RadPhysics 11_16_12 Chapter 6 KSW(3).txt

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RadPhysics 11_16_12 Chapter 6 KSW(3).txt
2012-12-20 23:32:16
Radiation Physics Bushong

RadPhysics 11/16/12 KSW
Show Answers:

  1. Never calla Radiation Technologist a ____________.
  2. The most frequent used support system for an xray tube is the
    ceiling support system
  3. A _______support system consists of two perpendicular sets of ceiling mounted rails. This allows for both longitudinal and transverse travel of the xray tube.
  4. When the xray tube is centered above the exam table at the standard  SID, the xray tube is in the ________ position. Other positions can be chosen and locked by the technologiwt. Some ceiling supported xray tubes have a single control that removes all locks allowing the tube to ______.
    • preferred detent
    • "float"
  5. SID stands for:
    source to image receptor distance
  6. Interventional radiology suites are often equipped with ______support systems, so called because the system is shaped like a ___.These systems are *ceiling mounted and provide for very flexible xray tube positioning.The xray tube is attached to the other end of the ___arm from the image receptor.
    • C-arm
    • C
    • C

    *The book says that C arms are celing mounted BUT then shows a picture of one that is clearly NOT. From what I can tell:some are some aren't
  7. The ________support system has a single column with rollers at eah end, one attced to a ceiling mounted rail adn the other attached to a floor mounted rail. The xray tube slides up and down the column as the column rotates.
    floor to ceiling
  8. isotropically
    • Identical in all directions
    • (Isotropic radiation has the same intensity regardless of the direction of measurement)
  9. Whe xrays are produced they are emitted ________, that is, with equal intensity in all directions.
  10. We use only xrays emitted through the  special section of the xray tube called the ________.
  11. The xrays emitted through the window are called the ______  ______.
    useful beam
  12. Xrays that leak through the protective housing are called___________.They contribute nothing in the way of diagnostic information and result in unnecessary exposure of the patient and the radiologic technologist. Properly designed protective housing reduces the level of leakage to less than _________ when operated at max conditions.
    • leakage radiation
    • 1mGya/hr at 1m
  13. Protective housing guards against excessive radiation exposure and ______ _________. They also provide some ______ support and protects the xray tube from ________. The protective housing around some xray tubes contains oil that serves both as an _______ against electric shock and as a thermal cushion to dissipate ____.
    • electric shock
    • mechnical
    • rough handling
    • insulator
    • heat

    • IN SUMMARY: the protective housing:
    • 1)protects against electric shock
    • 2)guards against rough handling
    • 3)guards against excess radiation exposure
    • 4)helps to dissipate heat-(oil in the protective housing acts as     a thermal cushion and some housings also have cooling fans to dissipate heat)
  14. What are the benefits of a metal enclosure xray tube?
    The maintain a constant electric potential between the electrons   of the tube current and the enclosure therefore, they have a longer life and are less likely to fail.

    *Virtually all high capacity xray tubes now use metal enclosures
  15. An xray tube is an electronic vacuum tube with components within a ____ or ______ enclosure.
    • glass (pyrex which withstans tremendous heat)
    • metal

    *METAL tubes maintain a constant electric potential between the electrons   of the tube current and the enclosure therefore, they have a longer life and are less likely to fail.
  16. The _______is the negative side of the xray tube and has two primary parts: a ____ and a _______ .
    • cathode
    • filament
    • focusing cup

    • Cathode is NEGATIVE because a C is  a negative(bad) grade
    • Anode is POSITIVE because an A is a good(positive) grade
  17. Early xray tubes, modifications of the ________ tube were NOT _______ but rather contained controlled quantities of ______ within  the enclosure. The modern xray tube, the _______tube, is a _________. If it becomes _____, xray production fails, and the tube can fail.
    • Crookes,vacuum tubes, gas
    • Coolidge, vacuum tube, gassy.fail

