Radiation Protection for Cardiologists

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Radiation Protection for Cardiologists
Part 2 The Nature of Ionising Radiation

• John Saunderson
• Radiation Protection Adviser
• PRH ext 6690

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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.

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Types of Ionising Radiation

• Electromagnetic
• X-rays
• Gamma rays
• Beta particles
• Annihilation radiation

• Particles
• Beta particles
• Positrons, electrons
• Alpha particles
• Neutrons
• Pions, etc.

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Electromagnetic Spectrum
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X-rays

• Electromagnetic radiation
• Short wave length
• 90 kV beam from 1.4 x 10-11 m (1/10th atom width)
• High frequency
• 2.2 x 1019 Hz (22 billion GHz)
• Photons
• 1.4 x 10-14 J (90 keV)

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Production of X-rays
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99 electron energy wasted as heat .
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Production of X-rays 3 Efficiency

• 99 of the electrons interact with the orbital
  electrons of the target resulting in
• 1 interact with the target nuclei producing

HEAT
X-rays
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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.

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200 kVp X-Ray Spectrum (Bremsstrahlung)
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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.

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80 kVp Diagnostic X-ray Beam
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Tc-99m
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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

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Production of X-rays 9 Physics
Radiation Intensity

• For a Tungsten target characteristic K lines are
  at 59keV and 69keV
• Low energy (lt20keV)
• X-rays are filtered out by the glass envelope of
  the tube
• (this is an exit spectrum)

K lines
L lines
X-ray Photon Energy
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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

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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 .

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Effect of filtration
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Tube to Patient Distance
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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 lt 30cm,
• preferably gt 45cm
• for chests gt 60 cm .

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Parameter Summary

• Parameter Quality/Penetration Intensity
• mA ? - ?
• kV ? ? ? (kV2)
• Filtration ? ? ?
• Distance - ? (1/r2)

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1.1 Properties of Radiation

• Attenuation of ionising radiation
• Scattering and absorption.

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Attenuation, Scattering and Absorption
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Attenuation, Scattering, Absorption
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No attenuation - adds to contrast .
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Absorption - adds to contrast .
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Scattering - adds to contrast, if it misses
imager .
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Scattering - adds to fog, if it hits imager .
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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.

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How attenuation varies

• Different energies
• Different materials

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From NIST Physical Reference Data
(http//physics.nist.gov/PhysRefData/XrayMassCoef/
cover.html)
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Photoelectric effect
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Photoelectric Absorption

• ? ? ?m x Z3 / E3
• ? linear attenuation coefficient for PE effect
• ?m mass density (kg/m3)
• Z atomic number
• E photon energy

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Compton Scattering
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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

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70
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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

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Density

• grams per c.c.
• Calcium carbonate 2.7 g/cm3
• soft tissue 1 g/cm3
• proportional to density, so calciumwater is
  about 31

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Atomic number

• Property of atoms of different elements

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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 (118)/3 31/3
• so calciumwater 203 31/33 2161
• BUT scattering not affected by Z

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Effect of increasing kV

• Higher average photon energy
• Less attenuation
• Greater proportion of scatter
• Less dependant on atomic number .

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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

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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).

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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).

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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).

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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.

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fin