Shielding

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Resident Physics Lectures

• Shielding

George David Associate Professor Department of
Radiology Medical College of Georgia
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Typical Shielding Problem
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Shielding Considerations

• Whom are we protecting?
• Workload
• Type of studies performed
• Distances
• Occupancy
• Primary / secondary
• Use

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Whom Are We Protecting?
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Exposure Limits

• Controlled areas
• 0.1 mGy / week
• 10 mrad / week
• 5 mGy / year
• Uncontrolled areas
• .02 mGy / week
• 2 mrad / week
• 1 mGy / year
• Film
• 0.1 mGy during storage period

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Controlled vs. Non-Controlled Areas
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Workload

• mA-min beam on at each kVp

OR

• patients
• images / patient

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Barrier Considerations

• Is beam directed at this barrier?
• Primary or Secondary?
• Distance?
• What fraction of time?
• Use factor
• Whats behind the barrier?

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Barriers
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Use Factor

• Fraction of time beam aimed at each barrier

Typical Primary Barriers
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Occupancy Factor

• Fraction of time the maximally exposed individual
  is present while beam on

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Locations Assumed to have Full Occupancy

• Offices
• Labs
• Reception
• Reading room
• Nurses station
• Control Room

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Occupancy Factor
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Shielding Design VERY Conservative

• Attenuation by patient ignored
• Perpendicular incidence assumed
• Shields not part of wall structure ignored
• Tube leakage normally far below assumed maximum
  allowable value

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Shielding Design VERY Conservative

• Field sizes often smaller than maximum assumed
  value
• Scatter levels considerably less for smaller
  field
• Occupancy factors are conservatively high
• 100 for an individual in an office
• 20 for an individual in a hallway
• Lead comes in specific thicknesses
• Ordered in next thicker size
• Distance to occupied area assumed to be 1 foot
  behind barrier

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Notes

• Shielding designs very conservative
• Designs must be approved by state
• Integrity must be tested
• Adequacy must be tested

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Notes

• Design only valid for given layout and workload
• Changes to equipment style or position or
  workload require new shielding calculation