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Resident Physics Lectures

Resident Physics Lectures. X-Ray Shielding. George David Associate Professor Department of Radiology Medical College of Georgia. Typical Shielding Problem. Shielding Considerations. Whom are we protecting? Workload Type of studies performed Distances Occupancy

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Resident Physics Lectures

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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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