1. A-LEVEL PHYSICS 15/09/2015 13:49 Medical Physics

2. A-LEVEL PHYSICS 15/09/2015 13:49 X-rays Basic concept X-rays can expose photographic film X-rays can pass through soft tissue X-rays can not pass through bone Put a body over some photographic film and then “shine” X-rays on them Develop the film Photograph of the inside of the patient

3. A-LEVEL PHYSICS 15/09/2015 13:49 Major applications Radiography –Diagnostic imagery Radiotherapy –Destroying dangerous cells

4. A-LEVEL PHYSICS 15/09/2015 13:49 Properties of X-rays Can not be reflected Can not be refracted Can not be focused

5. A-LEVEL PHYSICS 15/09/2015 13:49 Rotating-anode X-ray tube

6. A-LEVEL PHYSICS 15/09/2015 13:49 Attenuation –Intensity of X-rays reduced –Passage through matter Linear attenuation coefficient

7. A-LEVEL PHYSICS 15/09/2015 13:49 Half-value thickness HVT The thickness of material needed to reduce intensity to 50% of its initial value x II0I0 0 X I

8. A-LEVEL PHYSICS 15/09/2015 13:49 Half-thickness value X1/2 Half the original intensity Cancel I 0 Take natural logs Make x the subject

9. A-LEVEL PHYSICS 15/09/2015 13:49 Measuring  0 X Log I

10. A-LEVEL PHYSICS 15/09/2015 13:49 X-ray spectrum 0 2 4 6 8 10 Wavelength [x10exp11] Intensity [arbitary units] 0 2 6 4 8 10 Continuous radiation Bremsstrahlung Line spectrum min

11. A-LEVEL PHYSICS 15/09/2015 13:49 Continuous radiation Bremsstrahlung Energy of the emitted X-ray is equal to the energy lost by the electron –Some…why there is a range –All…why there is a minimum wavelength

12. A-LEVEL PHYSICS 15/09/2015 13:49 Line spectrum High energy incident electron Removes DEEP-LYING orbiting electron Outer electron falls into the space Emits X-ray photon –Much more energy than visible light –Light photons use higher orbital transitions

13. A-LEVEL PHYSICS 15/09/2015 13:49 Line spectrum

14. A-LEVEL PHYSICS 15/09/2015 13:49 Line spectrum

15. A-LEVEL PHYSICS 15/09/2015 13:49 Line spectrum

16. A-LEVEL PHYSICS 15/09/2015 13:49 Continuous spectrum

17. A-LEVEL PHYSICS 15/09/2015 13:49 X-ray spectrum 0 20 40 60 80 100 Photon energy [keV] Intensity [arbitary units] 0 2 6 4 8 10 Range of energies corresponding to frequency of X-ray Maximum value corresponding to min

18. A-LEVEL PHYSICS 15/09/2015 13:49 Quality & Intensity: Different supply voltage 0 20 40 60 80 100 Photon energy [keV] Intensity [arbitary units] 0 2 6 4 8 10 100kV 50kV More energy available to create X-rays from each incident electron

19. A-LEVEL PHYSICS 15/09/2015 13:49 Quality & Intensity: Different filement current 0 20 40 60 80 100 Photon energy [keV] Intensity [arbitary units] 0 2 6 4 8 10 Top 20mA Lower 10mA Greater number of electrons striking the anode so more X-rays produced

20. A-LEVEL PHYSICS 15/09/2015 13:49 Interaction with matter Simple scattering Photoelectric absorption Compton scattering Pair production

21. A-LEVEL PHYSICS 15/09/2015 13:49 Simple scattering Low energy Can’t remove an electron Incident x-ray is deflected No loss of energy Material scatter X-ray Without absorbing energy

22. A-LEVEL PHYSICS 15/09/2015 13:49 Photoelectric absorption Similar to the production of line spectra Incident high energy electron replaced by X-ray photon X-ray loses ALL energy as an inner electron is ejected –Photoelectron…ionizes other atoms Lower energy photon released as an electron falls into the vacant orbit

23. A-LEVEL PHYSICS 15/09/2015 13:49 Photoelectric absorption

24. A-LEVEL PHYSICS 15/09/2015 13:49 Compton effect

25. A-LEVEL PHYSICS 15/09/2015 13:49 Pair production High energy X-ray photons –1.022MeV [rest mass of electron + positron] Nucleus interaction –X-ray vanishes –Electron-positron pair created Electron ionizes atoms Positron annihilated by electron –Two photons created

26. A-LEVEL PHYSICS 15/09/2015 13:49 Filtration Limit X-rays to the “useful” energies –Remove unwanted low energy X-rays –Heterogeneous Range of values Absorbing filter material –Photoelectric absorption –All energy of incident X-ray absorbed –Low energy photon emitted

27. A-LEVEL PHYSICS 15/09/2015 13:49 Effect of filtering 0 20 40 60 80 100 Photon energy [keV] Intensity [arbitary units] 0 2 6 4 8 10

28. A-LEVEL PHYSICS 15/09/2015 13:49 Filtration HVT increases Higher proportion of high-energy X-ray photons

29. A-LEVEL PHYSICS 15/09/2015 13:49 Beam size and alignment 4 adjustable lead shields

30. A-LEVEL PHYSICS 15/09/2015 13:49 Beam size and alignment Mirror reflects light that is transparent to X-rays

31. A-LEVEL PHYSICS 15/09/2015 13:49 X-ray image quality Focal spot Angled target –Enlarged focus

32. A-LEVEL PHYSICS 15/09/2015 13:49 X-ray image quality

33. A-LEVEL PHYSICS 15/09/2015 13:49 Scattering To produce an image only use X-rays that have passed directly through the body Grid –Vertical lead strips 5mm deep 0.5mm apart 0.05mm thick –Moves from side to side prevent grid from casting its own shadow

34. A-LEVEL PHYSICS 15/09/2015 13:49 Contrast media How to take an X-ray of the stomach or other organ? –Barium meal –High Z [atomic number] –Absorb X-rays –So get a better contrast on the image

35. A-LEVEL PHYSICS 15/09/2015 13:49 Intensifying screens Intensifying cassette –Reduces patient’s exposure time –White plastic –Fluorescent crystals Absorb X-rays Give out light Film much more sensitive to light Metal prevents back scattering

36. A-LEVEL PHYSICS 15/09/2015 13:49 Fluoroscopy Real time images X-ray to light Nice idea but… –X-ray dose too high –Patient dies! Image intensifier

37. A-LEVEL PHYSICS 15/09/2015 13:49 Computer Tomography Need a powerful computer Compares signals from 2000+ tiny detectors X-ray source rotated around the patient –pules of X-rays produced Complex mathematical algorithm used to determine the attenuation at each point in the body Excellent resolution Able to be used on soft tissue

38. A-LEVEL PHYSICS 15/09/2015 13:49 Computer Tomography The NET for… –CT scanning –Principles –Images Encarta for… –Uses –Treatment

39. A-LEVEL PHYSICS 15/09/2015 13:49 Attenuation…again Mass attenuation co-efficient Consider water –ice –water –steam All have different densities But ALL have the same MAC