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Principles of Radiographic Exposure > Automatic Exposure Control/Phototiming > Flashcards

Automatic Exposure Control/Phototiming Flashcards

1
Q

What are AECs programmed to do?

A

Terminate radiographic exposure time

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2
Q

The AEC does NOT control any other exposure factors except…

A

The length of the exposure time

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3
Q

What is still under the control of the radiographer using an AEC?

A

mA and kVp

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4
Q

The AEC function is to:

_______ a ______ _______ of radiation and ____ the timer circuit when a radiation _____ of ______ quality is reached

A

The AEC function is to:

MEASURE a PRESET QUANTITY of radiation and BREAK the timer circuit when a radiation DOSE of SUFFICIENT quanity is reached

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5
Q

What are the 2 types of AECs?

A
  1. Photomultiplier
  2. Ionization Chamber
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6
Q

Which AEC is more popular?

A

Ionization Chamber

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7
Q

What does the photomultiplier consist of?

A

Consists of a fluorescent screen, a photomultiplier tube, and a complex electrical circuit

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8
Q

What is the photomultiplier connected to?

A

Connected to a timer designed to terminate the exposure time

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9
Q

Where is the photomultiplier placed?

A

Under the bucky tray

  • a large device
  • not radiolucent
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10
Q

With phototiming, what happens when the x-ray strikes the fluorescent screen?

A

It will glow

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11
Q

The glowing light is _____ by the ________ and is converted to __________.

A

The glowing light is DETECTED by the PHOTOMULTIPLIER and is converted to ELECTRICAL ENERGY.

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12
Q

In phototiming, the _____ passes through to the ______ to a ______, which will ______ the _____ when the ___________________ is reached.

A

In phototiming, the ENERGY passes through to the CIRCUIT to a CAPACITOR, which will TERMINATE the EXPOSURE when the PRE-DETERMINED ELECTRICAL CHARGE is reached.

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13
Q

What is the disadvantage of the phototiming system?

A

The cassettes cannot have lead foil so more backscatter radiation is produced which leads to fogging of the image

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14
Q

The ionization chamber

  • must be…
  • located…
  • plate is…
  • will…
A

The ionization chamber

  • must be RADIOLUCENT
  • located between patient and IR
  • plate is only 5mm in thickness
  • will reach a pre-determined charge and terminate exposure time
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15
Q
A
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16
Q

What is the basic design of the ionization chamber?

Ionization chambers (_____ or _____) and a complex _____________ connected to the ______ to ______ the exposure.

A

Ionization chambers (DETECTORS or CELLS) and a complex ELECTRICAL CIRCUIT connected to the TIMER to TERMINATE the exposure.

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17
Q

In ionization chambers, each chamber has ____________ separated by a _________________. These are encased within a ____________ (credit card size)

A

In ionization chambers, each chamber has 2 ELECTRODES separated by a THIN LAYER OF AIR. These are encased within a THIN ALUMINUM SHELL (credit card size)

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18
Q

With the ionization chamber, when x-rays ______ the ______ shell and their _________, ______ are released and _______ occurs.

A

With the ionization chamber, when x-rays PENETRATE the ALUMINUM shell and their AIR MOLECULES, ELECTRONS are released and IONIZATION occurs.

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19
Q

What is ionization?

A

Adding or removing electrons from their orbital shells

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20
Q

With ionization chambers, the ____ are attracted to ______ charged ________ and are carried through an _________ to a ____________.

A

With ionization chambers, the ELECTRONS are attracted to POSITIVELY charged ELECTRODES and are carried through an ELECTRICAL CIRCUIT to a CHARGE CAPACITOR.

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21
Q

A capacitor is also called a…

22
Q

What is a capacitor/condenser?

A

A device used to store specific quantity of electricity

23
Q

With an ionization chamber, once the amount of ________ is _______, any further influx of _______ will cause the _______ to _______.

This action will ___________.

A

With an ionization chamber, once the amount of ELECTRICITY is STORED, any further influx of ELECTRICITY will cause the CAPACITOR to DISCHARGE.

This action will TERMINATE THE EXPOSURE.

24
Q

What 6 things must radiographers select?

A
  1. Optimum kVp (least amount of radiation to penetrate patient)
  2. mA stations
  3. Density level (normally set as normal)
  4. Photocell, 3 (area of interest is what you select)
  5. Back-up timer (in case of malfunction)
  6. Table or vertical bucky
25
Q

What are your technique selections?

