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RA 221

RA 221. HEALTH PHYSICS Designing for Radiation Safety. CARDINAL PRINCIPLES. TIME DISTANCE SHIELDING. EXPOSURE TIME. MINIMIZE time of exposure Dose is directly proportional to exposure time Total exposure = rate X time Fluoroscopic time is critical Intermittent fluoro

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RA 221

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  1. RA 221 HEALTH PHYSICS Designing for Radiation Safety

  2. CARDINAL PRINCIPLES • TIME • DISTANCE • SHIELDING

  3. EXPOSURE TIME • MINIMIZE time of exposure • Dose is directly proportional to exposure time • Total exposure = rate X time • Fluoroscopic time is critical • Intermittent fluoro • 5 minute reset timer

  4. SHIELDING • MAXIMIZE shielding • Shields absorb radiation • Shielding is based on absorption characteristics of shielding material • HVL & TVL • Shielding is also based on energy of beam and area being shielded • Consider room, patient and personnel shielding

  5. DISTANCE • MAXIMIZE distance from source of radiation • “BEST” method of protection • Based on inverse square law

  6. ALARA • AS LOW AS RESONABLY ACHIEVABLE • APPLIES TO BOTH RADIATION WORKERS AND PATIENTS • EXPOSURE LEVELS SHOULD BE NO MORE THAN 10% OF THE DOSE LIMIT

  7. Radiation Safety Standards Organizations • ICRP • NCRP • UNSCEAR • NAS/NRC-BEIR

  8. US Regulatory Agencies • NRC • Agreement States • EPA • FDA • OSHA

  9. Radiation Safety Laws • Radiation Control for Health and Safety Act of 1968 • Consumer-Patient Radiation Health and Safety Act of 1981

  10. Radiographic Equipment Specifications • Protective tube housing-shields from leakage radiation • 100 mr/hr at 1 meter from tube • Control panel must have audible indication of exposure • SID indicator must be present • Must be accurate to within 2% of the SID • Variable aperture collimator must be present • Alignment must be accurate to within 2% of SID

  11. Radiographic Equipment Specifications • PBL must limit beam to size of image receptor • Must be accurate to within 2% of SID • Beam must be aligned with IR • Appropriate filtration must be present in tube • 2.5 mm Al for 70 KV and above • 1.5 mm Al for below 70 KV

  12. Radiographic Equipment Specifications • The exposure must be reproducible • Variation should not exceed 5% • Linearity must be accurate • Maximum variation 10% • Operator shield must be present –should not be possible to make exposures when outside the shield • Portable equipment should allow tech to remain 6 feet (2 meters) from patient or tube

  13. Fluoroscopic Equipment Specifications • Bucky slot should be shielded • .25 mm Pb • Protective curtain must be present • .25 mm Pb • Cumulative timer must be provided with an audible sound after 5 minutes • Exposure limits: • 2.1 R/min for each MA of operation • 10 R/min during fluoroscopy

  14. Fluoroscopic Equipment Specifications • Minimum source-to-skin distance • 15” on fixed units (38 cm) • 12” on mobile units (30 cm) • Image intensifier acts as primary barrier and must be aligned with the tube • Must be 2 mm Pb equivalent • Beam must have adequate filtration • 2.5 mm Al equivalent • Collimation must show an unexposed border around monitor • Exposure switch must be “dead man”

  15. Protective Barriers • Primary Barriers • Protect from primary radiation • 1/16” lead equivalent – 4 inches of masonry • Any wall in which the primary beam can hit – extend 7 feet from floor • Secondary Barriers • Protect from leakage and scatter radiation • 1/32 “ lead equivalent • All other areas of room

  16. Protective Barriers • Distance • Walls further from source need less shielding • Controlled vs uncontrolled areas • Controlled area: • Area occupied primarily by radiographers • maximum exposure 100 mrem (1ms) per week • Uncontrolled area: • Occupied by general population • 2 mrem (20 µSv) per week

  17. Protective Barriers • Use Factor (U) • Percent of time the beam is on and aimed at a particular wall • Walls = ¼ Floors = 1 • Occupancy Factor • Use of room adjacent to x-ray room • Greater use, greater shielding • Workload (W) • Use of x-ray room • MAmin/week

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