    • SUMMARY:
    • Early X-ray  Tubes (*old crooked farts in a glass tube)
    • Crookes        
    • NOT Vacuum
    • Glass enclosure 
    • Modern X-ray Tubes (*New Cool Metal vaccums)
    • Coolidge
    • Vacuum
    • Most are metal

    • old xray tubes-old Crooked farts in a glass tube
    • new xray tubes-new Cool metal vaccums
  18. The xray tube ______is an area of the glass or metal enclosure, aprx. 5x2cm, that is thin and through which the useful beam of xrays is emitted.
  19. The _______is the negative side of the xray tube; it has two primary parts, a ____ and a _________.
    cathode, filament, focusing cup

    • *DUAL filament cathodes are designed to provide for two focal spots
    • dual filaments in focusing cup
  20. The _____ is a coil of wire similar to that in a kitchen toaster, but is much smaller:2-5 cm(aprx.)
    cathode, filament, focusing cup

    *DUAL filament cathodes are designed to provide for two focal spots
  21. An xray tube emits electrons when it is _____. When the current through the filament is sufficiently high, the outer shell electrons of the  filament atoms are "boiled off" and are ejected fromm the filament. This phenomenon is known as _______________. Filaments are usually made out of _________. _____ provides forr higher _______ than other metals. Its melting point is ____therefor it is not likely burn out like the filament of a light bulb. It also does not ______ easily.
    • heated
    • thermionic emision
    • thoriated tungsten
    • themionic emission
    • 3410 degrees C
    • vaporize
  22. Why is thoriated tungsten used for the filament?
    • 1)It has higher thermionic emission than other metals(more electrons are ejected from the tungsten filament than other metals)
    • 2)Its not likely to burn out because it has such a high melting pont(3410 degrees C)
    • 3)does not vaporize easily

    *EVENTUALLY the tungsten DOES vaporize(also from the anode target) and  slowly plates the inside of the glass or metal enclosure.This can lead to arcing and tube failure.
  23. ______vaporization with deposition on the inside of the glss enclosure is the most common cause of tube failure.
  24. The filament is embedded in a metal shroud called a _______.Because all the electrons accelerated from the cathode to anode are electrically_____,the electron beam tends to spread out owing to _____. Some electrons can even miss the anode completely. The ______is negatively charged so that it electrostatically confines the electron beam to a small area of the anode. The effectiveness of the _______is determined by its size and shape, its charge, and the position of the filaments in the __________. Most rotating anode xray tubes have _____filaments mounted in the cathode assembly "side by side" creating large and small __________.
    • focusing cup
    • negative
    • eletrostatic repulsion
    • focusing cup
    • focusing cup
    • focusing cup
    • 2 focal spots
  25. Certain types of xrays tubes  called _________ tubes are designed to be turned on and off very rapidly. _______tubes are used in  portable capacitor discharge imaging systems and in digital subtraction angiography, digital radiography, and cineradiography, each which requires multiple exposures for precise exposure time. The term ____ is borrowed from vacuum tube electronics and refers to an element in the tube that acts as a switch. In a _______ xray tube, the focusing cup is the grid and therefore the exposure ______.
    • grid-controlled
    • grid-controlled
    • grid-controlled
    • grid
    • switch
  26. The filament cloud, called the ______, makes it difficult for subsequent electrons to be emitted by the filament because of __________.This phenomenon is called the __________. The major obstacle in producing xray tubes with currents that exceed 1000 mA is the design of adequete space charge compensating devices.
    • space charge
    • electrostatic repulsion
    • space charge effect

  27. At a any given filament current,the xray tube current rises with increasing voltage to a maximun value.A further increase in kVp does not result in a  increase in mA because all of the available electrons have been used. This is the _________ __________.It is is not reached at a lower kVp because of space charge limitation. When an xray tube is operated at saturation current it is said to be ________________.
    • saturation current
    • emission limited

    • Below the saturation point, The tube current is limited
    • by the space charge effect (space-charge-limited).
    • ↑↑ kV → significant ↑↑in x-ray tube current although filament heating is the same.

