AEC Flashcards
History of AEC
1942
Russell Morgan
Definition from Bushong
A device that measures the quantity of radiation reaching the IR and automatically terminates the exposure when the IR has received the required radiation intensity.
Sterling defines AEC
Radiographic density control device that terminates the exposure when a predetermined amount of radiation has been reached. The AEC is an automatic electronic timing device that replaces the conventional exposure timer.
What is the legacy term for AEC
Photo timer
What do the definitions refer to this exposure control, to be associated with?
The density or quantity of radiation reaching the IR.
How are we controlling the quantity of radiation reaching the IR
By controlling our exposure time.
AEC is only as good as? And requires?
The skill of the operator and requires a special understanding on the part of the RT
2 purposes of AEC
- To control the quantity of radiation reaching the IR
2. To consistently achieve properly exposed images, regardless of patient’s size, pathology, etc
Advantages over Fixed Time Technique 3
- Reduces repeats by producing radiographs with proper exposure to the IR
- Consistent images on all patients
- Consistency on comparison images of the same patient
3 types of AEC
- photodiode Detector Assembly (less common)
- Ionization Chamber (more common)
- Solid State Radiation Detector
Photodiode detector assembly
Light sensor located behind the cassette (requires special IR)
How does a photodiode detector assembly work?
Radiation is transmitted through the patient and IR to the sensor screen (covered with a phosphor) causing it to fluoresce. The light is picked up by the detector and an electric current is generated in direct proportion to the xRay exposure. The current is fed to an electronic device that terminates the exposure when the proper level of radiation has been reached.
Disadvantage of photodiode detector assembly
Can be over sensitive in the 50-60 kVp range
How does the ionization chamber control the length of the exposure time
Uses radiation
Describe the location of the ionization chamber
Located in front of the Bucky
It’s radiolucent
How does the ionization chamber work
A gas is ionized by radiation passing through the chamber. Ionization within the chamber produces a charge that terminates the exposure when a preset level/ charge has been reached.
A disadvantage of AEC Ionization Chamber
Can be over sensitive in lower kVp ranges i.e. Mammo.
Solid state radiation detectors are used for
Exposure-timer checks
Minimum Response Time
The shortest exposure time that the AEC can produce or control. If the minimum response time is greater than the actual time needed for a particular part, the image will be over exposed. Our MA should be low enough so that the exposure time utilized will be longer than the minimum response time.
Back-up Time
The maximum permissible exposure time that can be achieved before the exposure circuit is terminated.
Why is back up time a safety feature
It limits the maximum exposure possible in case of operator error or equipment failure
What should the back up MAS be set at
1.5 times The mAs expected for the part being imaged
Regulations require that AEC’s have what
A 600 mAs safety override.
Technique considerations re: mA and kVp
The kVp should be at the appropriate level for the part being radiographed
Technique considerations re: centering
Centering is crucial, as is proper photocell selection.
Technique considerations re: photocells
Photocells can be used individually or in combination with each other. When more than one cell is selected, the electrical charges or averaged, and is the resultant density on the radiograph.
Density settings are sometimes called
Density override
Density settings are used for
Overriding the AEC for pathologies, or when and increase or decrease in density is required.
Technique considerations re: AEC calibrating
AEC must be calibrated for the IR speed in use.
With our 400 speed rare earth, we should consider keeping our mA….
Relatively low, so that we avoid minimum response time problems.
With photodiode AEC’s which use a fluorescent screen, what do we need to remember
Our screens should match the output phosphor. This is called spectral matching.
Technique considerations re: collimation
Proper collimation is required to prevent the premature termination of our exposure from scatter.
Tech nique charts are recommended and should include
mA kVp Backup time Sensor selection OD setting
4 common errors or problems with AEC’s
- Activating the wrong sensor.
- Activating the table Bucky instead of the vertical Bucky for an upright exam
- . Use of AEC with cross table work
- Improper use of lead over photocell
What happens if you activate the table Bucky instead of the chessboard for a chest exam
Generally, you will end up with an over-exposed image here, as the table was waiting for a signal that it never received, so the maximum time was used or mAs.
What problem could you have if you use AEC with crosstable work
Quanta reset will always go off. If the fixed time technique selected was appropriate, the image should be OK. The photocell just never received the signal it was waiting for.
Oblique ribs upper, use—kVp
Higher
Oblique ribs upper. Use — kVp
Lower
APR aka
Anatomically Programmed Radiology
Auto pilot
If your 0.03 out of range, what do u do?
1/2 the mAs
Reciprocity law
As mA decreases, time increases
You can decrease the mA to force the machine to use a longer time
