Radiation Protection for Assistant Practitioners in Mammography

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Radiation Protection for Assistant Practitioners
in Mammography

• John Saunderson
• Radiation Protection Adviser
• (TPRH ext. 6690)

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IRMER Syllabus

• Production of X-rays
• Absorption and scatter
• Radiation hazards and dosimetry
• Special attention areas
• Radiation Protection
• Laws Guidelines
• Equipment .

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1. Fundamental Physics of Radiation
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Production of X-rays(4.1c)
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What are X-rays?

• Electromagnetic radiation .

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What are X-rays?

• Electromagnetic radiation
• Have
• wavelength
• frequency
• energy
• Photons .

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Making X-rays

• Electromagnetic waves are made when charged
  particles are decelerated
• For most things ELECTRONS are used .

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Electrons

• tiny negatively charge particles
• 1 gram of electrons would be 1,000,000,000,000,00
  0,000,000,000,000 (1000 trillion trillions)
• moving electrons in a wire electrical current .

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Electrons moving up and down an aerial make radio
waves
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99 electron energy wasted as heat .
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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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Tube voltage 30 kV
Molybdenum target Tungsten target
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Effect of filtration
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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 average mammogram
• 0.2 of x-ray energy reaches film
• 99.8 absorbed in breast.

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How attenuation varies

• Different energies
• Different materials

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20
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70
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Different Materials

• 5 cm of
• standard tissue 0.25 transmitted
• muscle 0.27 transmitted
• water 0.37 transmitted
• Perspex 0.87 transmitted
• 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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(End lecture 1 by now)