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L05 Interaction

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L05 Interaction

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

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