PHYSICS CH6 11/16 CH

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RadTherapy
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182970
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PHYSICS CH6 11/16 CH
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2012-11-12 13:29:55
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PHYSICS CH6 11 16
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PHYSICS CH6 11/16 CH
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  1. The xray tube and housing assembly are quite heavy; therefore, they require a ____ so the radiologic technologist can position them.
    Support Mechanism
  2. The ____ support system is probably the most frequently used
    Ceiling 
  3. A telescoping column attaches the xray tube housing to the rails, allowing for variable
    source to image receptor distance (SID)
  4. When the xray tube is centered above the exam table at the standard SID, the xray tube is in a
    preferred detent position
  5. Some ceiling supported xray tubes have a single control that removes all locks, allowing the tube to 
    float
  6. The ____ has a single column with rollers at each end, 1 attached to a floor mounted rail
    floor to ceiling support system
  7. On the floor to ceiling support system, what slides up and down the column as the column rotates
    xray tube
  8. A variation of the floor to ceiling support system has the column positioned on a single ___ with 1 or 2 floor mounted rails
    floor support system
  9. Which systems are ceiling mounted and provide for very flexible xray tube positioning
    C arm support system
  10. When xrays are produced, they are emitted ____ that is with equal intensity in all directions
    isotropically
  11. ____ is when xrays are produced with equal intensity in all directions
    isotropically
  12. We only use the xrays that are emitted thru a special section called the
    window
  13. The xrays emitted thru the window are called the
    useful beam
  14. xrays that escape thru the protective housing are called
    leakage radiation
  15. What contributes nothing in the way of diagnostic info and results in unnecessary exposure of patient and radiologic technologist
    leakage radiation
  16. what guards against excessive radiation exposure and electric shock
    protective housing guards
  17. Properly designed protective housing reduces the elvel of leakage radiation to less that ____ when operated at maximum conditions
    1mGy/hr at 1 m
  18. The protective housing protects against
    • leakage
    • electric shock
    • mechanical support
    • cooling abilities
  19. The protective housing also provides ___ for the xray tube and protects the tube from damage caused by rough handling.
    mechanical support
  20. The protective housing around some xray tubes contains oil that serves as an ____ against electric shock and as a thermal ____ to dissipate heat
    • insulator
    • cushion
  21. The xray tube is a special type of vacuum tube that contains 2
    • 2 electrodes
    • the cathode and the anode
  22. What are 2 electrodes within the xray tube
    • cathode
    • anode
  23. Why is the large 30-50 cm long and 20 cm in diameter glass enclosure made of pyrex glass?
    to enable it to with stand the tremendous amount of heat generated
  24. The glass/metal enclosure maintains a vacuum inside the tube which allows for ___ and ____
    more efficient xray production and a longer tube life
  25. What is an example of an early xray tube
    Crookes Tube
  26. What is an example of a modern xray tube
    Coolidge Tube
  27. The early xray tubes, modifications of the Crookes tube, were not vacuum tubes but rather contained controlled qtys of ____ within the enclosure
    gas
  28. The modern xray tubes, Coolidge, is a _____ tube. if it becomes gassy, xray production fails and the tube can fail.
    vacuum
  29. _____maintain a constant electric potential between the electrons of the tube current and the enclosure and therefore have a longer tube life and less liikely to fail
    metal enclosure tubes
  30. What are some benefits of a metal tube over a glass tube
    As the glass tube ages, some tungsten vaporizes and coats the inside of the glass enclosure, which alters the electrical properties allowing tube current to stray and interact with the glass and results in arcing and tube failure, metal tubes do not have this issue and maintain a constant electric potential between the electrons of the tube current and enclosure.
