Radiation Protection: Design of Protective Barriers

Equipment Design for Radiation Protection

Design of Protective Barriers

• A medical physicist must be consulted in designing proper radiation shielding.
• There are two basic types of barriers: Primary and Secondary barriers.
• Primary radiation is the most intense and difficult to shield.
• Any wall to which the useful beam can be directed is called a primary barrier.
• Three of the four walls in a fixed x-ray lab are primary barriers, as well as the floor.
• Lead bonded to sheet rock or wood paneling is often used as a primary barrier.
• Concrete or brick may be used instead, with 4 inches of masonry being equal to 1/16 inch of lead.

Primary Barriers

• The primary barrier is perpendicular to the central ray.
• Lead is often used, with a thickness of at least 1/16 inch (1.6 mm) and a height of 7 feet.

Secondary Barriers

• Secondary barriers are designed to stop leakage and scatter radiation.
• The primary beam should never be pointed at a secondary barrier.
• The patient is the source of most scatter radiation produced.
• Leakage radiation is emitted from the tube housing in all directions other than the primary beam.
• Secondary barriers often have a lead glass window with a thickness of 1.5 mm of Pb.
• The regulatory limit of leakage radiation is 100 mR/hr at 1m distance.

Influencing Factors

• Workload is measured in mA per min or week (the amount of activity in an x-ray room).
• Use factor is assigned to walls (1/4) and the floor (1), representing the amount of time the beam is directed at a wall or floor.
• Occupancy factor (T) has limits of:
  • 100 mR/week for controlled areas (occupied by radiation workers).
  • 10 mR/week for uncontrolled areas (e.g. corridors, waiting rooms, restrooms).

Radiation Barriers

• Three qualities of the Primary Barrier:
  • CR is perpendicular.
  • Thickness of at least 1/16 inch of Pb (1.6 mm).
  • Height of 7 feet.
• Three qualities of the Secondary Barrier:
  • CR is parallel.
  • Thickness of at least 1/32 inch of Pb (0.8 mm).
  • Height of 7 feet.

Types of Wearable Protective Devices

• Lead aprons: minimum 0.25 mm Pb for standard x-ray.
• Fluoro aprons: 0.5 mm Pb.
• Lead glasses: 0.35 mm Pb.
• Thyroid shields: 0.5 mm Pb.
• Lead gloves: 0.25 mm Pb.

Patient Shielding

• Flat contact shields.
• Shaped contact shields (best for patient dose).
• Shadow shields - used for procedures with a sterile field.

Units of Measurement

• Sieverts (Sv) - equivalent dose.
• Grays (Gy) - absorbed dose in tissue.
• Coulombs per kilogram (C/kg) - primary beam.
• Becquerels (Bq) - radioactive material.

Exposure Reduction Strategies

• Proper positioning.
• Higher kV and lower mAs.
• Higher filtration.
• Collimation.
• Shielding.