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Interaction of radiation with matter, X-ray production and X-ray beams

IAEA Regional Training Course on Radiation Protection of patients for Radiographers, Accra, Ghana, 11-15 July 2011. Interaction of radiation with matter, X-ray production and X-ray beams. Interaction of radiation with matter. 1. Electron-nucleus interaction (I). Bremsstrahlung :

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Interaction of radiation with matter, X-ray production and X-ray beams

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  1. IAEA Regional Training Course on Radiation Protection of patients for Radiographers, Accra, Ghana, 11-15 July 2011 Interaction of radiation with matter, X-ray production and X-ray beams

  2. Interaction of radiation with matter 5: Interaction of radiation with matter

  3. 1. 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

  4. 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

  5. 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

  6. 2. Characteristic x-rays • 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

  7. 3. Photoelectric effect • Incident photon with energy h •  all photon energy absorbed by a tightly bound orbital electron • ejection of electron from the atom • Condition: h > EB (electron binding energy) 5: Interaction of radiation with matter

  8. 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

  9. 4. Compton scattering • Interaction between photon and electron • Compton is practically independent of Z in diagnostic range • The probability of interaction decreases as h increases • Variation of Compton effect according to: • energy (related to X Ray tube kV) and material • lower E  Compton scattering process  1/E 5: Interaction of radiation with matter

  10. Beam characteristics: 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

  11. 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). 5: Interaction of radiation with matter

  12. 60 kV - 50 mAs 70 kV - 50 mAs 80 kV - 50 mAs X Ray penetration in human tissues 5: Interaction of radiation with matter

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

  14. 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

  15. Effect of Compton scattering Effects of scattered radiation on: • image quality • patient absorbed energy • scattered radiation in the room 5: Interaction of radiation with matter

  16. X-ray production 5: Interaction of radiation with matter

  17. Basic elements of the X Ray source assembly • Generator : power circuit supplying the required potential to the X Ray tube • X Ray tube and collimator: device producing the X Ray beam 6: X Ray production

  18. X Ray tubes 6: X Ray production

  19. X Ray tube components • Cathode: heated filament which is the source of the electron beam directed towards the anode • tungsten filament • Anode (stationary or rotating): impacted by electrons, emits X Rays • Metal tube housing surrounding glass (or metal) X Ray tube (electrons are traveling in vacuum) • Shielding material (protection against scattered radiation) 6: X Ray production

  20. housing cathode X Ray tube components 1: long tungsten filament 2 : short tungsten filament 3 : real size cathode 1: mark of focal spot 6: X Ray production

  21. Example of a cathode 6: X Ray production

  22. Cathode structure (I) • Modern tubes have two filaments • a long one : higher current/lower resolution • a short one : lower current/higher resolution • Coulomb interaction makes the electron beam divergent on the travel to the anode • lack of electrons producing X Rays • larger area of target used • focal spot increased lower image resolution Focalisation of electrons is crucial ! 6: X Ray production

  23. Anode angle (I) • The Line-Focus principle • Anode target plate has a shape that is more rectangular or ellipsoidal than circular • the shape depends on : • filament size and shape • focusing cup’s and potential • distance between cathode and anode • Image resolution requires a small focal spot • Heat dissipation requires a large spot • This conflict is solved by slanting the target face 6: X Ray production

  24. Anode heel effect (I) • Anode angle (from 7° to 20°) induces a variation of the X Ray output in the plane comprising the anode-cathode axis • Relative higher beam intensity on cathode side The heel effect is not always a negative factor • It can be used to compensate for different attenuation through parts of the body • For example: • thoracic spine (thicker part of the patient towards the cathode side of the tube) 6: X Ray production

  25. X-ray beam 5: Interaction of radiation with matter

  26. Radiation emitted by the X Ray tube • Primaryradiation: before interacting photons • Scattered radiation: after at least one interaction; need for Antiscatter grid • Leakage radiation: not absorbed by the X Ray tube housing shielding • Transmitted radiation: emerging after passage through matter 7: X Ray beam

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