1. Radiation Protection for Cardiologists John Saunderson Radiation Protection Adviser PRH ext 6690 Part 2 The Nature of Ionising Radiation

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. Types of Ionising Radiation Electromagnetic X-rays Gamma rays Beta particles Annihilation radiation Particles Beta particles Positrons, electrons Alpha particles Neutrons Pions, etc. 4. Electromagnetic Spectrum 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. Production of X-rays 7. 99% electron energy wasted as heat . 8. Filament (heats up on prep.) Target kV +- X-rays e- mA 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. 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. 200 kVp X-Ray Spectrum (Bremsstrahlung) 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. 80 kVp Diagnostic X-ray Beam 0 10 20 30 40 50 60 70 80 90 keV Intensity 14. 0 20 40 60 80 100 120 140 keV Intensity Tc-99m 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 electrons 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. Production of X-rays 9 Physics For a Tungsten target characteristic K lines are at 59keV and 69keV Low energy ( 45cm for chests > 60 cm . 22. Parameter Summary Parameter Quality/Penetration Intensity mA - kV (kV2 ) Filtration Distance - (1/r2 ) 23. 1.1 Properties of Radiation Attenuation of ionising radiation Scattering and absorption. 24. Attenuation, Scattering and Absorption 25. Attenuation, Scattering, Absorption 26. No attenuation - adds to contrast . 27. Absorption - adds to contrast . 28. Scattering - adds to contrast, if it misses imager . 29. Scattering - adds to fog, if it hits imager . 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. How attenuation varies Different energies Different materials 32. From NIST Physical Reference Data (http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html) x inout eII .).( . = 33. Photoelectric effect 34. Photoelectric Absorption m x Z3 / E3 = linear attenuation coefficient for PE effect m = mass density (kg/m3 ) Z = atomic number E = photon energy 35. Compton Scattering 36. Compton Scattering m x e / E = linear attenuation coefficient for PE effect m = mass density (kg/m3 ) e = electron density (e- per kg) E = photon energy 37. 1 10 100keV (log) Attenuationcoefficient(log) Absorption Scatter 20 30 70 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. 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. Atomic number Property of atoms of different elements 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. Effect of increasing kV Higher average photon energy Less attenuation Greater proportion of scatter Less dependant on atomic number . 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. 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. 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. 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. 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. fin