Chapter 7

• What is the first condition needed to produce X-rays?

  Vacuum tube
• What is the second condition needed to produce X-rays?

  A source of electrons
• What is the third condition needed to produce X-rays?

  High voltage to energize the electrons
• What is the fourth condition needed to produce X-rays?

  Target materials to decelerate the electrons
• What are the types of support systems for X-ray tubes?

  1. Ceiling support

  • Most frequently used
  • Tube attached to ceiling-mounted rails

  1. Floor support

  • Tube attached to a column on floor-mounted rails
  • Variation: floor-to-ceiling support

  1. C-arm support

  • Tube and image receptor attached
  • Mounted on ceiling or floor
  • L-arm and U-arm variations
• Why is the ceiling support the most frequently used type?

  It provides stability and flexibility in positioning
• What is the material composition of the protective housing for X-ray tubes?

  Cast steel structure lined with lead
• What is the function of the protective housing for X-ray tubes?

  Guards against excessive radiation exposure
• What is the maximum radiation leak allowed from the glass enclosure?

  1 mGy/hour at 1 meter
• What are the functions of the glass enclosure in an X-ray tube?

  Controls leaks, protects tube, isolates high voltage, cools tube
• What is the material composition of the focusing cup in an X-ray tube?

  Made of nickel
• What is the function of the focusing cup?

  Directs electrons toward the anode during exposure
• What is the material composition of the filament in an X-ray tube?

  Thoriated tungsten wire
• Why is thorium added to the filament?

  To extend the life of the filament
• What is the length and width of the filament?

  1. 2 centimeters long and 1-2 mm wide
• What is thermionic emission?


  Filament "boils off" electrons when heated
• What is the "space charge" in an X-ray tube?

  An electron cloud created by thermionic emission
• What type of glass is used for X-ray tube enclosures?

  Heat-resistant Pyrex glass
• What is the boiling off of electrons occurring at the filament called?

  Thermionic emission
• What is the maximum mA limit for general purpose tubes due to the space charge effect?

  1000 mA
• What is the maximum mA limit for specialty X-ray tubes?

  2000 mA
• What are the revolutions per minute (rpm) of general purpose X-ray tube anodes?

  3,400 rpm
• What are the revolutions per minute (rpm) of high capacity X-ray tube anodes?

  10,000 rpm
• Why is tungsten used in X-ray tubes?

  1. High melting point (3400 °C)
  2. High atomic number (74)
  3. High thermal conductivity
• What are the two types of anodes in X-ray tubes?

  1. Stationary anode

  • Fixed on a copper rod
  • Used in dental & mobile imaging

  1. Rotating anode

  • Target is the entire surface of the disc
  • Spreads heat over a wider area
• What is the target in an X-ray tube?

  The area struck by electrons
• What is the composition of a stationary target?

  Tungsten alloy embedded in copper
• What is the composition of a rotating target?

  Tungsten alloy is the entire disc
• What are the materials used for anodes in X-ray tubes?

  1. Tungsten

  • Melting point: 3400 °C

  1. Molybdenum

  • Low mass density, used in mammography

  1. Copper

  • Used in rotors, melting point: 1100 °C

  1. Graphite

  • Layered under tungsten for lighter anodes

  1. Rhenium and Rhodium

  • Mixed with target materials for thermal capacity
• What is the purpose of mixing Rhenium and Rhodium with target materials?

  To increase thermal capacity and strengthen the anode
• What alloy is used in general X-ray tubes?

  Tungsten-Rhenium alloy (10% Rhenium)
• What alloy is used in mammography X-ray tubes?

  Molybdenum-Rhodium alloy