    • Above the saturation voltage,The space charge effect has no influence on the x-ray tube current. The tube current is determined by the number of electrons
    • made available by the heated filament (emission-limited{all available electrons have been used)
    • or temperature-limited).↑↑ kV → very little
    • change in tube current
  28. What approximate percentage of the electrons in the tube current are converted to heat?
  29. Describe the anode heel effect.
    • Because of the angle of the tube, x-rays
    • have a greater intensity on the cathode side of the tube, with the
    • intensity diminishing toward the anode side

    *place cathode over thicker body part
  30. The smaller the _______the larger the heel effect
    • anode angle
  31. The difference in radiation intensity across the useful beam can vary as much as ______. The _______ of the useful beam is the imaginary line generated by the centermost xray in the beam.
    • 45%
    • central ray
  32. The effective focal spot is smaller on the ______ side of the xray field than on the ______ side.
    • anode
    • cathode
    • anode:smaller focal spot
    • cathode: larger focal spot M
    • *place cathode over thicker body part
  33. The ________results in a smaller effective focal spot and less radiation intensity on the _____ side of the xray beam.
    • heel effect
    • anode
  34. Most diagnostic xrays have____focal spots, one large and the other small. The small focal spot is used when better spatial resolution is required. The large focal spot is used when large body spots are imaged and when other techniques that produce_____ are required.
    • small
    • high heat
    • ________________________________________________
    • small focal spot=better spatial resolution
    • large focal spot=large body parts, when using techniques that produce high heat at aprx =>400mA
  35. The anode is the ______side of the xray tube.There are two types of anodes, ______ and __________.
    • positive
    • stationary
    • rotating
  36. Xray tubes are designed so that projectile electrons from the cathode interact with the______  only at the_______.However, some of the electrons bounce off of the focal spot and then land on other areas of the ______, causing xrays to be produced from outsiede the focal spot. These xrays are called ____________.
    • target, focal spot, target
    • off- focus radiation
    • *bad because it extends the size of focal spot&reduces image contrast
    • *FIX=fixed diapragm in the tube housing near the window of the xray tube
  37. Most of the reasons xray tubes fail are related to ______.
    Enormous ____ is generated in the  anode of the xray tube during xray exposure.The _____must be dissipated for the xray tube to continue to function.
  38. ______is the transfer of heat by the emission of infrared heat. Heat lamps emit not only by visible light but also by infrared ________.
    • radiation
  39. Heat can be dissipated in one of three ways: _________,__________, or___________.
    • radiation, convection conduction

  40. _________is the transfer of energy from one area of an object to another.The handle of a heated iron skillet becomes hot because of conduction.
    • conduction
  41. ___________is the transfer of heat by the movement of a heated substance from one place to another. Many homes are heated by the ________of hot air.
    • Convection
  42. Excessive____results in reduced xray tube life
  43. All three modes of heat transfer(______,______,______) occur in an xray tube. Most of the heat is dissipated by radiation during exposure.The may glow red hot. It always emits infrared radiation.
    radiation, conduction, convection
  44. Maximum radiographic techniques should never be applied to a _____________. If the temperature of the ______increases too rapidly, it may crack, becomung unstable during rotation and rendering the tube useless.
    • cold anode
    • anode
  45. The most frequent cause of abrupt tube failure is from electrons arcing from the filament to the enclosure because of ________.
    vaporized tungsten

    *slowly vaporizes and plates the inside of the enclosure
  46. Between exposures, heat is dissipated by _________to the oil bath in which the tube is immersed.
    • conduction
  47. 3 reasons for xray tube failure:
    • 1)a single excessive exposure
    • 2)long exposure time leading to excessive heating of anode(during exposures lasting from 1-3 seconds, the temperature of the node my be sufficient to caise it to glow like an incandescent bulb)
    • 3)vaporization of tunsten filament(and target to a lesser degree) plates the inside of the enclosure and leads to arcing
  48. 4 functions of protective housing:
    • 1)guards against electrical shock
    • 2) keeps radiation leakage ideally to 1mGa/hr at 1m
    • 3)protects against rough handling
    • 4)serves as a way to conduct heat away from target