  31. The ____ is the negative side of the xray tube and has 2 primary parts ____ and ____
    a filament and a focusing cup
  32. The ____ is a coil of wire similar to that in a kitchen toaster, but is much smaller
    filament
  33. What emits electrons when heated
    xray tube filament
  34. When the current thru the filament is sufficiently high, the outer electrons of the filament atoms are
    "boiled off" and ejected from the filament
  35. Filaments are usually made of
    thoriated tungsten
  36. ______ provides for higher thermionic emission than other metals
    tungsten
  37. What is the most common cause of tube failure
    Tungsten vaporization with deposition on the inside of the glass enclosure
  38. The filament is embedded in a metal shroud called the
    focusing cup
  39. Because all of the electrons accelerated from the cathode to the anode are electrically ____, the electron beam tends to spread out owing to electrostatic repulsion. Some electrons can even miss the anode
    negative
  40. The focusing cup is _____ charged so that is electrostatically confines the electron beam to a small area of the anode
    negatively charged
  41. The focusing cup is negatively charged so that it can electrostatically confines the _____ to a small area of the ____
    • electron beam
    • anode
  42. The effectiveness of the focusing cup is determined by
    • it shape and size
    • position of the filament in the focusing cup
  43. Most rotating anode xray tubes have ___ filaments mounted in the cathode assembly side by side creating large and small focal spot sizes
    2
  44. Most rotating anode xray tubes have 2 filaments mounted in the ____ assemble side by side creating ____ and______
    • cathode
    • large and small focal spot sizes
  45. The xray tube _____ is adjusted by controlling the filament current
    current
  46. The xray tube current is adjusted by
    controlling the filament current
  47. When the filament emits the electrons, they hang around before going to the anode causing a cloud of electrons around the filament, this is called a
    space charge
  48. Space charge makes it difficult for subsequent electrons to be emitted by the filament because of ____. this phenomenon is called
    • electrostatic repulsion
    • space charge effect
  49. What makes it difficult for subsequent electrons to be emitted by the filament because of electrostatic repulsion?
    space charge
  50. Thermonic emission at low kVp and high mA can be
    space charge limited
  51. What is filament current that has risen to its max value because all available electrons have been used
    saturation current
  52. Most diagnostic xray tubes have ____ focal spots
    • 2
    • 1 large
    • 1 small
  53. Which focal spot is used when better spatial resolution is required
    • small focal spot
    • like an ankle
  54. Whic focal spot is used when imaging a larger body part and when other techniques that produce high heat are required
    large focal spot
  55. Normally either filament can be used with the lower mA station-approx ____ or less
    300 mA
  56. At approx ____ and up, only the larger focal spot is allowed because the heat capacity of the anode could be exceeded if you use the small
    400 mA
  57. At what mA do you use the larger focal spot and why
    400 mA because  the heat capacity could be exceeded if you use the small
  58. Small focal spots range from ____to_____ mm
    0.1-1
  59. Large focal spots range from _____to______mm
    0.3-2mm
  60. What are the 2 types of anodes
    • stationary
    • rotating
  61. Where are stationary anode xray tubes used
    • dental xrays
    • portable imaging
    • special purpose units in which high tube current and power are not required
  62. Where are rotating anode xray tubes used
    • general purpose
    • because they must be capable of producing high intensity xray beams in a short time
  63. What is the positive side of the xray tube and conducts electricity and radiates heat and contains the target
    anode
  64. What 3 functions do the anodes serve in an xray tube
    • electrical conductor
    • mechanical support
    • thermal dissapater
  65. The anode is an/a _______ when it receives electrons emitted by the cathode and conducts them thru the tube to the connecting cables and back to the high voltage generator
    electrical conductor
  66. The anode also is a thermal dissipater when the projectile electrons from the cathode interact with the anode, more than ___ of their kinetic energy is converted to ____
    • 99%
    • heat
  67. What is the area of the anode struck by the electrons from the cathode
    target
  68. in stationary anode tubes, the target consists of a ____
    tungsten alloy embedded in the copper anode
  69. In rotating anodes, the target is the
    entire rotating disc
  70. Tungsten is the material of choice for the target for general radiography for what 3 reasons
    • atomic #
    • thermal conductivity
    • high melting point
  71. Tungsten's ____ results in high efficiency xray production and in high energy xrays
    high atomic #, 74
  72. Tungsten has a ____ nearly equal to that of copper which is why its an efficient metal fro dissipating the heat produced
    thermal conductivity
  73. Which anode xray tube allows the electron beam to interact with a much larger target area therefore the heating of the anode is not confined to 1 small spot
    rotating anode
  74. What are possible with the rotating anode
    • higher tube currents
    • shorter exposure times
  75. Higher tube currents and shorter exposure times are possible with the rotating anode
  76. What is used to turn the anode
    electromagnetic induction motor
  77. Rotating anode is powered by
    electromagnetic induction motor
  78. Where on the target are xrays emitted
    focal spot
  79. As the size of the focal spot decreases, the heating of the target is concentrated onto a smaller area. This is the ____ to the focal spot size
