The document discusses the interaction of radiation with matter. It covers topics including the atomic structure, quantities and units used in physics, production of bremsstrahlung and characteristic x-rays, photon interactions such as the photoelectric effect and Compton scattering, beam attenuation, and the principles of radiological image formation. The interaction of radiation depends on factors like the photon energy and atomic number of the absorbing material. Different interaction mechanisms dominate based on these factors and contribute to image contrast in medical imaging.

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.