CH 7 AEC in Radiography: Technology, Settings, and Positioning
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18 Terms
What does AEC stand for?
Automatic exposure control
What does AEC control?
Time
Which technology (phototimer or ionization chamber) is commonly used today?
Ionization chamber
How many photo cells are there and how are they arranged?
Three—one center cell and two outer cells slightly above the center
In what combination may photo cells be selected?
Any one cell, or any combination of two or all three cells
What is meant by density setting?
A setting that creates either more or less exposure; common settings are -2, -1, 0, +1, +2
In what situation would the density setting be increased or decreased from normal density?
For extreme cases involving patient size or pathology
When using AEC, what adjustment should be made for part thickness?
Increase kVp
Why is positioning so important when using AEC?
The area of interest must be over the photo cell since it determines the exposure
What is the definition of minimum response time and its numerical value?
The length of time required for AEC to respond and terminate exposure (0.001 s)
What type of mAs values does minimum response time most affect?
Small mAs values
What is the built-in backup time for machines that expose above 50 kVp? Below 50 kVp?
Above - 600 mAs; Below - 2000 mAs
What is the rule for setting a manual backup time?
150% of expected exposure
Why is it necessary to set a manual backup time?
To reduce patient exposure in case of user error
Explain the importance of positioning when using AEC.
The area of interest must be over the ionization chamber so the correct tissue determines image exposure, and the chamber must be fully covered
What effect on exposure will occur if an activated photo cell is collimated out of the exposure field?
The image will be overexposed
Given 400 mA × 0.0025 s = 1 mAs, calculate the actual technique with minimum response time when AEC is used.
0.0025 + 0.001 s = 0.0035 s; 400 mA × 0.0035 s = 1.4 mAs
An expected chest x-ray exposure is 110 kVp and 5 mAs. If phototimed, what manual backup time should be set?
7.5 mAs
Note 
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