Automatic Collimator/ Stem Radiation

• Automatic Collimator has two sets of horizontal lead shutters
• Automatic Collimator is adjustable- allows for infinite number of field sizes

  • Upper shutters
  • Lower shutters
• Upper shutters control stem radiation (off-focus)
• Lower shutters reduce blur
• Automatic collimator contributes to "added" filtration
• Automatic collimator provides a light field
• Stem Radiation AKA: off focus, extra-focal
• Stem radiation refers to photons not produced at the focal spot (target) of the anode
• Stem radiation involves

  • tube housing
  • evaporate tungsten on envelope
• Light field (light bulb and mirror) represents the X-ray field
• Cross-hairs represent the CR (Central ray)
• Poor field-light congruency may lead to parts of the anatomy being cut off