Beam Limitation

• Beam limitation is how we control field size of the x-ray beam
• Beam limitation:

  1. Reduced patient exposure (main advantage)
  2. Increase image quality by decreasing scatter to the IR
  3. Increases visibility of detail
• • Decrease in field size
  • Decreases scatter
  • Decreases density (fog)
  • Increases contrast
  • Increases detail
• 50-90% of density on the radiograph is a result of scatter radiation
• Penumbra (blur) is greater at the edges of the beam
• Types of Beam Limiting Devices:

  1. Aperture Diaphragm
  2. Cones (flared and cylinder)
  3. Variable Aperture
• Beam limiting devices are constructed of lead or lead-lined because of its characteristic to attenuate
• 1. Aperture Diaphragm: Lead plates with windows cut out in various sizes

  • not in use anymore
• Aperture Diaphragms Advantage:

  • simplest, least expensive

  Aperture Diaphragms Disadvantage:

  • change for each size
  • large amount of blur
  • dedicated units
• 2.1. Flared Cone Advantage:

  • Matches the divergence of the beam

  
  

  2.1. Flared Cone Disadvantage:

  • Flare is usually larger than the beam
• 2.2. Cylinder cone Advantage:

  • Blur is reduced because it truly limits the field size

  2.2. Cylinder cone Disadvantage:

  • Circular field
• What is the disadvantage of cones with a circular field?

  Excessive radiation for larger body parts
• 3. Variable Aperture

  • Collimator
  • Automatic collimater --> Involves a Positive Beam Limiting Device (PBL)
• What is a Positive Beam Limiting Device (PBL)?

  assures the size of the field does not exceed the size of the IR