  x : element of track
    • for distant collisions: the lower the electron energy, the higher the amount transferred
    • most Bremsstrahlung photons are of low energy
    • collisions (hence ionization) are the main source of energy loss
    • except at high energies or in media of high Z
  • 24. Linear Energy Transfer
    • Biological effectiveness of ionizing radiation
    • Linear Energy Transfer (LET): amount of energy transferred to the medium per unit of track length of the particle
    • Unit: e.g. [keV.  m -1 ]
  • 25. Part 5: Interaction of radiation with matter Topic 6: Photoelectric effect and Compton scattering IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 26. Photoelectric effect
    • Incident photon with energy h 
    •  all photon energy absorbed by a tightly bound orbital electron
      • ejection of electron from the atom
      • Kinetic energy of ejected electron: E = h  - E B
    • Condition: h  > EB (electron binding energy)
    • Recoil of the residual atom
    • Attenuation (or interaction) coefficient
      •  photoelectric absorption coefficient
  • 27. Factors influencing photoelectric effect
    • Photon energy ( h  ) > electron binding energy E B
    • The probability of interaction decreases as h  increases
    • It is the main effect at low photon energies
    • The probability of interaction increases with Z 3 (Z: atomic number)
    • High-Z materials are strong X Ray absorber
  • 28. Compton scattering
    • Interaction between photon and electron
    • h  = E a + E s (energy is conserved)
      • E a : energy transferred to the atom
      • E s : energy of the scattered photon
      • momentum is conserved in angular distributions
    • At low energy, most of initial energy is scattered
      • ex: E s > 80% (h  ) if h  <1 keV
    • Increasing Z  increasing probability of interaction. Compton is practically independent of Z in diagnostic range
    • The probability of interaction decreases as h  increases
  • 29. Compton scattering and tissue density
    • Variation of Compton effect according to:
      • energy (related to X Ray tube kV) and material
      • lower E  Compton scattering process  1/E
    • Increasing E  decreasing photon deviation angle
    • Mass attenuation coefficient  constant with Z
      • effect proportional to the electron density in the medium
      • small variation with atomic number (Z)
  • 30. Part 5: Interaction of radiation with matter Topic 7: Beam attenuation and Half value thickness IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 31. Exponential attenuation law of photons (I)
    • Any interaction  change in photon energy and or
    • direction
    • Accounts for all effects: Compton, photoelectric,…
      • dI/I = -  dx
      • I x = I 0 exp (-  x)
        • I : number of photons per unit area per second [s -1 ]
        •  : the linear attenuation coefficient [m -1 ]
        •  /  [m 2 .kg -1 ]: mass attenuation coefficient
        •  [kg.m -3 ]: material density
  • 32. Attenuation coefficients
    • Linear attenuation depends on:
    • characteristics of the medium (density  )
    • photon beam energy
    • Mass attenuation coefficient:  /  [m 2 kg -1 ]
      •  /  same for water and water vapor (different  )
      •  /  similar for air and water (different µ)
  • 33. Attenuation of an heterogeneous beam
    • Various energies  No more exponential attenuation
    • Progressive elimination of photons through the matter
    • Lower energies preferentially
    • This effect is used in the design of filters
    •  Beam hardening effect
  • 34. Half Value Layer (HVL)
    • HVL: thickness reducing beam intensity by 50%
    • Definition holds strictly for monoenergetic beams
    • Heterogeneous beam  hardening effect
    • I/I0 = 1/2 = exp (-µ HVL) HVL = 0.693 / µ
    • HVL depends on material and photon energy
    • HVL characterizes beam quality
    •  modification of beam quality through filtration
    •  HVL (filtered beam)  HVL (beam before filter)
  • 35. Photon interactions with matter Annihilation photon Incident photons Secondary photons Secondary electrons Scattered photon Compton effect Fluorescence photon (Characteristic radiation) Recoil electron Electron pair E > 1.02 MeV Photoelectron (Photoelectric effect) Non interacting photons (simplified representation)
  • 36. Dependence on Z and photon energy
    • Z < 10  predominating Compton effect
    • higher Z increase photoelectric effect
      • at low E: photoelectric effect predominates in bone compared to soft tissue
      • (total photon absorption)
    • contrast products  photoelectric absorption
      • high Z (Barium 56, Iodine 53)
    • use of photoelectric absorption in radiation protection
    • ex: lead (Z = 82) for photons (E > 0.5 MeV)
  • 37. Part 5: Interaction of radiation with matter Topic 8: Principle of radiological image formation IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology
  • 38. X Ray penetration and attenuation in human tissues
    • Attenuation of an X Ray beam:
    • air: negligible
    • bone: significant due to relatively high density (atom mass number of Ca)
    • soft tissue (e.g. muscle,.. ): similar to water
    • fat tissue: less important than water
    • lungs: weak due to density
        • bones can allow to visualize lung structures with higher kVp (reducing photoelectric effect)
        • body cavities are made visible by means of contrast products (iodine, barium).
