Week 8: Automatic Exposure Control (AEC) Flashcards
Automatic exposure control (AEC), automatic exposure device (AED), phototimer
programmed to terminate the radiographic exposure time once the IR has been exposed appropriately
Automatic Exposure Control (AEC)
• AEC functions by measuring a present quantity of radiation and breaking the timer circuit when a dose sufficient to produce the desired exposure has been reached
• AEC provides consistent IR exposures and reduces the chance of overexposing patients, regardless of patient size, tissue thickness, or tissue density
• With AEC, the radiographer can adjust mA and kVp, but not the exposure time
Ionization chamber
• Also called AEC cells, photo-timers, or radiation detectors
• Use a thin, 5 mm thick parallel-plate chamber positioned immediately above the IR. It serves to measure the exposure to the IR.
• Ionization within the gas or air-filled chamber(s) creates a charge, and when the appropriate charge has been reached for the selected patient part, the exposure is terminated
Some AEC systems use _______________ radiation detectors instead of ionization chambers, but they both work similarly
Solid-state
Minimum reaction time (minimum response time)
Length of time necessary for the AEC to respond to the radiation and for the generator to terminate the exposure
The minimum reaction time is the __________ exposure time an AEC unit can provide
quickest
Modern ionization chambers have a minimum response time of _____ seconds
.001
AEC backup time
A feature that will terminate the exposure in instances when the AEC fails to do so
Backup times cannot exceed the tube limit and should be set at ____ of the anticipated manual exposure mAs
150%
By law, backup times must terminate the exposure at _____ mAs for exposures above 50 kVp, and _______ mAs for exposures below 50 kVp
600, 2,000
T or F: Most AEC units automatically set the back time
True
T or F: Adjustments in kVp or mA will affect the overall exposure to the IR
False, it will not
lowering kVp or mA will create a __________ exposure, and raising the kVp or mA will create a __________ exposure
longer, shorter
Increasing kVp beyond the optimal setting will __________ scatter on the image
increase
___________ kVp beyond the optimal setting can create image noise due to under penetration of the part
Decreasing
Because changes in mA or kVp by the technologist do not affect the overall exposure to the IR, changes in the ________ control setting allow changes in IR exposure
density
Typical AEC density control labels
Most labels use the center control as the normal density (0 and N in the two
examples below). Increases and decreases above normal density affect the overall exposure to the IR. For example, increasing the density setting from 0 to 1, 2, or 3 will increase the IR exposure. Decreasing the density setting from 0 to -1, -2, or -3 will decrease the IR exposure.
Major density controls operate ________ exposure changes, whereas minor controls operate _____ adjustments
large, fine
T or F: The density control setting should not normally be used to compensate for patient part thickness or kVp changes, as the AEC is designed to calculate this compensation automatically
True
When can the density control setting be adjusted?
When increases and decreases in the overall IR exposure are needed, such as when the patient and ionization chambers are properly positioned but the image still needs to be repeated due to under or overexposure
Common detector configurations
• Some newer AEC systems have five ionization chambers, but AEC ionization chambers are usually in the three-chamber configuration, with the center cell at the center of the IR, and the right and left cells positioned slightly higher
• It is important that the exact size, shape, and position of the ionization chambers be known to the radiographer. These usually rectangular boxes are marked on the wall bucky and can be seen on an image taken with low kVp
• Some manufacturers provide plastic inserts for collimators that project an image of the chamber location, size, and shape with the positioning light beam
Appropriate detector selection
• The technologist can select 1, 2, or all 3 ionization chambers for an exposure, although these are usually preprogrammed for each body part to be imaged when using APR (anatomically programmed radiography). These preprogrammed options can be changed by the technologist if needed
• AEC provides diagnostic quality exposures only for structures positioned directly over the selected ionization chambers. Positioning exactly the right amount of tissue over the correct combination of ionization chambers is important to ensure proper exposure for the part and to the IR.
AEC- Alignment & Positioning Considerations: Lateral Spine
AEC- Alignment & Positioning Considerations: PA chest