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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology. RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY. L 5: Interaction of radiation with matter . Topics. Introduction to the atomic basic structure Quantities and units

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RADIATION PROTECTION IN DIAGNOSTIC AND INTERVENTIONAL RADIOLOGY

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  1. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology RADIATION PROTECTION INDIAGNOSTIC ANDINTERVENTIONAL RADIOLOGY L 5: Interaction of radiation with matter

  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 5: Interaction of radiation with matter

  3. Overview • To become familiar with the basic knowledge in radiation physics and image formation process. 5: Interaction of radiation with matter

  4. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 5: Interaction of radiation with matter Topic 1: Introduction to the atomic basic structure

  5. Electromagnetic spectrum E keV 1.5 0.12 keV 1 10 102 103 3 eV 104 X and  rays IR UV light 100 10 1 0.1 0.01 4000 0.001 8000  Angström IR: infrared, UV = ultraviolet 5: Interaction of radiation with matter

  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 5: Interaction of radiation with matter

  7. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 5: Interaction of radiation with matter Topic 2: Quantities and units

  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 5: Interaction of radiation with matter

  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 5: Interaction of radiation with matter

  10. Atom characteristics A, Z and associated quantities • Hydrogen A = 1 Z = 1 EK= 13.6 eV • Carbon A = 12 Z = 6 EK= 283 eV • Phosphor A = 31 Z = 15 EK= 2.1 keV • Tungsten A = 183 Z = 74 EK= 69.5 keV • Uranium A = 238 Z = 92 EK= 115.6 keV 5: Interaction of radiation with matter

  11. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 5: Interaction of radiation with matter Topic 3: Bremsstrahlung production

  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. 5: Interaction of radiation with matter

  13. N N Bremsstrahlung spectrum E E n(E) n1E1 n2E2 n1 n3E3 n2 n3 Emax E1 E1 E2 E2 E3 E3 Electrons strike the nucleus 5: Interaction of radiation with matter

  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 5: Interaction of radiation with matter

  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 5: Interaction of radiation with matter

  16. Bremsstrahlung spectra dN/dE dN/dE (spectral density) E0 E E0 E From a “thin” target From a “thick” target E0= energy of electrons, E = energy of emitted photons 5: Interaction of radiation with matter

  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 E Bremsstrahlung Bremsstrahlung after filtration keV keV 50 100 150 200 5: Interaction of radiation with matter

  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 5: Interaction of radiation with matter

  19. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 5: Interaction of radiation with matter Topic 4: Characteristic X Rays

  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 5: Interaction of radiation with matter

  21. Spectral distribution of characteristic X Rays (II) Energy (eV) K1 100 80 60 40 20 - 20 - 70 - 590 - 2800 - 11000 - 69510 6 5 4 3 2 0 P O K2 N K1 M L L L K2 L K 0 10 20 30 40 50 60 70 80 (keV) 5: Interaction of radiation with matter

  22. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 5: Interaction of radiation with matter Topic 5: Primary and secondary ionization

  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 5: Interaction of radiation with matter

  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·µm-1 5: Interaction of radiation with matter

  25. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 5: Interaction of radiation with matter Topic 6: Photoelectric effect and Compton scattering

  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 - EB • Condition: h > EB (electron binding energy) • Recoil of the residual atom • Attenuation (or interaction) coefficient  photoelectric absorption coefficient 5: Interaction of radiation with matter

  27. Factors influencing photoelectric effect • Photon energy (h) > electron binding energy EB • The probability of interaction decreases as h increases • It is the main effect at low photon energies • The probability of interaction increases with Z3 (Z: atomic number) • High-Z materials are strong X Ray absorber 5: Interaction of radiation with matter

  28. Compton scattering • Interaction between photon and electron • h = Ea + Es(energy is conserved) • Ea: energy transferred to the atom • Es: energy of the scattered photon • momentum is conserved in angular distributions • At low energy, most of initial energy is scattered • ex: Es > 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 5: Interaction of radiation with matter

  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) 5: Interaction of radiation with matter

  30. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 5: Interaction of radiation with matter Topic 7: Beam attenuation and Half value thickness

  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 • Ix = I0 exp (- x) • I: number of photons per unit area per second [s-1] • : the linear attenuation coefficient [m-1] •  / [m2.kg-1]: mass attenuation coefficient •  [kg.m-3]: material density 5: Interaction of radiation with matter

  32. Attenuation coefficients Linear attenuation depends on: • characteristics of the medium (density ) • photon beam energy Mass attenuation coefficient:  / [m2kg-1] •  / same for water and water vapor (different ) •  / similar for air and water (different µ) 5: Interaction of radiation with matter

  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 5: Interaction of radiation with matter

  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) 5: Interaction of radiation with matter

  35. Photon interactions with matter Scattered photon Compton effect Secondary photons Fluorescence photon (Characteristic radiation) Annihilation photon Incident photons Non interacting photons Recoil electron Secondary electrons Photoelectron (Photoelectric effect) Electron pair E > 1.02 MeV (simplified representation) 5: Interaction of radiation with matter

  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 e.g., lead (Z = 82) for photons (E > 0.5 MeV) 5: Interaction of radiation with matter

  37. IAEA Training Material on Radiation Protection in Diagnostic and Interventional Radiology Part 5: Interaction of radiation with matter Topic 8: Principle of radiological image formation

  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 than water • lungs: weak due to density • bones can allow to visualize lung structures can be visualized with higher kVp (reducing photoelectric effect) • body cavities are made visible by means of contrast products (iodine, barium). 5: Interaction of radiation with matter

  39. 60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs X Ray penetration in human tissues 5: Interaction of radiation with matter

  40. X Ray penetration in human tissues Improvement of image contrast (lung) 5: Interaction of radiation with matter

  41. X Ray penetration in human tissues Improvement of image contrast (bone) 5: Interaction of radiation with matter

  42. X Ray penetration in human tissues 70 kV - 25 mAs 70 kV - 50 mAs 70 kV - 80 mAs 5: Interaction of radiation with matter

  43. X Ray penetration in human tissues 5: Interaction of radiation with matter

  44. X Ray penetration in human tissues 5: Interaction of radiation with matter

  45. Purpose of using contrast media • To enhance the contrast within a specific organ • To improve the image quality • Main used substances • Barium: abdominal parts • Iodine: urography, angiography, etc. 5: Interaction of radiation with matter

  46. X Ray absorption characteristics of iodine, barium and body soft tissue 100 Iodine 10 Barium X Ray ATTENUATION COEFFICIENT (cm2 g-1) Soft Tissue 1 (keV) 0.1 20 30 40 50 60 70 80 90 100 5: Interaction of radiation with matter

  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 5: Interaction of radiation with matter

  48. 10 1.0 Total X Ray ATTENUATION COEFFICIENT (cm2 g-1) 0.1 Compton + Coherent Photoelectric (keV) 0.01 20 40 60 80 100 120 140 Contribution of photoelectric and Compton interactions to attenuation of X Rays in water (muscle) 5: Interaction of radiation with matter

  49. 10 1.0 Total X Ray ATTENUATION COEFFICIENT (cm2 g-1) 0.1 Compton + Coherent Photoelectric (keV) 0.01 20 40 60 80 100 120 140 Contribution of photoelectric and Compton interactions to attenuation of X Rays in bone 5: Interaction of radiation with matter

  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: bodycavities can be made visible by means of contrast media: iodine, barium 5: Interaction of radiation with matter

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