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25 Cards in this Set
- Front
- Back
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what are automatic exposure controls? (AEC)
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devices designed to reproduce images w/ consistent density and contrast regardless of differences in anatomical sizes or textures of the same anatomical part
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what is the goal of AEC?
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reproducibility and consistency between patients and technologists
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according to Carlton what is the single function of AEC
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to eliminate the need for the radiographer to set an exposure time
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what are automatic exposure devices (AED)
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phototiming (old method)
ionization chambers (current method) |
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what is a phototimer?
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use a photomultiplier tube to detect amount of remnant radiation exiting from the patient
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what 3 things does the radiation have to pass through to reach the phosphor screen?
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patient
film cassette |
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where are ionization chambers placed?
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between the patient and the film
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what do ionization chambers measure?
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measures the remnant radiation that leaves the patient
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what are ionization chambers filled with?
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air
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when do ionization chambers terminate exposure?
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when a predetermined amount of air has been ionized
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what is a typical configuration of the ionization chambers
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3 chamber on table or wall bucky
projected or imprinted on wall bucky |
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AED provide a diagnostic quality density ONLY for?
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structures positioned directly above the ionization chamber
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the use of AEC requires the art of?
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positioning
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when multiple cells are selected, what does the AEC do?
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averages the signals from the cells, when appropriate diagnostic quality is reached, exposure is terminated
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what controls the penetrability of the beam and the scale of contrast of the image>
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kVp
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do we collimate close to areas of the ionization chamber?
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NO
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too wide of collimation will produce what?
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scatter radiation that may undercut the patient
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minimal response time?
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time necessary for AEC to respond to ionization and send a signal to terminate the exposure (.001 sec)
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back up timer?
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used to prevent EXTREME over exposure of the patient to radiation
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what do you set back up timer to?
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150% of the anticipated manual exposure time
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extremity cassettes used in conjunction w/ AEC calibrate for par screens will result in what?
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underexposed films
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dont use AEC on..?
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extremeties
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we don't increase what to increase density of image?
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mA
kV |
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what is the range of optimal density on images?
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0.25-2.5 H&D scale
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when the thickness is 10 cm or greater, which should be used?
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AEC and a bucky
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