RAD 253 Chapter 7 The Xray Tube

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RAD 253 Chapter 7 The X-ray Tube

• Two primary components cathode and anode

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Tube must be supported ceiling/floor
mounted/c-arm, etc

• SIDs
• Detents centers and SIDs
• (over ride exposure if not accurate)

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Protective housing

• X-rays are produced isotropically (all
  directions) protective housing allows only
  those directed at the window to escape
• Leakage radiation less than 100 mR/hr at 1 m in
  any direction other than the window

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Filament (thoriated tungsten)

• 3410 degree melting point
• Tungsten wire usually 1-2 cm long and 2mm in
  diameter
• Dual filament (equals dual target size) for
  better detail
• Thermionic emission
• Negatively charged focusing cup

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Anode (positive charge target)

• Most are rotating to give more surface area for
  heat dissipation
• 3400-10,000rpm rotation
• Angled to give larger actual BUT smaller
  effective focal spot
• Made of Tungsten, rhenium
• Support arm assists to rid anode of heat and is
  made of composite molybdnium/rhenium, tungsten,
  copper

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Brems/Breaking Radiation
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Characteristic Radiation
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Other anode attributes

• Line-focus principle actual/effective focal
  spot
• Heel effect
• Angles (smaller anode angel LARGER heel effect)
• More penetrating beam on cathode side of beam due
  to heel effect

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Heat enemy 1!!!

• Tube warm up is required if one hour has lapsed
  since the last exposure (check PRIOR to putting
  patient in the room)
• Results in primary reason for all tube failures!

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Tube rating charts

• Exposure tube rating charts
• Variable kVp/fixed mAs not too practical as
  kVp is driven by atomic mass density of the
  patient or contrast agent
• Variable mAs/fixed kVp - most practical as
  atomic mass number is the driving force of kVp
  and mAs amount of radiation produced
• Anode cooling
• Housing cooling

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