    limiting factor
  80. Radiology requires ___ because the ____ the focal spots, the better the spatial resolution of the image.
    • small focal spots
    • smaller
  81. The ____ is the actual xray source
    focal spot
  82. Before the rotating anode, the ____ was incorporated into xray tube targets to allow a large area for heating while maintaining a small focal spot
    line-focus principle
  83. Diagnostic xray tubes have ____ that vary from approx 5-20 degress
    target angles
  84. What results in an effective focal spot size much less than the actual focal spot size
    Line focus principle
  85. What is an unfortunate consequence of the line focus principle where the radiation intensity on the cathode side of the xray is greater than that on the anode side
    heel effect
  86. The heel effect is when the radiation intensity on the ____ side of the xray field is greater than that on the _____ side due to the line focus principle
    • cathode
    • anode
  87. The ____ the anode angle, the ____ the heel effect
    • smaller
    • larger
  88. The difference in radiation intensity across the useful beam of an xray field can vary by as much as
    45%
  89. The ____ of the useful beam is the imaginary line generated by the centermost xray in the beam
    central ray
  90. If radiation intensity along the central ray is 100%, then the intensity of the cathode is ____ and that on the anode is ____
    • as high as 120%
    • as low as 75%
  91. What is important when imaging anatomical structures that differ greatly in thickness or mass density
    heel effect
  92. Positioning the ____ side of the xray tube over the thicker part of the anatomy provides more uniform radiation exposure of the image receptor
    cathode side
  93. Where are the cathode and anode directions usually indicated
    on the protective housing, sometimes near the cable connectors
  94. What results in smaller, effective focal spot and less radiation intensity on the anode side of the xray beam
    heel effect
  95. The heel effect results in smaller effective focal spot and less radiation intensity on the ____ side of the xray beam
    anode
  96. When electrons bounce off the focal spot and then land on other areas of the target, causing xrays to be produced from outside of the focal spot it is called
    off focus radiation
  97. Heat from the xray tube can be dissipated in what 3 ways
    • radiation
    • conduction
    • convection
  98. What is the transfer of heat by the emission of infared radiation
    Radiation
  99. What is the transfer of energy from 1 area of an object to another
    Conduction
  100. What is the transfer of heat by the movement of a heated substance from 1 place to another
    convection
  101. Excessive heat results in
    reduced xray tube life
  102. What results in reduced xray tube life?
    excessive heat
  103. All 3 modes of heat transfer occur in an xray tube. Most of the heat is dissipated by radiation during
    exposure
  104. What should never be applied to a cold anode
    maximum radiographic technique
  105. How does a 2nd type of xray tube failure result
    from maintaining the anode at elevated temps for prolonged periods
  106. What is the most frequent cause of abrupt tube failure
    electron arcing from the filament to the enclosure because of vaporized tungsten
  107. The filament wire becomes thinner and eventually breaks, producing
    an open filament
  108. What is the primary support structure for the xray tube which allows the greatest ease of movement and range of position
    Ceiling support system

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