  • 39. X Ray penetration in human tissues 60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs
  • 40. X Ray penetration in human tissues Improvement of image contrast (lung)
  • 41. X Ray penetration in human tissues Improvement of image contrast (bone)
  • 42. X Ray penetration in human tissues 70 kV - 25 mAs 70 kV - 50 mAs 70 kV - 80 mAs
  • 43. X Ray penetration in human tissues
  • 44. X Ray penetration in human tissues
  • 45. Purpose of using contrast media
    • To make visible soft tissues normally transparent to X Rays
    • To enhance the contrast within a specific organ
    • To improve the image quality
    • Main used substances
      • Barium: abdominal parts
      • Iodine: urography, angiography, etc .
  • 46. X Ray absorption characteristics of iodine, barium and body soft tissue 100 20 30 40 50 60 70 80 90 100 10 1 0.1 Iodine (keV) X Ray ATTENUATION COEFFICIENT (cm 2 g -1 ) Barium Soft Tissue
  • 47. Photoelectric absorption and radiological image
    • In soft or fat tissues (close to water), at low energies (E< 25 - 30 keV)
    • The photoelectric effect predominates
    •  main contributor to image formation on the radiographic film
  • 48. Contribution of photoelectric and Compton interactions to attenuation of X Rays in water (muscle) 20 40 60 80 100 120 140 10 1.0 0.1 0.01 Total Compton + Coherent Photoelectric (keV) X Ray ATTENUATION COEFFICIENT (cm 2 g -1 )
  • 49. Contribution of photoelectric and Compton interactions to attenuation of X Rays in bone 20 40 60 80 100 120 140 10 1.0 0.1 0.01 Total Compton + Coherent Photoelectric (keV) X Ray ATTENUATION COEFFICIENT (cm 2 g -1 )
  • 50. X Ray penetration in human tissues
    • Higher kVp reduces photoelectric effect
    • The image contrast is lowered
    • Bones and lungs structures can simultaneously be visualized
    • Note : body cavities can be made visible by means of contrast media: iodine, barium
  • 51. Effect of Compton scattering
    • Effects of scattered radiation on:
      • image quality
      • patient absorbed energy
      • scattered radiation in the room
  • 52. Summary
    • The elemental parts of the atom constituting both the nucleus and the extranucleus structure can be schematically represented .
    • Electrons and photons have different types of interactions with matter
    • Two different forms of X Rays production Bremsstrahlung and characteristic radiation contribute to the image formation process.
    • Photoelectric and Compton effects have a significant influence on the image quality.
  • 53. Where to Get More Information (1)
    • Part 2: Lecture on “Radiation quantities and Units”
    • Attix FH. Introduction to radiological physics and radiation dosimetry. New York, NY: John Wiley & Sons, 1986. 607 pp. ISBN 0-47101-146-0.
    • Johns HE, Cunningham JR. Solution to selected problems form the physics of radiology 4th edition. Springfield, IL: Charles C. Thomas, 1991.
  • 54. Where to Get More Information (2)
    • Wahlstrom B. Understanding Radiation. Madison, WI: Medical Physics Publishing, 1995. ISBN 0-944838-62-6.
    • Evans RD. The atomic nucleus. Malabar, FL: R.E. Kriege, 1982 (originally 1955) ISBN 0-89874-414-8.