AEC Flashcards
What is AEC also known as?
Phototiming
When was AEC developed and why?
Developed in 1942 as a way to achieve more consistent exposures and reduce repeats
How do AEC systems work?
They convert the xrays detected into an electrical signal to control exposure
What do modern AEC systems use to control the exposure?
A gas ion chamber
What are the components of the gas ion chamber?
- double plate of aluminum with enclosed gas
- positively charged anode plate
- capacitor for charge accumulation
- thyraton for charge release
- electromagnet for exposure switch control
What is the process of the gas ion chamber?
- Xray photons enter the chamber and ionize the gas atoms
- The negatively charged ions are pulled towards the postively charged anode plate
- The ions then accumulate in the capacitor where they are counted against the threshold set
- Once the threshold is met the thyraton releases the charge and the electromagnet turns off the exposure
When utilizing AEC, what needs to be set on the console by the tech?
kV, mA and detector selection
If sufficient kVp is not set when using AEC what is the result?
- elongated exposure times
- potential image failure
- risk of over exposure
What is MRT?
Minimum response time and is how long the exposure circuit will take to initiate turn-off based on radiation received by the detectors
What general rule should be applied to MRT?
Shortest MRT time possible should be used to ensure the AEC function responds faster
What is the typical MRT range for new and old units?
0.002 for new units
0.02 for old units
What happens if your planned exposure time is less than the MRT?
Its best to not use AEC, otherwise the patient will be overexposed because the patient will be exposed longer than necessary
What patient group should be considered when setting MRT?
Pediatric patients as AEC should not be used for them since they are small and have a high risk of being overexposed
How does mA align with AEC?
mA should be increased when using AEC and compensated when the time element is adjusted
What are the safety features of AEC?
A backup time/mAs
Why does a backup timer need to be used for AEC?
If the AEC circuit fails to shut off after the threshold is met, the backup timer will turn the exposure off to limit exposure
What should the backup timer be set to?
2 times the expected exposure time or mAs for a particular projection
What are the downsides of pre-set manufacturer automatic backups?
They are often set too high and lead to overexposure
How high are the preset manufacturer backups set?
2-4 times the exposure
What are the most common AEC errors?
Choosing the incorrect bucky when imaging
What happens if the wrong bucky is selected?
With the wrong bucky selected the exposure will go on indefinitely until the backup timer kicks in and will overexpose
What is important to always do before imaging when using AEC?
A thorough review of the console to make sure all technical factors are set properly and correct detector elements are activated
What is the AEC intensity control?
An adjustment of the threshold of the capacitor so the exposure time can be increased or decreased
What happens when the threshold is increased?
It will signal the machine to turn off later and allow more radiation to pass
What happens if you lower the threshold?
The signal turns off sooner and allows for less radiation to pass
What are the built in console formats that affect intensity?
Small, medium and large settings
What is the minimum change in intensity that will make a noticeable difference in images?
35%
What is an example of a part that would benefit from the intensity control adjusements?
Hip radiographs since they require increased intensity
Why is positioning important when using AEC?
If the part is not positioned correctly over the detector it can result in the exposure from being terminated too quickly or not quick enough
If a spine is off centered by 3/4 inches what affect will it have on the image?
The detector cell will be outside the area of interest (bone) and the exposure will hit the soft tissue instead which is more penetrating and shutoff the AEC too soon, creating an underexposure
What are the AEC not suirable for?
AEC is not suitable for the following:
- small/narrow anatomy (pediatric extremeties)
- peripheral anatomy near body edges
- cases with large radiopaque prosthetics
What are the critical requirements for AEC?
- proprer alignment and positioning
- appropriate collimation
- careful detector cell selection
How are the detector cells configured?
Visible as 3 rectangular boxes on the bucky
Where are the detectors located?
In front of the grid
What are the detector systems made of?
Aluminum with wires that may be visible on images
What detector cells should be selected for CXR?
Top two cells so the lungs can be visulaized well
What detector cells should be selected for Spine, skull, hips, knees?
The center cell
What general rule should be followed when placing anatomy over the detector cells?
Thickest anatomy should be placed over the cell
What are common causes of overexposure?
- Wrong bucky activated
- Needed exposure time less than minimum time set as part of AEC
- Density control left on plus setting from previous patient
- Electronic malfunction of the AEC (backup buzzer sounds)
- Incorrect detector cell configuration, such that activated cells lie under tissues denser or thicker than the tissue of interest
- Presence of radiopaque artifacts or appliances within the anatomy
- Presence of external radiopaque artifacts such as lead sheets or sandbags over the sensor
What are common causes of underexposure?
- Backup time shorter than needed exposure time (esp on larger patients)
- Density control left on minus setting from previous patient
- Inadequate collimation (excessive scatter radiation reaching sensors)
- Incorrect detector cell configuration, sich that activated cells like under tissue less dense or thinner than the tissue of interest
- Detector cells not fully covered by the tissue of interest
Whare are the best practices when using AEC?
Always verify:
- Correct bucky selection
- Proper positioning
- Appropriate collimation
- Suitable detector cell configuration
