12. Automatic Exposure Control Flashcards
what is the purpose of AECs
achieve consistent exposures between examinations by controlling the amount of radiation reaching the IR
what do AECs do to the patient dose and repeat exposures
reduces both
what are the 3 hardware requirements of AEC
minimise any interferences with radiation on the IR
convert incident radiation to an electric signal
use the signal to control timing
what are 3 forms of radiation detection devices and what types of detection are they used for
gas filled detectors
scintillators = indirect detection
solid state detectors = direct detections
what is indirect radiation detection in terms of what the radiation turns into
radiation hits material and converts it to light and then electrical signals
what do scintillations do
3 steps
crystal converts x-ray to visible light
photocathode converts photons to photoelectrons
signal amplified by photomultiplier tube
what is the intensity of the scintillators
Pretty low intensity in terms of number of photons produced
what does the PMT do in the scintillator
Magnifies signal
how does the photocathode in the scintillator produce a current
in terms of photon hitting the scintillating material
Photon hits scintillating material and light photon hits photocathode converter into an e- and electrodes accelerates e- with positive and negative charge so e- builds up energy and imparts energy on diode and produce more e-
results in cascade effect to produce a current you can detect
what levels of radiation are scintillators used to accurately measure
Useful for accurate measurements of low level radiation
what does the x-ray photon do when it interacts with atoms or molecules
what is produced
it potentially ionises them and produces an ion pair part of it is negatively charged and part is positively charged
how are Ion pairs produced attracted to voltage source
the cathode attracts __pairs and anode attracts the ___ pair
what does this produce overall
oppositely attracted
Cathode attract positive ion pairs and anode attract negative pair
Produce current in circuit as charges are moving
what are phototimers
uses light paddle and photomultiplier tubes
where are phototimers placed
what kind of device are they
after the IR
exit type device
what do exit type devices do
radiation and x ray photons and feeds signal back to dictate how much radiation is being detected
why are measurements from exit type devices such as the phototimers not as high as it could be
some energy is attenuated by IR itself
what are ionisation chambers and what do they do
series of gas-filled ion chamber that measures the radiation before the IR
what are ionisation chambers made of
made of thin alumnium
what kind of device are ionisation chambers
entrance type device
which primary exposure factor does the AEC control
what are the other 2 set by
exposure time
mAs and kV are set by the MIT
what effect does low kV have on the contrast and exposure time
higher contrast
longer exposure time
what 2 things does the exposure timer do
what is its effect on the patient dose and image quality
Turns whole thing on and off in terms of whether it produces x rays or not
also controls how long its on for
Making sure system can pick up what exposure is on the receptor and switch it off at the appropriate time to reduce dose to patient but have good image quality
where do info go to in terms of the exposure timer
from the IR
what are the 6 components of the AEC circuit
x-ray generator
x-ray tube
dose area product meter
amplifier
AEC selector
backup timer
what does the DAP meter do in the AEC circuit
used to compare diagnostic risk between radiographic procedures
what is the DAP meter measured with and is it independent of
ionisation chamber
independent of distance from the source
why is the DAP independent of the distance from the source
Due to the divergence of a beam emitted from a “point source”, the area irradiated (A) increases with the square of distance from the source (A ∝ d2), while radiation intensity (I) decreases according to the inverse square of distance (I ∝ 1/d2).
Consequently, the product of intensity and area, and therefore DAP, is independent of distance from the source.
the pre-amplifier do in the AEC circuit
signal from the ionisation chamber is small so amplification is required to generate higher signal input into the comparator circuit
what are the 2 voltages in the reference voltage and what is another component in this part of the circuit
reference and accumulating voltage
comaparator
what does the AEC selector do in the AEC circuit
Setting how many photons hit the receptor and tells system to turn off
As signal comes in from AC and amplified as exposure gets going its builds up amount of voltage produced and at a particular point want system to stop,
Voltage collected from AEC builds up as it stays on, desired exposure vs what we have and when they match it turns off
what are the 2 things can be prioritised by the AEC selector section of the AEC circuit
Prioritise whether its more about image quality or image dose, have to prioritise one over the other
what is the signal to noise ratio a measure of
measure of image quality
what does the backup timer do in the AEC circuit
prevents over exposure to the patient and protect the tube from exceeding the heat loading capacity by specifying a max duration
the back up timer must be low enough and high enough to do what
low enough to protect the patient
high enough to exceed the required exposure
what % range is appropriate for the back up timer in the AEC circuit
150-200%
what is the minimum response time in the AEC circuit
the minimum time it takes for the AEC system to detect and react to the incident rays
what 3 things is the minimum response time limited by
electronic response time
focal spot size
anode heating
what are the 4 types of practical considerations for AEC
chamber selection
chamber position
radio-opaque objects
improper beam restriction
what is chamber selection in AEC
chambers must be appropriate for the anatomy of interest
what is chamber position in AEC
patient must be positioned appropriately relative to the AEC chambers
need to be cautious of overlapping tissues
what are radio-opaque objects in terms of AEC and why they need to be considered
presence of metal implants and radio-opaque objects attenuates the x-ray beam and affects the radiation detected by the AEC
what is improper beam restriction in AEC
too much scatter reaching the detector results in early termination of the exposure
what are the 2 consequences of not picking the correct AEC chambers
poor image quality and increase patient dose
if your focus is on soft tissue where should the AEC chamber you choose be over
why
what about if the focus is on bone
over the soft tissue so you dont overexpose the patient
for bone you want AEC exposure through the bone otherwise you’ll underexpose the image as itll pass through the soft tissue and reach desired dose much more quickly result in decreased dose but bad quality
what happens if you have bad AEC chamber positioning
if off centre and chamber is over tissue of different densities may potentially be over or under exposed if the tissue is more/less attenuating
what would shorter exposure times be good for and what does it limit
Shorter exp time means good for dose
scatter means terminates exposure earlier than needed and image quality is compromised
what should you do in terms of AEC if you need repeat exposures
why
switch to manual no AEC for repeats to ensure more predictability with exposure factors and minimizes the risk of further repeats needed
