AEC - Ch. 33 Flashcards
What does AEC stand for?
Automatic Exposure Control
What other factors does the RT still have control of when AEC is activated?
KVP Focal Spot Size mA Time
What would increasing mA do to time?
Increasing mA would decrease time
What is the function of AEC?
It measures a preset amount of radiation and then breaks the timer circuit
The single function of an AEC is to eliminate the need for the radiographer to do what?
To eliminate the need for the radiographer to set an exposure time
Who determines the preset amount of radiation that will break the circuit timer?
It is determined by the techs that set up the equipment during install
What action stops the exposure?
When the AEC system breaks the timer circuit.
What is the older term for AEC? The newer term?
Older is photodiode Newer is phototiming (still a relatively old term) AEC is a general term
Digital IRs are also…
Solid state detectors
The AEC panel in a solid state detector is very..
thin and effecient
What are some key qualities of the solid state detector’s AEC panel?
It absorbs less radiation and it produces less scatter making it very effecient
Is this the newer most common AEC system or older?
Newer
What is the order of operations with the older AEC system?
- Tube
- Primary beam
- IR
- Light paddles
What do light paddles do?
receive radiation that would exit the IR & convert it to light
After light paddles convert radiation to light, what happens to the light?
the light gets converted to an electronic signal which when it reaches its preset amount would end the exposure
New or old AEC?
Old
What does the term phototiming actually refer to?
the use of ionization chambers
What do ionization chambers do?
measure the exposure to the receptor
What is the most critical element in using AEC?
the exact positioning of the chambers
Be careful because different brands offer different positions of chambers
The art of using AEC is the art of..
positioning
The most common configuration of ionization chambers is..
one in the middle and 2 chambers on either side and slightly higher
Where does the center chamber lie in a configuration that is most common?
below the duodenum and transverse colon for most abdominal examinations
Why is the placement of the center cell (below the duodenum and transverse colon) helpful in abdominal examinations?
Because it eliminates problems with gastric and bowel gas being placed over the chamber and causing the AEC to terminate the exposure early
How is the configuration of the outer AEC cells helpful in a CXR?
They are placed away from the mediastinum and within the lobes of the lungs so the exposure doesn’t burn out the lungs trying to go through the mediastinum.
How many different combinations of AEC cell formats are available to the radiographer?
7
When more than one cell is activated, what will have the most incluence on the overall exposure?
the cell receiving the most radiation will contribute the greatest electrical signal
Scenario: If 3 cells are activated and an abdomen is positioned so the barium is over one cell and normal tissue over the other 2 cells, what is the result of the image? Why?
It will be slightly overexposed because the operational amplifier was dividing the incoming voltage by all 3 cells but only 2 cells were contributing to the signal. If the cell under the barium was the only one selected, it would be greatly overexposed.
What are typical density control labels?
-3, -2, -1, 0, 1, 2, 3
or
1/4, 1/2, N, 1 1/4, 1 1/2
normal being 0 or N
What are density controls used for?
the adjustment of the amount of radiation necessary to send the exposure termination signal. They regulate the image receptor exposure
Density controls should not be used…
to compensate for patient part thickness or kVp changes
What causes an increased repeat rate when using AEC?
Poor positioning skills
What is the protocol for the newer more common AEC systems?
- tube
- primary beam
- thru table
- thru chambers
- strike IR
When a timer circuit is used in AEC, what does the ionization chamber do?
sends a signal to the xray circuit timer that is hooked to the tube when it gets the signal it stops the exposure
ionization is proportional to…
the amount of xrays needed to reach the preset
In the ionization chamber, what is measuring the preset amount?
an electrode
the ionization chamber is..
hollow
Study this
This is with the timer circuit
After the xrays pass thru the pt and chambers,what happens?
the xrays will ionize the air molecules in the chambers inside the chambers
An electric charge is created & the electrode senses the preset is reached, the wire sends the signal & stops the exposure
Between what and what are the AEC cells located?
Between the grid and IR
How does the machine calculate the amount of exposure when there are more than one cell chosen?
the charge is totaled and averaged. When the amount is met, the system will stop the exposure
What are the 3 most common combinations of cells used?
- the 2 outer
- the one center
- All 3 cells
How does higher kVp = decreased dose?
with a higher kVp more xrays exit the pt (due to higher energy) which causes the preset to be reached faster which in turn causes the exposure to be terminated faster which equals a decrease in pt dose
What is the most common cause of suboptimal images with AEC?
positioning and centering errors
What could happen if collimation is too tight and the cells are not completely covered by the exposure field?
overexposure, increased pt dose, decreased image contrast, decreased image quality
This is because the field size is reduced but the preset is for a larger field so the smaller field gets too much radiation and overexposes.
What happens if the collimation is too wide with AEC?
Underexposure, undercutting scatter
This is because the exposure field is bigger than expected and the preset amount is reached faster causing the exposure to be cut off too soon
Pathology could cause what with AEC?
under or over exposure depending on the pathology
What could pleural effusion do to an exposure when using AEC?
It could cause overexposure since the tissue would be dense and take longer to reach the IR to reach the preset amount.
What could a pneumothorax do to an exposure using AEC?
The exposure could be underexposed since there isn’t as much tissue as normal and radiation reaches the IR too fast
Name the parts
a: Electrode
b: Aluminum
c: insulator
d: graphite
e: air
f: graphite
g: insulator
h: Aluminum
i: damping material
When would you use all 3 cells?
for a SBS or BE
When would you use the 2 outer cells?
CXR
What are some compensation issues for AEC?
Pt size (esp pedi)
Exp field size (errors related to calibration to exp field size)
IR variations
What could happen if there is an IR variation with AEC?
could cause under or over exposure
Density controls can have what % of change in the amount of present radiation?
25% either more or less depending on which way you go
What is the proper use of the density controls?
- Correct positioning of the part over the cells
- uncontrolable factors cause increase/decrease densities w/in patient
What could be done if we know ahead of time that a pt has additive or destructive pathology?
- use density controls
or
- set a manual technique
If density controls need to be set on a regular basis, what could this indicate?
That the equipment needs to be serviced
What does minimum response time mean?
the time needed for AEC to respond and terminate radiation exposure
What is the minimum response time for modern equipment?
1 millisecond (0.001 sec)
What is BUT
Back Up Timer
What does a back up timer do?
Safety feature in case AEC fails to terminate or if the wrong detector is used
What if the back up timer is set too short?
it can terminate the exposure prematurely.
Per the law, what is the preset time for the back up timer?
150% of manual exposure (600 mAs for above 50 kVp)
