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Radiology Physics > AEC - Ch. 33 > Flashcards

AEC - Ch. 33 Flashcards

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1
Q

What does AEC stand for?

A

Automatic Exposure Control

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2
Q

What other factors does the RT still have control of when AEC is activated?

A

KVP Focal Spot Size mA Time

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3
Q

What would increasing mA do to time?

A

Increasing mA would decrease time

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4
Q

What is the function of AEC?

A

It measures a preset amount of radiation and then breaks the timer circuit

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5
Q

The single function of an AEC is to eliminate the need for the radiographer to do what?

A

To eliminate the need for the radiographer to set an exposure time

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6
Q

Who determines the preset amount of radiation that will break the circuit timer?

A

It is determined by the techs that set up the equipment during install

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7
Q

What action stops the exposure?

A

When the AEC system breaks the timer circuit.

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8
Q

What is the older term for AEC? The newer term?

A

Older is photodiode Newer is phototiming (still a relatively old term) AEC is a general term

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9
Q

Digital IRs are also…

A

Solid state detectors

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10
Q

The AEC panel in a solid state detector is very..

A

thin and effecient

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11
Q

What are some key qualities of the solid state detector’s AEC panel?

A

It absorbs less radiation and it produces less scatter making it very effecient

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12
Q

Is this the newer most common AEC system or older?

A

Newer

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13
Q

What is the order of operations with the older AEC system?

A
  1. Tube
  2. Primary beam
  3. IR
  4. Light paddles
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14
Q

What do light paddles do?

A

receive radiation that would exit the IR & convert it to light

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15
Q

After light paddles convert radiation to light, what happens to the light?

A

the light gets converted to an electronic signal which when it reaches its preset amount would end the exposure

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16
Q

New or old AEC?

A

Old

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17
Q

What does the term phototiming actually refer to?

A

the use of ionization chambers

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18
Q

What do ionization chambers do?

A

measure the exposure to the receptor

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19
Q

What is the most critical element in using AEC?

A

the exact positioning of the chambers

Be careful because different brands offer different positions of chambers

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20
Q

The art of using AEC is the art of..

A

positioning

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21
Q

The most common configuration of ionization chambers is..

A

one in the middle and 2 chambers on either side and slightly higher

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22
Q

Where does the center chamber lie in a configuration that is most common?

A

below the duodenum and transverse colon for most abdominal examinations

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23
Q

Why is the placement of the center cell (below the duodenum and transverse colon) helpful in abdominal examinations?

A

Because it eliminates problems with gastric and bowel gas being placed over the chamber and causing the AEC to terminate the exposure early

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24
Q

How is the configuration of the outer AEC cells helpful in a CXR?

A

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.

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25
Q

How many different combinations of AEC cell formats are available to the radiographer?

A

7

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26
Q

When more than one cell is activated, what will have the most incluence on the overall exposure?

A

the cell receiving the most radiation will contribute the greatest electrical signal

27
Q

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?

A

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.

28
Q

What are typical density control labels?

A

-3, -2, -1, 0, 1, 2, 3

or

1/4, 1/2, N, 1 1/4, 1 1/2

normal being 0 or N

29
Q

What are density controls used for?

A

the adjustment of the amount of radiation necessary to send the exposure termination signal. They regulate the image receptor exposure

30
Q

Density controls should not be used…

A

to compensate for patient part thickness or kVp changes

31
Q

What causes an increased repeat rate when using AEC?

A

Poor positioning skills

32
Q

What is the protocol for the newer more common AEC systems?

A
  1. tube
  2. primary beam
  3. thru table
  4. thru chambers
  5. strike IR
33
Q

When a timer circuit is used in AEC, what does the ionization chamber do?

A

sends a signal to the xray circuit timer that is hooked to the tube when it gets the signal it stops the exposure

34
Q

ionization is proportional to…

A

the amount of xrays needed to reach the preset

35
Q

In the ionization chamber, what is measuring the preset amount?

A

an electrode

36
Q

the ionization chamber is..

A

hollow

37
Q

Study this

A

This is with the timer circuit

38
Q

After the xrays pass thru the pt and chambers,what happens?

A

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

39
Q

Between what and what are the AEC cells located?

A

Between the grid and IR

40
Q

How does the machine calculate the amount of exposure when there are more than one cell chosen?

A

the charge is totaled and averaged. When the amount is met, the system will stop the exposure

41
Q

What are the 3 most common combinations of cells used?

A
  1. the 2 outer
  2. the one center
  3. All 3 cells
42
Q

How does higher kVp = decreased dose?

A

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

43
Q

What is the most common cause of suboptimal images with AEC?

A

positioning and centering errors

44
Q

What could happen if collimation is too tight and the cells are not completely covered by the exposure field?

A

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.

45
Q

What happens if the collimation is too wide with AEC?

A

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

46
Q

Pathology could cause what with AEC?

A

under or over exposure depending on the pathology

47
Q

What could pleural effusion do to an exposure when using AEC?

A

It could cause overexposure since the tissue would be dense and take longer to reach the IR to reach the preset amount.

48
Q

What could a pneumothorax do to an exposure using AEC?

A

The exposure could be underexposed since there isn’t as much tissue as normal and radiation reaches the IR too fast

49
Q

Name the parts

A

a: Electrode
b: Aluminum
c: insulator
d: graphite
e: air
f: graphite
g: insulator
h: Aluminum
i: damping material

50
Q

When would you use all 3 cells?

A

for a SBS or BE

51
Q

When would you use the 2 outer cells?

A

CXR

52
Q

What are some compensation issues for AEC?

A

Pt size (esp pedi)

Exp field size (errors related to calibration to exp field size)

IR variations

53
Q

What could happen if there is an IR variation with AEC?

A

could cause under or over exposure

54
Q

Density controls can have what % of change in the amount of present radiation?

A

25% either more or less depending on which way you go

55
Q

What is the proper use of the density controls?

A
  1. Correct positioning of the part over the cells
  2. uncontrolable factors cause increase/decrease densities w/in patient
56
Q

What could be done if we know ahead of time that a pt has additive or destructive pathology?

A
  1. use density controls

or

  1. set a manual technique
57
Q

If density controls need to be set on a regular basis, what could this indicate?

A

That the equipment needs to be serviced

58
Q

What does minimum response time mean?

A

the time needed for AEC to respond and terminate radiation exposure

59
Q

What is the minimum response time for modern equipment?

A

1 millisecond (0.001 sec)

60
Q

What is BUT

A

Back Up Timer

61
Q

What does a back up timer do?

A

Safety feature in case AEC fails to terminate or if the wrong detector is used

62
Q

What if the back up timer is set too short?

A

it can terminate the exposure prematurely.

63
Q

Per the law, what is the preset time for the back up timer?

A

150% of manual exposure (600 mAs for above 50 kVp)

Radiology Physics (10 decks)

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