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2014-04-22 17:27:02

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  1. what is a device used to maesure body part thickness
  2. what is apr
    • anatomically programmed radiography
    • systems are commonly utilized computerized exposure control units that combine an automatic exposure control (AEC) system w/ exposure factors to correspond to specific anatomical procedures (body part button = tech factors for that part)
  3. AEC controls what
    the length of the exposure (time)
  4. what happens when CR and DR are underexposed
    the image will look grainy
  5. List the two types of exposures systems and explain them
    variable kvp system (kvp is changed and manipulated) and fixed kvp systems (kvp does not change mas is changeable)
  6. what is a variable kvp system
    a system in which the kvp to be used for a particular projection is varied depending on measured body part thickness
  7. what is a fixed kvp system
    the kvp is held constant and uses and optimal kvp for each body part selected for a given range of subject densities and contrasts while the mAs is varied to achieve an appropriate image density
  8. a fixed kvp system chooses the ____ kvp for the procedure selected
  9. what are the two main methods in part measurement
    • part thickness
    • and measurement of central entering to cr exiting
  10. apr is controlled by what
    a integrated circuit or computer chip that has been programmed with exposure factors for different projections and positions of different anatomic parts
  11. what is the most common exposure system
    fixed variable system
  12. which body parts would would need the aec to be turned off
    • anybody part that requires a a breathing technique of a long exposure time
    • sternum ap scapula transthoracic for prox humerus lateral thoracic
  13. what does aec or automatic exposure control do
    what device is used to do this
    • it stops the exposure at the right time so that a radiograph with good density is produced
    • a photocell
  14. AEC controls ___
    the time or the length of the exposure
  15. where are the photocells
    in bucky or near it and cannot be seen
  16. a photocell is either a _____ or a ______
    ionization chamber or photomultiplier
  17. what is exit radiaiton
    small angle scatter and penetration photons
  18. list the structure of the photocells in the table
    • table top
    • grid
    • sensors
    • then the bucky tray
  19. how does the ir produce a preset OD (optical density)
    through an electronic feedback circuit radiation exposure is terminated when a sufficient number of xrays have reached the IR
  20. what is the usual range of the radiographic densities
    .25 and 2.5 above base + fog
  21. what is od range
    which is lightest and darkest
    • 0-4
    • 0 lightest
    • 4 darkest
  22. what would happen if AEC was used on a large patient
    it would take a long time for enough exit radiation to get through the patient and into the photocell to charge the xray timer to the predetermined amt of charge exposure length would be long
  23. what would happen if aec was used on a smaller patient
    the patient would cause a shorter length of exposure
  24. what is the most common photocell to activate
    the two outer or the center cell
  25. what is the backup timer
    what is it used as
    • when aec is used the ordinary timer used for an exposure made without aec is the back up
    • it is use a safety device for the equipment to save the xray tube life
  26. if a ordinary pa chest exposure time is set for .20 second but the backup time is .15 what will happen to the image
    • the amt of radiation receive will stop faster and produce an underexposed image because the appropriate time is .20
    • there the backup timer should be set to a time longer than what is needed
  27. what happens if aec fails to work
    what happens if the backup time is set to high
    • the backup time will kick in n stop the exposure
    • then the patient will receive a larger dose of radiation
  28. what do radiographers do to prevent backup timing making their image too light faster backup time?
    they set the backup time very high so it does not cut off earlier but this is not good practice because it increase pt dose
  29. can backup timers exceed tube limit and what is there percentage of the anticiapated manual exposure mAs
    • no
    • 150%
  30. according to us public law 90-602 generators must terminate the exposure at ______ for exposures above ____ kvp and ______ mAs for exposures under ____kvp
    • 600 mAs; 50
    • 2000 mAs;50 kvp
  31. AEC affects only _____ ______ and time controls _____ exposure
    exposure time; IR
  32. what happens to the aec once kvp is changed
    the aec will compensate woth an adjustment in exposure time to adjust Ir exposure
  33. If kvp is decreases IR exposure ______ so the AEC will _____ time to compensate
    decreases; increases (longer time)
  34. If kvp is increases IR exposure ______ so the AEC will _____ time to compensate
    increases; decrease (shorter faster time)
  35. an increase or decrease in kvp when using aec changes ______ and _____ _____
    contrast; IR exposure
  36. high or increase in ma causes a _____ exposure time
    a lower or decrease in ma causes a _____ iexposure time to produce adequate density
    which is best to pick
    • decrease (faster) shorter
    • increase (slower) longer time
    • high ma = short exposure time to beat pt motion
  37. when should we use a two photo cell arrangement
    and a 1 center cell
    • pa chest kub ribs pelvis
    • shoulder lat chest femur hip lat spine
  38. what would happen if the center cell was chosen for a pa chest instead of a two cell arrangement
    the image will be over exposed because the body part centered over the cent4er cell will be exposed appropriately (the thoracic vertebrae) and the lung field will be way too black because the outer cell was not selected
  39. how does a photocell determine the length of the exposure
    by averaging the tissue densities over a certain cell
  40. what happens to an image when using a center cell but the body part was not position correctly in the middle of the cell
    the photocell will detect some of the body part and some parts that werent over the center (blank spots) so when the photocell averages the length thru tissue thickness it will become underexposed
  41. what happens to your image  a pa chest was centered to low or too high
    • if it was too low the photocell will average the radiation for the abdomen and the chest will be overexposed
    • if it was too high the photocell will average the radiation and thickness for the upper chest (which is thinner) and the lungs will be underexposed
  42. when should collimation be avoided
    too close collimation would result in
    not enough collimation would result in
    • when the collimation is close to the activated chambers
    • overexposed image
    • in a light image underexposed
  43. when using aec with barium where should the body be positioned
    the body part with the barium should be positioned over the photocell
  44. when would we need to adjust density options with aec
    when patient is extremely large or patient is really small
  45. density controls:
    -1 =
    +1 =
    0 =
    -2 =
    • -1 = 30% decrease in relative exposure
    • +1 = 30% increase
    • 0 = normal density
    • -2 = 60% decrease in dens
    • +2 = 60% increase in dens
  46. what is minimum response time
    modern scr's may have a minimum reaction tim of less than _____ second
    • shortest amt of time it takes ionization chambers or photocells and tube to turn off  exposure
    • .001 seconds
  47. what are scr's
    silicon controlled rectifiers are electronic timers (most used today) capable of accurate exposures short as 0.001 sec with only a millsecond delay
  48. an ionization device is located between what
    tabletop and the image receptor
  49. if the back up time  is set at 0.5 second, what will happen tothe radiograph if the required exposure time is 0.8
    the image will be underexposed because not enough radiation will make it to meet 0.8 it will cut off at 0.5 which is not enough radiation needed to make the image
  50. if the minimum reaction time is 0.01 second, what will happen to the radiograph if the required exposure is 0.008
    the image will be overexposed because 0.008 is shorter than 0.01 thus the exposure will not stop until it passes 0.008 and reaches 0.01 which is a longer time so more radiation will occur
  51. which of the following changes decrease the length of exposure when using AEC
    decrease mA
    decrease kVp
    change density -1
    change density setting +1
    change density setting -1 because it gives you 25 percent less radiation less radiation less length of exposure
  52. when using aec, if the centering point for a c spine is at c7 instead of c4, how would the radiographic density be affected
    it will be overexposed