Unit 4 (Grids)

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CoLinRadTechs
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180395
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Unit 4 (Grids)
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2012-10-28 17:01:55
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Imaging Principles
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Unit 4: Grids
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  1. What are the two main purposes of a grid?
    • improves contrast
    • absorbs scatter
  2. Is absorption or transmission responsible for the dark areas of the radiograph?
    transmission
  3. If you use a grid without compensating by increasing technical factors, will the density increase or decrease?
    decrease
  4. What is defined as a process in which x-rays interact and change direction?
    scatter
  5. Name two things scatter can produce?
    • fog
    • lower contrast
  6. As kVp increases, does scatter increase or decrease?
    increase
  7. If the field size decreases, does the scatter increase or decrease?
    decreases
  8. As the thickness of the part decreases, does the scatter increase or decrease?
    decrease
  9. If the atomic number decreases, does the scatter increase or decrease?
    increase
  10. You should consider using a grid when the part thickness is:
    greater than 10cm
  11. Would a KUB or a hand produce a greater amount of scatter?
    KUB
  12. Consider using a grid if you use a kVp:
    greater than 60
  13. What is the most common metal that interspace material is made of?
    aluminum
  14. Name the two main components of grid construction.
    • radiopaque lead strips
    • radioluscent interspace material
  15. Who created grids and when?
    • Dr. Gustav Bucky
    • 1913
  16. Name three types of lead strip positioning.
    • linear parallel
    • linear focused
    • cross hatched
  17. Which grid produces the maximum amount of scatter?
    A. linear parallel
    B. cross hatched
    C. linear focused
    B. cross hatched
    (this multiple choice question has been scrambled)
  18. Grids allow ____________ to reach the IR.
    primary radiation
  19. Name the two largest disadvantages of grid use?
    • increase in patient dose
    • grid lines on film
  20. Who made improvments to the use of grids?
    Dr. Hollis Potter
  21. How did Dr. Potter improve the use of grids?
    • realigned lead strips to run in one direction
    • moved grid during exposure to make lines invisibile on image
  22. When considering grid dimensions, "h" stands for:
    height of radiopaque strips
  23. When considering grid dimensions, "D" stands for:
    distance between the strips (thickness of interspace material)
  24. What is the grid ratio formula?
    grid ratio = h/D
  25. Does a higher grid ratio mean more or less lead content?
    more
  26. Is a higher grid ratio more or less efficient in removing scatter?
    more
  27. What is the typical grid ratio range?
    5:1 to 16:1
  28. What is defined as the number of lead strips per inch or cm?
    grid frequency
  29. Common frequency ranges:
    • 60-200 lines/in
    • 25-80 lines/cm
  30. Typically, higher frequency grids have thicker or thinner lead strips?
    thinner
  31. Do thicker lines result in more or less grid clean-up?
    more
  32. Do grid ratio and frequency have a direct or indirect relationship?
    indirect
  33. Digital imaging systems can use very high frequency grids with ranges of:
    • 103-200 lines/in
    • 41-80 lines/cm
  34. What is the most important factor in a grid's efficiency?
    lead content
  35. What unit is used to measure lead content of a grid?
    mass per unit area (g/cm2)
  36. What type of grid tends to have the highest lead content?
    high ratio, low frequency grid
  37. As lead content increases, does contrast increase or decrease?
    increase
  38. How are the lead strips of a cross hatched grid pattern positioned?
    both horizontal and vertical strips
  39. Name two positioning requirements when using a cross hatched grid.
    • primary beam must be centered to grid
    • grid must remain flat
  40. How are the lead strips of a linear grid pattern positioned?
    the lead strips run the length of the cassette
  41. Name the two types of linear grids?
    • parallel
    • focused
  42. In a focused linear grid, lead strips are angled to match the __________ of the beam.
    divergence
  43. When using a focused linear grid, the primary beam will align with the:
    interspace material (scatter absorbed by lead strips)
  44. What type of positioning lattitude does a focused linear grid have?
    narrow
  45. Name three aspects of narrow positioning lattitude.
    • improper centering results in peripheral cut-off
    • only useful at preset SID
    • higher ratio grid require careful alignment with tube
  46. Improper centering results in:
    peripheral cut-off
  47. A parallel linear grid absorbs a large amount of _____________ resulting in ____________.
    • primary beam
    • some cut-off
  48. Type of grid that can be attached to a cassette for use:
    stationary grid (aka grid caps)
  49. What size is the Potter-Bucky diaphragm grid?
    17"X19"
  50. Name two types of grid movement.
    • reciprocating
    • oscillating
  51. Describe a reciprocating motor.
    a motor that drives the grid back and forth during exposure.
  52. Describe the movement of an oscillating grid.
    moves in a circular motion.
  53. When a grid is placed in the beam to remove scatter, will the density of the radiograph increase or decrease?
    decrease
  54. What formula is used to find the grid conversion factor or Bucky factor?
    GCF = mAs with grid/ mAs without grid
  55. In the grid conversion formula, are you dealing with a direct or indirect relationship between mAs and GCF?
    direct
  56. Who evaluates grid performance?
    International Commission on Radiologic Units and Measurements (ICRU)
  57. What two criteria are used by the ICRU to evaluate grid performance?
    • selectivity
    • contrast improvment ability
  58. What describes the grids ability to allow primary radiation to reach the IR and prevent scatter?
    selectivity
  59. Are highly selective grids better or worse at removing scattered radiation?
    better
  60. High lead content grids are more or less selective?
    more
  61. What compares radiographic contrast of an image with a grid to radiographic contrast of an image without a grid?
    contrast improvement ability (the K factor)
  62. What are the typical ranges of the K factor?
    1.5-3.5
  63. The higher the K factor number, the higher or lower the contrast?
    higher
  64. If you are dealing with the capital letter K, you are dealing with:
    contrast
  65. Proper ___________ between x-ray tube and grid is very important.
    alignment
  66. Improper alignment will result in:
    cut-off
  67. The grid error off-level refers to:
    the receptor itself
  68. The grid error off-center refers to:
    the central ray
  69. The grid error off-focus refers to:
    angulation of the tube
  70. Name five common grid errors.
    • off-level
    • off-center
    • off-focus
    • upside down
    • Moire effect
  71. Which grid error is a result of not being lined up with the convergence point?
    off-level
  72. Where will you see cut-off with an off-level grid?
    on the peripheral edges
  73. Which grid error affects one side of the image more than the other?
    off-center
  74. What is the Moire effect and how can it be prevented?
    • when grid lines are parallel to scan lines (digital systems)
    • high frequency grids can prevent it
  75. What is an alternative to grid use?
    air-gap technique
  76. A 10" air-gap has a similar clean-up of what grid ratio?
    15:1
  77. What amount of air-gap is needed for a clean-up similiar to a 15:1 grid?
    10"

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