X-ray Tube Review

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Author:
Anonymous
ID:
237809
Filename:
X-ray Tube Review
Updated:
2013-09-29 23:36:55
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xray tube RAD
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Description:
Lecture weeks 4 and 5
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  1. Positive electrode of xray tube
    Anode
  2. Negative electrode of xray tube
    Cathode
  3. Process of boiling off electrons from the filament
    Thermionic emission
  4. Define space charge
    Cloud of electrons around filament
  5. Composition of filament
    Thoriated tungsten
  6. Two parts of cathode
    Filament and focusing cup
  7. Three functions of anode
    • Electrical conductor
    • Mechanical support
    • Good thermal dissipater
  8. Define line focus principle
    Effective focal spot is smaller than actual focal spot
  9. Area on target where electrons strike
    Actual focal spot
  10. Two types of anodes
    Stationary and rotating
  11. Composition of stationary anode
    Tungsten rhenium alloy embedded in copper anode
  12. Composition of rotating anode
    Tungsten rhenium alloy disc with molybdenum and graphite
  13. Three types of tube supports
    • Ceiling
    • Column supported by floor or ceiling
    • C-arm
  14. Part of glass enclosure where xrays are emitted
    Tube window
  15. Federal regulation for amount of leakage radiation
    100 mR per hour at distance of 1 meter
  16. How is heat dissipated from xray tube?
    Oil cooled and air cooled
  17. Area on patient or image receptor where electrons are directed
    Effective focal spot
  18. Three reasons tungsten is used for anode
    • High atomic number
    • High melting point
    • Thermal conductivity
  19. Type of motor used to rotate anode
    Induction/inductor
  20. Two parts of motor
    Rotor and stator
  21. Two speeds anode rotates
    • Regular: 3,600 rpm
    • Fast: 10,000 rpm
  22. What happens when you press the rotor button (boost)?
    • Anode rotates up to speed
    • Filament heated to boil off electrons
  23. What happens when you press the exposure button?
    Electrons are accelerated across the tube and abruptly stopped to produce xrays
  24. Angle of anode
    5-15 dgrees
  25. Define heel effect
    Radiation intensity is greater on the cathode side of the xray field than on the anode side
  26. Most common reason xray tube fails
    Tungsten vaporization on the inside of glass enclosure leads to arching
  27. Define target
    Area of anode struck by electrons
  28. Three methods to dissipate heat
    • Conduction
    • Convection
    • Radiation
  29. Term for emitted in all directions (360 degrees)
    Isotropically
  30. Three essential items needed to produce xrays
    • 1. Hot filament for thermionic emission
    • 2. kVp application to accelerate electrons across xray tube
    • 3. Abrupt stoppage of electrons at target of anode
  31. Purpose of glass envelope
    Creates a vacuum
  32. Why is a metal envelope preferred over glass (pyrex)?
    • 1. Maintains constant electric potential between electrons and enclosure
    • 2. Longer life
    • 3. Less likely to fail
  33. Advantage of rotating anode
    Heat of anode spread across larger surface area (1000 times); increased heating capacity
  34. Composition of stem anode? Why is that material used?
    Molybdenum - poor heat conductor
  35. List reasons anode fails in xray tube
    • Surface melting and pitting
    • Bearing and motor damage
    • Tungsten vaporization
    • Cracking of anode
  36. List reasons cathode fails in xray tube
    • Tungsten vaporization
    • Open filaments (wire breaks from thinning due to use)
  37. Purpose of radio graphic rating chart
    Conveys which techniques are safe or unsafe for tube operation
  38. Purpose of anode cooling curve
    • Shows amount of heat units anode can hold
    • Shows time required for heated anode to cool
    • Shows how long you have to wait before you can take next exposure
  39. Formula for calculating heat units
    HU=mA x time x kVp  Single phase

    HU=mA x time x kVp x 1.4  Three phase/high frequency
  40. Can one chart be transferred to all units
    No, each machine has a rating chart specific to that unit
  41. List the two main byproducts of xray production
    • Heat
    • Xrays
  42. How much heat is produced when electrons strike the target? How much xray?
    99% heat, <1% xray
  43. The efficiency of xray production increases with what?
    Increasing kVp
  44. What is emitted when an outer shell electron fills an inner shell void
    Characteristic radiation
  45. Why is characteristic radiation called that?
    Characteristic of binding energy of target element
  46. 'Bremsstrahlung' means what?
    Slowing down or braking
  47. In the diagnostic range, most xrays produced are what type?
    Bremsstrahlung
  48. The type of radiation that is not emitted below 70 kVp
    Characteristic
  49. Define xray emission spectrum
    Relative number of xrays emitted, plotted as a function of the energy of each xray
  50. What type of xrays have a range of energies and form a continuous emission spectrum?
    Bremsstrahlung
  51. What type of xrays have fixed energies and form a discrete emission spectrum?
    Characteristic
  52. Function of filtration
    Remove low energy or soft rays from xray beam that would only add to patient's skin dose
  53. Total filtration in xray unit operating at 90 kVp
    2.5 mm Al eq
  54. Define HVL
    Thickness of designated absorber required to reduce or decrease the intensity of the primary beam by 50% of one half of it's initial value
  55. List types of compensating filters
    • Wedge
    • Trough
  56. Amount of added filtration in xray unit
    2.0 mm Al eq
  57. What makes up inherent filtration
    Glass envelope, insulating oil
  58. Advantage of three phase power generation
    Voltage never drops to zero
  59. Purpose of autotransformer
    Steps up the voltage in the kVp circuit
  60. Purpose of filament circuit
    Provide low voltage and high amperage to heat the filament of the xray tube
  61. Purpose of kVp circuit
    Produce the kilo-voltage needed to penetrate the body part being xrayed
  62. Two types of automatic exposure control
    • Ionization chamber
    • Photo timer
  63. Two limitations for AEC
    • Positioning is critical
    • Minimum time exposure
  64. Purpose of AEC
    Machine decides when to terminate the exposure
  65. Define back up timer
    Setting place on timer to shut off machine in case the machine doesn't shut off before hand
  66. Where is the ion chamber located for AEC? Photo timing?
    Ion chamber - between patient and IR

    Photo timing - below image receptor

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