finals

• 1. X- ray tube failure


  Have several causes, most of which are related to the thermal characteristics of the x-ray tube
• 1. x-ray tube


  • Heat must be dissipated for the x-ray tube to continue to function

  
• Methods of heat dissipation

  1. Radiation

  2. Conduction

  3. Convection
• Radiation


  Transfer of heat by the emission of infrared radiation
• Conduction


  Transfer of heat from one area of an object to another
• Convection


  Transfer of heat by the movement of a heated substance from one place to another
• Excessive heat


  Reduced tube life
• Most of the heat is dissipated through radiation during exposure
• Excessive temperature of the anode during a single exposure


  May result to localized surface melting and pitting
• Too rapid increase in temperature of the anode


  May result to cracking of the anode
• Maximum radiographic technique should not be applied to a cold cathode
• Maintaining the anode at elevated temperatures for prolonged times


  Second type of tube failure

  
• Tube arcing


  Caused when vaporized tungsten coats the inside of the glass or metal enclosure is interacted with electrons, causing disturbed electrical balance on the x-ray tube, causing abrupt, intermittent changes in tube current
• Rating charts


  Guides the radiologic technologists in using the x-ray tube

  
• Types of rating charts

  • Radiographic Rating Chart
  • Anode Cooling Chart
  • Housing Cooling Chart
• Radiographic Rating Chart


  • Most important of the three types, conveys which radiographic techniques are safe and unsafe for x-ray tube operation
  • X and y axis show scales of mA and kVp
  • For a given mA, any combination of kVp and time that lies below the mA curve is safe

  
• Anode Cooling Chart


  • Contains the thermal capacity of the anode and its heat dissipation characteristics

  
• Housing Cooling Chart


  • Has a shape similar to that of the anode cooling chart and is used precisely the same way
  • Complete cooling after maximum heat capacity requires from 1 to 2 hours

  
• Linear energy transfer (LET)


  • The average energy deposited per unit path length along the track of an ionizing particle
  • Unit: keV/um
  • Describes the energy deposition of a particular type of radiation, which largely determine the biological consequence of radiation

  
• Linear energy transfer

  • Proportional to the square of the charge of the particle
  • Inversely proportional to the particle's kinetic energy
• Particles with high linear energy transfer

  • Alpha particles
  • Protons
  • Neutrons
• High linear energy transfer radiations

  • Greater density of interactions at cellular level
  • More likely, than low linear energy transfer, to produce biological damage in a given volume of tissue
• Particles with low linear energy transfer

  • Electrons
  • Positrons
  • Gamma rays
  • X-rays

  
• Low linear energy transfer radiations

  • Less likely than high linear energy transfer to produce tissue damage in the same volume of tissue

  
• Linac x-rays (6-15 MeV)

  0.3 keV/um
• Beta Particle (1 MeV)

  0.3 keV/um
• cobalt-60 y-rays

  0.2
• 250 kVp X-rays (standard)

  2 keV/um
• 150 MeV protons (therapy energies)

  0.5 keV/um
• neutrons

  0.5-100
• alpha particles

  50-200
• carbon ions (in spread out bragg peak)

  40-90