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Comprehensive Guide to X-ray Shielding Design Parameters in Radiology

This lecture covers X-ray shielding principles including exposure limits, barrier considerations, occupancy factors, and conservative design practices to ensure radiation safety in medical facilities. Topics include workload calculation, barrier identification, use factors, and practical considerations for shielding design.

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Comprehensive Guide to X-ray Shielding Design Parameters in Radiology

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  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 vs secondary barriers • Use factor

  4. Whom Are We Protecting?

  5. Definitions • Controlled areas • Occupied by employees / staff whose occupational dose is monitored • Uncontrolled areas • Occupied by • Patients • Visitors • Non-radiation employees • Areas adjacent to but not part of x-ray facility

  6. 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

  7. Shielding Parameters • Identification of barriers • Image field size • Distances • Tube to protected person • Scatterer (patient) to protected person • Tube to image receptor

  8. Weekly Workload • mA-min beam on at each kVp OR • Types of exams • # patients for each exam type • # images / exam

  9. These Do Not Have Same Shielding Requirements

  10. Barrier Considerations • Is beam directed at this barrier? • Primary or secondary radiation striking this barrier? • Distance? • Protected person to tube • Protected person to patient • Tube to image receptor • What fraction of time is beam aimed at this barrier? • Use factor • What’s behind the barrier?

  11. Barriers • Primary barrier • Radiation from source • Secondary • Scattered radiation from patient • Leakage radiation from tube

  12. Barriers • Primary barrier • Radiation from source • Secondary • Scattered radiation from patient • Leakage radiation from tube

  13. Use Factor • Fraction of time beam aimed at each barrier Default Use Factors if no better information

  14. Occupancy Factor • Fraction of time the maximally exposed individual is present while beam on • Only busiest shift counted • Protecting person, not area

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

  16. Occupancy Factor

  17. Shielding Design VERY Conservative • Attenuation by patient ignored • Perpendicular incidence of radiation to barrier assumed • Shields not part of wall structure ignored • Tube leakage typically a small fraction of assumed maximum allowable value

  18. 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

  19. Notes • Shielding designs very conservative • Designs must be approved by state • Integrity must be tested • Looking for voids / gaps • Adequacy must be tested • Did the installer put in no less than specified lead thickness? • Does installed shielding result in doses below regulatory limits?

  20. Notes • Shielding design only valid for given layout and workload • Changes to equipment style or position or workload require new shielding calculation

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