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 Primary / secondary Use

Whom Are We Protecting?

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

Controlled vs. Non-Controlled Areas

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

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

Barriers

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

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

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

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

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