1 / 26

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 :

scorp
Télécharger la présentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related