1. Resident Physics Lectures
X-Ray
Shielding
George David
Associate Professor
Department of Radiology
Medical College of Georgia

2. Typical Shielding Problem

3. Shielding Considerations
Whom are we protecting?
Workload
Type of studies performed
Distances
Occupancy
Primary / secondary
Use

4. Whom Are We Protecting?

5. Exposure Limits Controlled areas Uncontrolled areas Film
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

6. Controlled vs. Non-Controlled Areas

7. Workload
mA-min beam on at each kVp OR
# patients
# images / patient

8. Barrier Considerations
Is beam directed at this barrier?
Primary or Secondary?
Distance?
What fraction of time?
Use factor
What’s behind the barrier?

9. Barriers

10. Typical Primary Barriers
Use Factor
Fraction of time beam aimed at each barrier
Typical Primary Barriers
Barrier
Use Factor
Floor
0.89
Cross Table Wall
0.09
Other Walls
0.02
Chest Bucky Wall
1.00

11. Occupancy Factor
Fraction of time the maximally exposed individual is present while beam on

12. Locations Assumed to have Full Occupancy
Offices
Labs
Reception
Reading room
Nurses station
Control Room

13. Occupancy Factor Location Occupancy Factor Exam / treatment ½
Corridors, lounges
1/5
Corridor doors
1/8
Public toilets, storage, outdoor seating
1/20
Outdoor, parking lots
1/40

14. 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

15. 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

16. Notes Shielding designs very conservative
Designs must be approved by state
Integrity must be tested
Adequacy must be tested

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