    *top three are the MAIN 3 FUNCTIONS
  49. What is the purpose of the focusing cup?
  50. The heel effect is important when imaging anatomical structures that differ greatly in_____ or ______.
    thickness or mass density

    • *FAT CAT   or
    • cathode over thick
  51. Diagnostic xray tubes have target angle that vary from  aprx.__________ degrees.
    5-20 degrees
  52. Filament current that has risen to its maximum value because all the electrons have been used:
    saturation current
  53. When an xray tube is operated at saturation current it is said to be ________________.
    emission limited
  54. Thermionic emission at_____kVp and ___mA can be _____________.
    • low, high
    • space charge limited

    • kVp = filament current(the potential or voltage between the anode and cathode)kilovolt peak or kilovolt potential
    • mA=tube current(the flow of electrons)milliamperes
  55. Heat form the ______is dissipated through____,_______, and _______,
    • anode, conduction, radiation, convection
  56. The rotating anode is powered by a ________________.
    • electromagnetic induction motor
  57. What are the three main reasons tungsten is the material of choice for the target for general radiography?
    • 1)Tungsten's high atomic number:74, results in high-efficiency xray production.
    • 2)Tungsten has a a thermal conductivity nearly equal to that of copper which makes it an efficient heat dissipator.
    • 3Because of its high melting point (3400) it can stand up under high tube current without pitting or bubbling.

    • *NOTE-thoriated tungsten(tungsten with the addition of 1-2% thorium to enhance thermionic emission) has a melting point of 3410 C
    • regular(NON thoriated) tungsten has a melting point of 3400 C
    • thoriated tungsten=3410C
    • tungsten=3400 C
  58. The atomic number of tungsten:

    results in high effeciency xray production
  59. Melting point of tungsten?
    3400 degrees Celsius
  60. Melting point of thoriated tungsten?
    3410 degrees Celsius
  61. 3 functions of the anode:
    • 1)electrical conductor-recieves the electrons emitted by the cathode and conducts them through the tube to the connecting cables and back to the high voltage generator.
    • 2)provides mechanical support for the target
    • 3)Acts as a thermal dissipator
  62. LABEL:
  63. Label the layers of the rotating anode:
    • Top layer(target):Tungsten
    • Middle layer: Molybdenum
    • Bottom layer: Graphite

    layered surface gives increased heat capacity
  64. The forcal spot is the actual xray _______.
  65. The focal spot is the area of the target from which the xrays are emitted. Radiology requires small focal spots because the smaller the focal spot the better the spatial resolution of the image.Unfortunately, as the size of the focal spot decreases, the heating of the target is concentrated on to a smaller area. This is the limiting factor to focal spot size.Before the rotating anode was developed,another design was incorporated to allow a large area for heating while maintaining a small focal spot. This design is known as the ______.By angling the target, one makes the effective area of the target much smaller than the actual area of electron interqaction.
    • line-focus principle
  66. The primary support structure for the xray tube which allows the greatest ease of movement and range of position, is the _____________.
    • ceiling support system
  67. Protective housing covers the xray tube and provides the following functions:
    • 1)reduces radiation leakage to 1mGya/hr at 1m
    • 2)provides mechanical support, thereby protecting tube from damage
    • 3)serves as a way to conduct heat away from xray tube
  68. A _____ or ___ enclosure surrounds the _____ (-)  and the ____(+) which are the electrodes of the vaccuum tube. The cathode contains the _____filaments  which is the source of ______. The rotating anode is the ________disc which serves as a target for the accelerated electrons fron the _________. The ______results from angled targets. The _____ is the variation in xray intensity across the xray beam from absorption of xrays in the heel of the target.
    • metal, glass
    • cathode
    • anode
    • electrons
    • tunsten-rhenium
    • cathode
    • line focus principle
    • heel effect
  69. The ______results from angled targets.
    line focus principle
  70. The _____ is the variation in xray intensity across the xray beam from absorption of xrays in the heel of the target.
    heel effect