DI quiz 3 - AEC Flashcards
With AEC, you never select what ?
time - mAs (because AEC controls the time based on how long it takes to receive adequate radiation)
What is AEC?
automatic exposure control - automatically adjusts the amount of radiation used
What 2 things are important when using AEC?
anticipated values and actual values
Main purpose of AEC?
To ensure patient isn’t overexposed to radiation and image produced has the right level of brightness / detail (regardless of thickness) and provide consistency with exposures
Location of AEC with normal digital radiography?
BEFORE IR
What happens once AEC detectors sense that enough radiation has been received to create a good-quality image?
The exposure is terminated (send signal)
Pros of AEC
provides consistency (because x-ray tech isn’t picking a technique) and reduces overexposure risk to pt
In order for AEC to work correctly, it’s important to:
properly center / position patient to correct detectors and select correct detectors (alignment*)
Which detector do you always centre to?
center detector
Purpose of exposure adjustment switch? (density selector)
extend (lengthen) or shorten the exposure more than it normally would
With exposure adjustment, what occurs at the positive side? (+)
Will go past the cut off, it will make the exposure longer (extend)
With exposure adjustment, what occurs at the negative side? (-)
Won’t go all the way to the cut off, it will terminate the exposure sooner
Why are exposure adjustments only with AEC?
You don’t extend or shorten with fixed techniques (the technique is what it is with fixed technique)
What is “fixed mode”
algorithm doesn’t fix the image - image stays the same (similar to raw data - can be dark)
What is the purpose of algorithm?
to adjust image after exposure
When would we use exposure adjustment / density selectors? (when would you want to shorten/lengthen exposure)
high (larger pt) or low (smaller pt) scatter situations (ex. larger body part - large pt, large abdomen, larger pelvis)
Do IR and AEC communiate?
no
What doesn’t AEC know?
if you properly positioned/centered patient, the difference between good or bad (scatter) radiation
Larger field size sees more?
scatter = IR sees more radiation = shorter exposure time = lower mAs
Imaging a larger body part will lead to
longer exposure time, higher mAs
What is always an option with AEC?
turning it off and going to a fixed technique, use exposure adjustments (the +/-)
When are exposure adjustment / density selectors done - before or after exposure?
before exposure
Using small focal spot with AEC, what do you need to be cautious about?
you’ll be using a longer time to get the mAs (if you’re doing a exp that motion is an issue - can be an issue)
Limitations of AEC? Shouldn’t use AEC:
if anatomy doesn’t cover cells, with peripheral anatomy (anatomy near outside of body-clavicle), if pt has radiopaque material (hip replacement)
AEC won’t compensate for
bad centring / positioning
Why is using AEC with radiopaque material not good?
the metal material has higher atomic Z / more dense, AEC detector will see less radiation = longer exp and higher mAs
Improper centring can cause
underexposure (because AEC will call for a shorter exp time based on centring of anatomy), premature image/cut off, changes in mAs and DI
What indicates you may need to retake an image?
looking at the image (not the DI)
What is the most repeated?
chest x-ray
Be cautious with what gender for chest xray, and why?
female - because breast tissue is more dense, so will absorb more radiation
What does AEC control?
the exposure time (exposure will terminate once adequate radiation received)
What is an anticipated value with AEC?
What value/mAs you’re expecting to see
What is an actual value with AEC?
What the value/mAs actually was after exposure
Why is it important to look at mAs after exposure with AEC?
To know how much radiation pt received and to compare the anticipated/actual value
Why are the outlines not on the table?
because the table floats and IR moves with the upright bucky
What type of radiation does AEC measure?
remnant radiation
How much radiation needs to hit IR for a good image
1 mR
What is AEC detector inbetween?
grid and IR
With AEC, regardless of pt thickness, IR should
receive the same amount of radiation, might just take longer for thicker body (same rad needed for a good image for that body part)
Why does a thicker body part take longer?
because thicker body part absorbs more radiation = less radiation per time reaching IR = longer exposure time
When does AEC compensate for changes in radiation intensity?
If it occurs prior to detector, NOT after detector
What does AEC use for their detectors?
ionization chambers
Explain ionization chamber purpose in AEC
x-ray photons hit the ionization chamber, creating a charge, once a certain amount of radiation has reached, signal is sent, and exposure is terminated
What techniques do you select with AEC?
kvp and mA
Purpose of ionization chambers
to measure the radiation
What happens with a small pt / body part regarding AEC?
AEC will shut down (terminate) exposure sooner, shorter exposure time and lower mAs
What happens with a large pt / body part regarding AEC?
AEC will take longer to shut down (terminate), longer exposure time and higher mAs
What detectors do you select for spine
center
What detectors do you select for abdomen
all 3
What detectors do you select for chest
R / L lateral
What anatomy shouldn’t you use AEC with?
clavicle, lateral Y scapula, patella, mandible, sternum and any anatomy that won’t cover the entire detector (ex. finger)
Important factors for using AEC
collimation, anatomy knowledge, proper position/centring, technique understanding
When would you use: center detector
80% of the time - Shoulder, spine, hip, femur, skull work, single knee, lateral chest
When would you use: lateral detector
Standing knees, chest AP/PA
When would you use: all 3 detectors
Abdomen, pelvis area
IR gets a fixed amount of radiation regardless of the following
kVp, mA, SID, pt thickness
Define minimum response time
shortest possible exposure time (only with AEC)
Purpose of having a back up mAs (time) with AEC?
