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Lecture 2

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Lecture 2

  1. 1. Radiation Protection for Cardiologists John Saunderson Radiation Protection Adviser PRH ext 6690 Part 2 – The Nature of Ionising Radiation
  2. 2. Ionising or Non-Ionising? • Ionising radiation – X-rays – Gamma rays – Beta particles – Positrons, electrons – Alpha particles – Neutrons – Pions, etc. • Non-ionising – Ultrasound – MRI – Lasers – Ultraviolet – Infra-red.
  3. 3. Types of Ionising Radiation • Electromagnetic – X-rays – Gamma rays – Beta particles – Annihilation radiation • Particles – Beta particles – Positrons, electrons – Alpha particles – Neutrons – Pions, etc.
  4. 4. Electromagnetic Spectrum
  5. 5. X-rays • Electromagnetic radiation • Short wave length – 90 kV beam from 1.4 x 10-11 m (1 /10 th atom width) • High frequency – 2.2 x 1019 Hz (22 billion GHz) • Photons – 1.4 x 10-14 J (90 keV)
  6. 6. Production of X-rays
  7. 7. 99% electron energy wasted as heat .
  8. 8. Filament (heats up on prep.) Target kV +- X-rays e- mA
  9. 9. • 99% of the electrons interact with the orbital electrons of the target resulting in • 1% interact with the target nuclei producing Production of X-rays – 3 Efficiency
  10. 10. Bremstrahlung radiation • “braking radiation” • +ve nucleus attracts –ve electron and slows it down • Energy lost as a photon • Produces continuous spectrum from zero to e x kV.
  11. 11. 200 kVp X-Ray Spectrum (Bremsstrahlung)
  12. 12. Characteristic Radiation • Incoming electron knocks an orbital electron out of orbit (1,2) • An electron falls from a higher level into the gap (3) • The energy lost in falling is released as a photon (4) • Energy depends on target material • i.e. “characteristic” of the target.
  13. 13. 80 kVp Diagnostic X-ray Beam 0 10 20 30 40 50 60 70 80 90 keV Intensity
  14. 14. 0 20 40 60 80 100 120 140 keV Intensity Tc-99m
  15. 15. Production of X-rays – 6 Physics The spectrum will have a max energy of kVp (the high voltage set up between anode and cathode) This happens when ALL of the electron’s kinetic energy is transferred to the X-ray kVp (i.e. kilo-voltage- potential, peak) is one of the main parameters which can be changed to affect image quality
  16. 16. Production of X-rays – 9 Physics For a Tungsten target characteristic K lines are at 59keV and 69keV Low energy (<20keV) X-rays are filtered out by the glass envelope of the tube (this is an exit spectrum) RadiationRadiation IntensityIntensity X-ray Photon EnergyX-ray Photon Energy K linesK lines L linesL lines
  17. 17. Production of X-rays – 10 Physics Changing parameters alters spectrum: High tube current → more electrons thermionically emitted from cathode → more electrons reach target → More electrons create X- rays More X-rays = more photons = higher intensity
  18. 18. Effect of Tube Currant (mA) and Tube Voltage (kV) • mA effects number of electrons per second, therefore number of x-ray photons per second • mAs effects total number of x-ray photons • kV effects how much energy the photons have, and how many per second • In prep., filament is heated and anode spins .
  19. 19. 0 10 20 30 40 50 60 70 80 keV Intensity 3mmAl 0 0.1mmCu + 3mmAl Effect of filtration
  20. 20. Tube to Patient Distance
  21. 21. 24 Tube to Patient Distance •Greater FSD = lower patient dose •e.g. ↑ from 50 to 70 cm ⇒ ↓ 49% skin dose •Greater FSD = less magnification • (so fewer distortions) •Tube to patient distance for general radiology • never < 30cm, • preferably > 45cm • for chests > 60 cm .
