Resident Physics Lectures
X-Ray
Shielding
George David
Associate Professor
Department of Radiology
Medical College of Georgia
Typical Shielding Problem
George
David
Shielding Considerations
•
•
•
•
•
•
•
Whom are we protecting?
Workload
Type of studies performed
Distances
Occupancy
Primary / secondary
Use
George
David
Whom Are We Protecting?
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David
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
Controlled vs. NonControlled Areas
Workload
• mA-min beam on at each kVp
• # patients
• # images / patient
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David
Barrier Considerations
• Is beam directed at this barrier?
• Primary or Secondary?
• Distance?
• What fraction of time?
 Use factor
• What’s behind the barrier?
Barriers
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David
Use Factor
• Fraction of time beam aimed
at each barrier
Typical Primary Barriers
Barrier
Floor
Cross Table Wall
Other Walls
Chest Bucky Wall
Use Factor
0.89
0.09
0.02
1.00
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David
Occupancy Factor
• Fraction of time the maximally
exposed individual is present while
beam on
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David
Locations Assumed to
have Full Occupancy
• Offices
• Labs
• Reception
• Reading room
• Nurses station
• Control Room
Occupancy Factor
Location
Exam / treatment
Corridors, lounges
Corridor doors
Public toilets, storage,
outdoor seating
Outdoor, parking lots
Occupancy
Factor
½
1/5
1/8
1/20
1/40
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
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
Notes
• Shielding designs very
conservative
• Designs must be approved by
state
• Integrity must be tested
• Adequacy must be tested
Notes
• Design only valid for given layout
and workload
• Changes to equipment style or
position or workload require new
shielding calculation