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Lecture 2

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  • As kV increases, peak hump of spectrum moves to a higher energy thereby changing the beam quality, skin dose may rise slightly. However, imparted energy is less as more X-rays pass through the patient and do not contribute to dose (energy in patient volume) Rule of Thumb: Increase by 10kV: Decrease mA by 2; Reduction in dose by 20-25% As the attenuation coefficient, u, is energy dependent; increasing kV will decrease the range and spread of u coefficients thereby decreasing contrast 1 electron volts = 1.6x10 -19 Joules energy of X-ray
  • Thermionic emission = heated wire filament, electrons near surface gain enough energy to leave the metal and form a cloud Concentration of these electrons forms a negative charge which can repel further electron emissions Thus use potential to draw away the cloud of electrons (<30kvp) you get space charge effect significance which can reduce effective tube mA Photon Number proportional to mA, kV, Z (anode)
  • Entrance Surface Dose proportional to 1/kv 2 Energy imparted proportional to 1/kv 2

Lecture 2 Lecture 2 Presentation Transcript

  • Radiation Protection for Cardiologists John Saunderson Radiation Protection Adviser PRH ext 6690 Part 2 – The Nature of Ionising Radiation
  • 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 .
  • Types of Ionising Radiation
    • Electromagnetic
      • X-rays
      • Gamma rays
      • Beta particles
      • Annihilation radiation
    • Particles
      • Beta particles
      • Positrons, electrons
      • Alpha particles
      • Neutrons
      • Pions, etc .
  • Electromagnetic Spectrum                                                                                                      
  • 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 10 19 Hz (22 billion GHz)
    • Photons
      • 1.4 x 10 -14 J (90 keV)
  • Production of X-rays
  •  
  • 99% electron energy wasted as heat .
  • Filament (heats up on prep.) Target kV + - X-ray s e - mA
  •  
    • 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 X-rays HEAT
  • 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 .
  • 200 kV p X-Ray Spectrum (Bremsstrahlung)
  • 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 .
  • 80 kV p Diagnostic X-ray Beam
  • Tc-99m
  • 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
  • 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)
    Radiation Intensity X-ray Photon Energy K lines L lines
  • 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
  • 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 .
  • Effect of filtration
  • Tube to Patient Distance
  •  
  • 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 .
  • Parameter Summary
    • Parameter Quality/Penetration Intensity
    • mA  - 
    • kV    (kV 2 )
    • Filtration   
    • Distance -  (1/r 2 )
  • 1.1 Properties of Radiation
    • Attenuation of ionising radiation
    • Scattering and absorption .
  • Attenuation, Scattering and Absorption
  • Attenuation, Scattering, Absorption
  • No attenuation - adds to contrast .
  • Absorption - adds to contrast .
  • Scattering - adds to contrast, if it misses imager .
  • Scattering - adds to fog , if it hits imager .
  • 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 .
  • How attenuation varies
    • Different energies
    • Different materials
  • From NIST Physical Reference Data (http://physics.nist.gov/PhysRefData/XrayMassCoef/cover.html)
  • Photoelectric effect
  • Photoelectric Absorption
    •    m x Z 3 / E 3
    •  = linear attenuation coefficient for PE effect
    •  m = mass density (kg/m 3 )
    • Z = atomic number
    • E = photon energy
  • Compton Scattering
  • Compton Scattering
    •    m x  e / E
    •  = linear attenuation coefficient for PE effect
    •  m = mass density (kg/m 3 )
    •  e = electron density (e - per kg)
    • E = photon energy
  •  
  • 20 30 70
  • 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
  • Density
    • grams per c.c.
    • Calcium carbonate 2.7 g/cm 3
    • soft tissue 1 g/cm 3
    • proportional to density, so calcium:water is about 3:1
  • Atomic number
    • Property of atoms of different elements
  •  
  • Atomic number (Z)
    • Property of atoms of different elements
    • Absorption proportional to Z 3
    • Calcium Z = 20
    • Hydrogen Z = 1; oxygen Z = 8;
      • so water (H 2 O) Z = (1+1+8)/3 = 3 1 / 3
    • so calcium:water = 20 3 : 3 1 / 3 3 = 216:1
    • BUT scattering not affected by Z
  • Effect of increasing kV
    • Higher average photon energy
    • Less attenuation
    • Greater proportion of scatter
    • Less dependant on atomic number .
  • 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 %
  • 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) .
  • 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) .
  • 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) .
  • 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 .
  • fin