  22. 22. Parameter Summary Parameter Quality/Penetration Intensity mA ↑ - ↑ kV ↑ ↑ ↑ (kV2 ) Filtration ↑ ↑ ↓ Distance - ↓ (1/r2 )
  23. 23. 1.1 Properties of Radiation • Attenuation of ionising radiation • Scattering and absorption.
  24. 24. Attenuation, Scattering and Absorption
  25. 25. Attenuation, Scattering, Absorption
  26. 26. No attenuation - adds to contrast .
  27. 27. Absorption - adds to contrast .
  28. 28. Scattering - adds to contrast, if it misses imager .
  29. 29. Scattering - adds to fog, if it hits imager .
  30. 30. Attenuation is absorption + scatter • Absorption adds to contrast • Scatter can add to contrast, but can also add to fog • For typical cardiological procedure; – 98% of x-ray energy absorbed by patient.
  31. 31. How attenuation varies • Different energies • Different materials
  32. 32. From NIST Physical Reference Data (http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html) x inout eII .).( . ρρ µ− =
  33. 33. Photoelectric effect
  34. 34. Photoelectric Absorption ∀τ ∝ ρm x Z3 / E3 ∀τ = linear attenuation coefficient for PE effect ∀ρm = mass density (kg/m3 ) • Z = atomic number • E = photon energy
  35. 35. Compton Scattering
  36. 36. Compton Scattering ∀σ ∝ ρm x ρe / E ∀σ = linear attenuation coefficient for PE effect ∀ρm = mass density (kg/m3 ) ∀ρe = electron density (e- per kg) • E = photon energy
  37. 37. 1 10 100keV (log) Attenuationcoefficient(log) Absorption Scatter 20 30 70
  38. 38. Different Materials (90 kVp) • 1 cm of soft tissue = 71% transmitted • 1 cm adipose = 77% transmitted • 1 cm bone = 27% transmitted • PMMA, water = 73% • density, atomic number
  39. 39. Density • grams per c.c. • Calcium carbonate 2.7 g/cm3 • soft tissue 1 g/cm3 • proportional to density, so calcium:water is about 3:1
  40. 40. Atomic number • Property of atoms of different elements
  41. 41. Atomic number (Z) • Property of atoms of different elements • Absorption proportional to Z3 • Calcium Z = 20 • Hydrogen Z = 1; oxygen Z = 8; – so water (H2O) Z = (1+1+8)/3 = 31 /3 • so calcium:water = 203 : 31 /3 3 = 216:1 • BUT scattering not affected by Z
  42. 42. Effect of increasing kV • Higher average photon energy • Less attenuation • Greater proportion of scatter • Less dependant on atomic number .
  43. 43. Transmission through 10 cm tissue • 80 keV → 16 % • 60 keV → 13 % • 50 keV → 10 % • 40 keV → 7 % • 30 keV → 2 % • 20 keV → 0.04 % • 15 keV → 0.000008 % • 10 keV → 10-21 %
  44. 44. Tube Voltage (kV) •Higher kV = lower patient dose •e.g. changing from 100 to 110 kV leads to 12% reduction in skin dose •Higher kV = less contrast • e.g. changing from 100 to 110 kV reduces spine/soft tissue contrast from 1.48 to 1.34 (9% drop).
  45. 45. Filtration •More filtration = lower patient dose •e.g. ↑ 0.1 mm Cu ⇒ ↓ 33% skin dose •More filtration = less contrast • e.g. ↑ 0.1 mm Cu ⇒ ↓ spine/soft tissue contrast at 80 kV from 2.76 to 2.46 (11% drop).
  46. 46. Tube to Patient Distance •Greater FSD = lower patient dose •e.g. ↑ from 50 to 70 cm ⇒ ↓ 49% skin dose •Greater FSD = less magnification • (so fewer distortions).
  47. 47. Still to do . . . • Image formation, image intensifiers, flat plates, nuclear medicine imaging • Practical radiation protection – Staff – Patients – X-ray & nuclear medicine – Assessing doses • Regulations and Guidelines • Practical Session.
  48. 48. fin

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