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20 Cards in this Set
- Front
- Back
what are the 2 parts of the AEC?
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• Detector
• Comparator |
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What is the Function of AEC Systems
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to monitor the amount of radiation reaching
the image receptor, and to terminate the exposure at a preset level normally measure product of intensity (I) and time (t) |
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Ionisation chamber measures radiation exposure ____ it reaches the image receptor
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before
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Photo-timers/Scintillator Counters are exit-type devices, meaning,
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they measure radiation exposure after it passes through cassette
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where is the Photo-timers/Scintillator Counters commonly found?
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Commonly found in
fluoroscopic units |
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What type of external mode is this?
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side on
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What type of external mode is this?
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head on
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Spectral sensitivity of
photocathode MUST match wavelength of light from scintillator crystal for optimal conversion |
Only 10-30% of photons
cause emission of electrons |
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) As radiation passes
through the chamber, the gas becomes ____. Ion-pairs form. Negative electrons flow to the _____. Current generated. |
ionised; anode
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Study
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study
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difference between ion chamber & fluorescent screen
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ion chamber & fluorescent screen
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where is the detector placed?
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Placed between patient and
film |
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advantages of detector
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• Allows actual mAs used to be displayed immediately after
exposure • Allows radiographer to become more familiar with manual exposure techniques • Use it to verify exposure charts in department • If image is suboptimal, allows of adjustment of exposure using manual technique |
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what is Minimum Response Time (M.R.T.)?
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• the length of time necessary for the AEC to respond to the
ionisation & send a signal to terminate the exposure. • Between 3 – 30 milliseconds may elapse before all the electronics & relays actually stop the exposure. |
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what are Guard/Backup timers?
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• all AEC devices are fitted with a guard timer
• A safety device which prevents the patient from receiving an excessive dose of radiation should the automatic timer fail due to mechanical or operator errors. • These terminate exposures that exceed 600mAs above 50kVp, and 2000mAs at lower potentials. |
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for density settings:
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-2: 50% decrease in density from 0 (Neutral)
-1: 25% decrease in density +1: 25% increase in density +2: 50% increase in density |
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the dominant zone is where ________.
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Positioning where a specific patient part (ROI)
should be placed over the detectors |
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If the sensor is under a body part for which k is
incorrect, then an over or ______-exposure will result |
under
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one of the largest sources of error in using AEC devices is ____.
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poor positioning
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Sufficient collimation demonstrates that as you increase the amount of
collimation and shielding, the amount of scattered radiation reaching the detector _____, resulting in a darker diagnostic radiograph. |
decreases
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