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202 MEDIMAGE > 12. Automatic Exposure Control > Flashcards

12. Automatic Exposure Control Flashcards

1
Q

what is the purpose of AECs

A

achieve consistent exposures between examinations by controlling the amount of radiation reaching the IR

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2
Q

what do AECs do to the patient dose and repeat exposures

A

reduces both

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3
Q

what are the 3 hardware requirements of AEC

A

minimise any interferences with radiation on the IR

convert incident radiation to an electric signal

use the signal to control timing

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4
Q

what are 3 forms of radiation detection devices and what types of detection are they used for

A

gas filled detectors

scintillators = indirect detection

solid state detectors = direct detections

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5
Q

what is indirect radiation detection in terms of what the radiation turns into

A

radiation hits material and converts it to light and then electrical signals

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6
Q

what do scintillations do

3 steps

A

crystal converts x-ray to visible light

photocathode converts photons to photoelectrons

signal amplified by photomultiplier tube

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7
Q

what is the intensity of the scintillators

A

Pretty low intensity in terms of number of photons produced

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8
Q

what does the PMT do in the scintillator

A

Magnifies signal

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9
Q

how does the photocathode in the scintillator produce a current

in terms of photon hitting the scintillating material

A

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

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10
Q

what levels of radiation are scintillators used to accurately measure

A

Useful for accurate measurements of low level radiation

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11
Q

what does the x-ray photon do when it interacts with atoms or molecules

what is produced

A

it potentially ionises them and produces an ion pair part of it is negatively charged and part is positively charged

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12
Q

how are Ion pairs produced attracted to voltage source

the cathode attracts __pairs and anode attracts the ___ pair

what does this produce overall

A

oppositely attracted

Cathode attract positive ion pairs and anode attract negative pair

Produce current in circuit as charges are moving

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13
Q

what are phototimers

A

uses light paddle and photomultiplier tubes

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14
Q

where are phototimers placed

what kind of device are they

A

after the IR

exit type device

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15
Q

what do exit type devices do

A

radiation and x ray photons and feeds signal back to dictate how much radiation is being detected

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16
Q

why are measurements from exit type devices such as the phototimers not as high as it could be

A

some energy is attenuated by IR itself

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17
Q

what are ionisation chambers and what do they do

A

series of gas-filled ion chamber that measures the radiation before the IR

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18
Q

what are ionisation chambers made of

A

made of thin alumnium

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19
Q

what kind of device are ionisation chambers

A

entrance type device

20
Q

which primary exposure factor does the AEC control

what are the other 2 set by

A

exposure time

mAs and kV are set by the MIT

21
Q

what effect does low kV have on the contrast and exposure time

A

higher contrast

longer exposure time

22
Q

what 2 things does the exposure timer do

what is its effect on the patient dose and image quality

A

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

23
Q

where do info go to in terms of the exposure timer

A

from the IR

24
Q

what are the 6 components of the AEC circuit

A

x-ray generator

x-ray tube

dose area product meter

amplifier

AEC selector

backup timer

25
Q

what does the DAP meter do in the AEC circuit

A

used to compare diagnostic risk between radiographic procedures

26
Q

what is the DAP meter measured with and is it independent of

A

ionisation chamber

independent of distance from the source

27
Q

why is the DAP independent of the distance from the source

A

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.

28
Q

the pre-amplifier do in the AEC circuit

A

signal from the ionisation chamber is small so amplification is required to generate higher signal input into the comparator circuit

29
Q

what are the 2 voltages in the reference voltage and what is another component in this part of the circuit

A

reference and accumulating voltage

comaparator

30
Q

what does the AEC selector do in the AEC circuit

A

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

31
Q

what are the 2 things can be prioritised by the AEC selector section of the AEC circuit

A

Prioritise whether its more about image quality or image dose, have to prioritise one over the other

32
Q

what is the signal to noise ratio a measure of

A

measure of image quality

33
Q

what does the backup timer do in the AEC circuit

A

prevents over exposure to the patient and protect the tube from exceeding the heat loading capacity by specifying a max duration

34
Q

the back up timer must be low enough and high enough to do what

A

low enough to protect the patient

high enough to exceed the required exposure

35
Q

what % range is appropriate for the back up timer in the AEC circuit

A

150-200%

36
Q

what is the minimum response time in the AEC circuit

A

the minimum time it takes for the AEC system to detect and react to the incident rays

37
Q

what 3 things is the minimum response time limited by

A

electronic response time

focal spot size

anode heating

38
Q

what are the 4 types of practical considerations for AEC

A

chamber selection

chamber position

radio-opaque objects

improper beam restriction

39
Q

what is chamber selection in AEC

A

chambers must be appropriate for the anatomy of interest

40
Q

what is chamber position in AEC

A

patient must be positioned appropriately relative to the AEC chambers

need to be cautious of overlapping tissues

41
Q

what are radio-opaque objects in terms of AEC and why they need to be considered

A

presence of metal implants and radio-opaque objects attenuates the x-ray beam and affects the radiation detected by the AEC

42
Q

what is improper beam restriction in AEC

A

too much scatter reaching the detector results in early termination of the exposure

43
Q

what are the 2 consequences of not picking the correct AEC chambers

A

poor image quality and increase patient dose

44
Q

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

A

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

45
Q

what happens if you have bad AEC chamber positioning

A

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

46
Q

what would shorter exposure times be good for and what does it limit

A

Shorter exp time means good for dose

scatter means terminates exposure earlier than needed and image quality is compromised

47
Q

what should you do in terms of AEC if you need repeat exposures

why

A

switch to manual no AEC for repeats to ensure more predictability with exposure factors and minimizes the risk of further repeats needed

202 MEDIMAGE (18 decks)

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