A

kVp and mA stations

26
Q

You choose the kVp optimal for…

A

kVp optimal for body part

27
Q

With mA stations, what’s the range of mA for a medium station and what body parts is it used for?

A

Medium station = 300-400 mA

Used on= body cavity and head

28
Q

What are lower mA stations used for?

A

Extremities

29
Q

What kind of mA station (high, medium, low) do you use for breathing techniques?

30
Q

What kind of mA station (high, medium, low) do you use for the shortest time?

A

Highest mA

31
Q

What are the density controls and is there an increase or decrease in relative exposure?

A
  • 0= normal density
  • -1 = 25% decrease in relative exposure
  • -2 = 50% decrease in relative exposure
  • +1 = 25% increase in relative exposure
  • +2 = 50% increase in relative exposure
32
Q

Why should density controls be changed?

A

Pathology

ex) pleural effusion, emphysema

33
Q

Density controls should not be used to…

A

Compensate for changes in part thickness

(the AEC does this automatically)

34
Q

How do you determine the configuration of the cell(s)?

A

Center part of interest over corresponding cell(s)

35
Q

What are the 2 time requirements for the AEC or AED?

A
  1. Minimal reaction timer (MRT) or response time
  2. Back-up timer
36
Q

What does the MRT/response time represent?

A

Represents the time required for operation of the AEC system to work

37
Q

What are the MRT of photomultipliers and ion chambers?

A
  • Photomultipliers= MRT of 0.05 seconds
  • Ion Chambers= MRT of 0.001 (quicker response time)
38
Q

Radiographers ______ adjust the setting of the MRT

A

Radiographers CANNOT adjust the setting of the MRT

39
Q

What time setting can the radiographers adjust?

A

Back-up timer

40
Q

What is the back-up timer?

A

A safety time if the AEC fails to terminate (stop) the exposure time

41
Q

The back-up timer aids in the safety of what 2 things?

A
  • Radiation safety for patient
  • Equipment safety
42
Q

Equipment often allows technologists to set a _____

43
Q

What should backup times not exceed?

A

Backup times should NOT exceed 150% (1.5x) of anticipated manual technique

44
Q

U.S Public Law 90-602

  • The back-up timer cannot exceed….. OR…
A
  • Back-up timer cannot exceed: 600 mAs for exposures using above 50 kVp

or

  • 2000 mAs for below 50kVp (normally for mammography)
45
Q

Poor positioning skills result in…

A

Increased repeat rate when using AEC

46
Q

Changes in _______ and ________ may affect outcome of image

A

Changes in TISSUE DENSITY and CONTRAST may affect outcome of image

47
Q

What are 4 reasons to use the AEC?

A
  • Allows for more patient time
  • Allows radiographers more positioning time
  • Allows for more consistent quality of radiographic images
  • Allows for radiation safety and protection
48
Q

What must be accurate when using the AEC?

A

Positioning

49
Q

What are the typical causes of dark AEC exposures?

  • Wrong…
  • Anatomy…
  • Density…
  • Electronic…
  • Incorrect…
A
  • Wrong bucky activated
  • Anatomy too small (requiring shorter time exposure than MRT)
  • Density control left on a + setting
  • Electric malfunction
  • Incorrect detector configuration
50
Q

Continued… What are the typical causes of dark AEC exposures?

  • Collimation…
  • Presence…
  • Presence…
  • Thicker…
A
  • Collimate field size so small that scatter is significantly less than field of use when AEC was calibrated or activated cell is clipped
  • Presence of radiopaque artifacts or appliances
  • Presence of external radiopaque artifacts
  • Thicker body part centered over cell
51
Q

What is a common cause of dark AEC exposure?

A

PA chest

  • centering the film and cells too low (below diaphragm)
52
Q

What are the typical causes of light AEC exposures?

  • Wrong…
  • Back-up…
  • Released…
  • Density…
  • Inadequate…
  • Incorrect…
    *
A
  • Wrong X-ray tube activated
  • Back-up time too short
  • Released exposure switch before AEC cut off exposure
  • Density control left on -setting
  • Inadequate collimation (excessive scatter)
  • Incorrect detector

Principles of Radiographic Exposure (18 decks)

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