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
AEC- Alignment & Positioning Considerations: T-Spine
AEC- Alignment & Positioning Considerations: Abdomen
Radiation detector sensitivity
• Ionization chambers do NOT know if the patient’s anatomy is positioned over it or not. Ionization chambers are programed to terminate an exposure once the appropriate IR exposure is received for that body part. If no tissue, or partial tissue is positioned over the ionization chamber, it will terminate the exposure quickly, resulting in an underexposure for the part
• If, for example, a lead shield is partially positioned over an ionization chamber during an exposure, the exposure to the patient will be increased as the lead is attenuating some of the radiation that should be reaching the ionization chamber
Contrast agents
• Positive contrast agents (barium and iodine) will attenuate more radiation, so the AEC will respond by increasing the exposure time until the IR is appropriately exposed
• Negative contrast agents (air and CO2) will attenuate less radiation, so the AEC will respond by decreasing the exposure time until the IR is appropriately exposed
Patient size
AEC will adjust the exposure time depending upon patient size, tissue thickness, and tissue density. Larger patients will require a longer exposure time, with smaller patients requiring a shorter exposure time.
Pathology
• Destructive pathologies, or pathologies that destroy tissue, will contribute to a shorter AEC exposure time. Examples of destructive pathologies include osteoporosis and emphysema
• Additive pathologies, or pathologies that create more attenuation due to the increase in subject density, will contribute to a longer AEC exposure time. Examples of additive pathologies include pneumonia and ascites
Prosthetics/implants
-Medical devices in the body will increase attenuation of the beam, therefore requiring a longer exposure time
-Often with these devices, the density control setting may need to be adjusted upwards from the normal setting in cases where the first image was underexposed
Collimation
• Collimating too tightly should be avoided near ionization chambers. If the beam is collimated from an activated chamber, the chamber operates as if the tissue is extremely dense. This will result in a longer exposure time and overexposed image. Potentially, the backup time may be needed to terminate the exposure
• Wide open collimation can create extra scatter which can terminate the exposure prematurely, resulting in some areas of the image that may be underexposed
Image receptor variations
When an AEC imaging system is installed, it is calibrated. This calls for making exposures of a test object or phantom and adjusting the AEC for the range of x-ray intensities required for a specific IR. The service engineer usually takes care of this calibration
- If a different IR is used with a calibrated AEC, the exposure may not be correct for the new IR if the detective quantum efficiency (DQE) differs between the two IRs
Anatomically Programmed Radiography (APR)
• APR is a system of preprogrammed exposure technique settings that is organized by position and examination and set through the control panel of the radiography unit
• AEC works with APR so that the control console permits the choice of an anatomical region and the projection. This choice results in the computer entering in the suggested average technique and ionization chamber(s) activated
• The radiographer may override the suggested technique when patient condition, pathology, or other factors allow for this
AEC- Technique Charts
• AEC technique charts differ from non-AEC charts because ionization chamber(s) location will need to be stated, but time settings will need to be eliminated.
• At a minimum, the AEC chart should include:
• Backup time or mAs (if set by radiographer)
• SID
• kVp
• Filament/focal spot size
• mA
• Use of a grid and grid ratio
• Ionization chamber(s)
• Acceptable EI range
AEC- Potential Disadvantages & Problems
-Incorrect ionization chamber selection can affect patient and/or IR exposure
-The image demonstrates overexposed lungs because only the center cell was activated instead of the right and left cells needed for the PA chest
AEC- Potential Disadvantages & Problems
-Patient motion in the image to the right caused the patient’s cervical spine not to be positioned properly over the center cell, resulting in an underexposed cervical spine
-For upright cervical spine images where motion may be an issue, a manual kVp and mAs set by the technologist may be more appropriate
When an exposure time is needed that is less than or quicker than the minimum reaction time, the image may be ____________
overexposed
An exposure time less than the minimum reaction time may be needed when:
-Using high mA stations
-Imaging small body parts
-When using high-speed imaging systems
To create an exposure time greater or longer than the minimum reaction time, the technologist can:
-Use a lower mA station
-Reduce kVp
-Turn off the AEC and use a manual exposure
If the backup time terminates an exposure and the image is underexposed, the technologist must problem solve on the repeat image. If AEC is used on the repeat, to avoid triggering the backup time again, a higher kVp can be used to avoid terminating the exposure at _____ mAs when using 50 kVp or higher. If the backup time is triggered, it may also be necessary to use a manual ____ and ____ instead of using AEC on the repeat
-600
-kVp, mAs