System failure, Technologist error
What does having a back up mAs with AEC prevent?
accidentally overexposing patient and prevents tube overload
Backup mAs (time) rule of thumb?
1.5 – 2x the expected manual exposure time / mAs
Pro of having a back up mAs option with AEC
it can be changed
Con of having a back up mAs option with AEC
it can terminate sooner / quicker, has to be selected first
What happens with image if not properly collimated with AEC?
longer exposure time because the detectors aren’t in the proper collimation
Explain falling load
starts the exposure with highest mA for selected kvp - mA drops during exposure (so, mA is always changing)
What do you select with falling load?
kvp
What does the falling load ensure? with mA
that the highest possible mA is used for the shortest exposure time allowed (maximizes mA)
Reasons why underexposure would occur with AEC
Back-up time less than needed exposure time, density left on the minus setting, Improper collimation (scatter), Incorrect detector cell selection, Detector cell not completely covered by area (tissue) of interest
Factors that are important to consider when using AEC for chest
Gender, body habitus, heart, diaphragm, centered up high enough, correct detectors, full inspiration
Issue with detectors being too low with AEC
will have a longer exposure time and higher mAs because of diaphragm and heart being in the way
Reasons why overexposure would occur with AEC
Needed exposure time less than MRT, Density left on plus setting (higher mAs), Incorrect detector cell selection, Radiopaque material, Electronic malfunction
Detector cell not completely covered by area (tissue) of interest can cause
lower mAs
Will AEC compensate for filtration?
yes, filters occur before detector
What does back up time set up?
the MAX exposure time (MAX mAs)
What do you select with AEC technique wise?
kvp, mA (can be adjusted)
List what you select for AEC, on the monitor?
grid, kvp, mA, IR type, detector
Do we know our mAs prior to our exposure with AEC?
no, because mAs is determined after the exposure with mAs
What can you do if you don’t know what your anticipated mAs will be for an exposure?
switch to a fixed technique to see what your mAs value should be after exposure
Why wouldn’t you use AEC for a finger?
because a finger doesn’t completely cover the AEC detector (large area around the finger - IR will see a LOT of radiation)
What exposure time would a 180 cm SID have?
longer - because less radiation reaching IR and less intensity
exposure time between 50 mA and 500 mA
50 mA would have longer exposure time
Will AEC compensate for thicker / thinner patient?
yes, may just take longer / shorter time
With AEC, regardless of SID or patient thickness, in the end IR should _________________
see the same amount of radiation, might just take longer or shorter time
With grids, going from 8:1-12:1 ratio, what happens?
12:1 absorbs more x-ray photons so longer exposure time until proper radiation received
What does improper detector selection do?
provides inaccurate amount of radiation for image, can cause longer exposure time
Detectors are designed for?
consistent exposures / intensity, fewer repeats
Once ion capacitor (ion chamber) is charged, what happens?
exposure terminates
look at mAs value when during AEC?
after exposure
Would AEC compensate for an extra grid left on the IR by mistake during an exposure?
no, because the extra grid on the IR comes AFTER the exposure
If you were to image a finger using AEC, what would happen?
detector would see a lot of radiation
Using the two lateral detectors, do they see the same radiation or different?
50/50 (same)
What would happen if you were off center with AEC and soft tissue was an issue?
increased radiation exposure to IR = shorter exposure time than needed
What would happen if you were off center with AEC detector for chest and you centered over the spine?
Spine absorbs more radiation so higher mAs and longer exposure time
How will overexposure affect mAs?
will be higher than anticipated value
How will underexposure affect mAs?
will be lower than anticipated value
“fixed” image is the
raw data
Why 3 detectors with abdomen?
helps average out abdomen since there’s a lot of organs, ST, air, gas
Issue with using AEC with barium?
detectors may be under barium (similar to metal)
Purpose of filters?
get rid of low E photons
Will AEC compensate for filtration?
yes, because it’s before the AEC detector
Will AEC compensate for new IR having a different DQE?
No, because that doesn’t occur before the IR
When anatomy is over less dense anatomy than needed, what happens to the image?
underexposure
When anatomy is over more dense anatomy than needed, what happens to the image?
overexposure
Purpose of falling load
to give us shorter exposure times
How do you shorten exposure with falling load?
go from 80% setting to 100% setting
If you’re concerned with motion and you’re using falling load with AEC, would you use 80% setting or 100% setting?
100% to get shorter time
what is the minimum response time
shortest exposure time possible with AEC
What causes changes in mAs (DI stays the same)
lateral decentering, increased SID, filtration, grid
What causes changes in mAs and DI
Improper centering, scatter, field size, wrong AEC detector, back up time, density selection
If you have to repeat an image after using AEC, what should you do?
use a fixed technique (UNLESS you know why - example: picked the wrong AEC detector)
