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SEM II - Exposure > AEC/Technique charts/Exposure Systems > Flashcards

AEC/Technique charts/Exposure Systems Flashcards

0
Q

AEC Function

A

A tool used to consistently reproduce images of uniform density.
Must be used correctly in order to be effective.

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1
Q

AEC

A

Automatic Exposure Control

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2
Q

2 Types of AEC

A

Photo timer type

Ionization chamber type

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3
Q

Ionization Chamber AEC

A

Most common type of AEC used today.
Uses two positively charged electrodes with a thin layer of air between.
Located in the Bucky between patient and IR.

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4
Q

Capacitor in AEC

A

An electrical component that temporarily stores a charge.

When capacitor filled, it discharges sending a signal that terminates the exposure.

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5
Q

3 Chambers

A

All connected to each other, left, right and center (.)

When more than one cell is selected the exposure is averaged between them.

Exposure terminated when total preset exposure value reached.

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6
Q

True photo timer

A

Exposure timed due to visible light emission.

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7
Q

Basic design of a photo timer AEC

A

Fluorescent screen
Lucite tubing
Photomultiplier tube (PMT)
An electric circuit to terminate exposure

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8
Q

Located behind the Bucky

A

The photo timer

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9
Q

Factors influencing AEC accuracy

A 
Part position
Detector selection
Density setting
Minimum response time
Back up timer
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10
Q

Positioning w/AEC

A

Imperative, must be perfect.

Anatomical part to be imaged must be right over the cells selected.

When properly positioned, the machine will produce the correct, consistent density while minimizing patient dose.

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11
Q

Detector selection

A

The correct cell(s) must be selected for the anatomical location of the part or interest.

3 cells total but most exposures require use of 1 or 2 cells only.

If using a 10x12 or smaller IR only the center should be selected.

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12
Q

Density setting

A

Setting that should normally be left alone and not used to compensate for patient thickness, the AEC device compensates for part thickness.

Most often used to increase or decrease density to visualize certain anatomy correctly.

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13
Q

Minimum response time

A

The shortest exposure possible for AEC function.

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14
Q

AEC processing time

A

Very fast but does take time - milliseconds.

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15
Q

Backup timer

A

A safety device.
Prevents the accidental overexposure of a patient due to timer switch failure or tech error. Reduces chance of destroying an X-ray tube due to overloading.

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16
Q

What is the maximum exposure allowed by law?

A

600 mAs

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17
Q

Recommended backup timer setting

A

150% of the expected mAs

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18
Q

Backup timer can be reached by

A 
Selecting the wrong Bucky on console
MA set too low can cause longer times
Too long of an SID
KVP set too low can cause longer times
Artifacts in image
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19
Q

Is AEC a substitute for technical skills?

A

NO!

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20
Q

Limitations of AEC

A

AEC does not compensate for all pathological conditions.

example is unilateral pleural effusion - results in adequate in the affected lung yet overexposure in the rest of the image.

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21
Q

Prosthetics with AEC

A

NO - results are overexposure of surrounding tissue and bone but adequate density of the metallic component.

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22
Q

AEC and collimation

A

Essential

The more tissue irradiated the more scatter produced.

  • scatter is X-ray energy, every X-ray reaching AEC is counted as part of the whole
  • May cause the exposure to terminate prematurely - underexposure
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23
Q

AEC and fast screen speed

A

Overexposure if a faster screen speed used than that normally used.

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24
Q

AEC and slow screen speed

A

Under exposure if slower than normal screen speed used.

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25
Q

APR

A

Automatically programmed radiography

  • exposures preset when specific anatomy selected
  • can be overridden by tech
26
Q

AEC Error with wrong Bucky

A

Make sure correct Bucky selected

- patient on table whereas wall Bucky selected

27
Q

AEC error with incorrect chamber selected

A

Results in the wrong density for desired anatomy

28
Q

AEC error with over collimation

A

Coming off selected chambers

29
Q

AEC error with undercollimation

A

Undercutting the anatomy of interest

30
Q

AEC with too low of an Ma station

A

The longer exposure time will result in unwanted motion

31
Q

AEC error with too high of an mA station

A

results in an overexposure reached prior to X-ray shutting off

32
Q

Function of technique charts

A

Keep image quality consistent
Minimize repeat exposures
Minimize patient exposure

33
Q

Are technique charts required by law?

A

Yes! They must be specific to the individual X-ray room.

34
Q

What are technique charts based upon?

A

Patient part/thickness as measured in centimeters w/calipers

35
Q

Where do you measure for patient/part thickness?

A

Along the path of the central ray.

36
Q

What should technique charts include?

A 
Body part
Part thickness
KVP
MA
S
37
Q

Fixed KVP technique charts

A

KVP remains constant with variation of thickness of patient anatomy.
- changes in mAs for variable patient thickness

38
Q

Advantages of fixed KVP technique charts

A

Constant KVP equals constant scale contrast
Decrease patient dose
Increase exposure latitude

39
Q

Disadvantage of fixed KVP technique chart

A

Produce more scatter

40
Q

Variable KVP Technique Charts

A

KVP is adjusted to compensate for variation of thickness of patient anatomy.

41
Q

Advantages with variable KVP technique charts

A

Higher contrast images (pretty to look at)

Increased perception of resolution

42
Q

Disadvantages with variable KVP technique charts

A

Changes in KVP result in changes on contrast

Lower exposure latitude

43
Q

Extrapolating techniques

A

Phantom testing

44
Q

Anthropomorphic phantom testing

A

Produce phantom images
Select optimal phantom image
Allows to extrapolate technique chart info

45
Q

Extrapolating technique with fixed KVP system

A

+/- 20% MAS per cm
- if 60 KVP and 5 mAs works for a 10cm ankle…
60 KVP and 6 MAS should work for a 11cm ankle

46
Q

Extrapolating technique with variable KVP system

A

+/- 2 KVP per cm
- if 60 KVP and 5 mAs works for a 10cm ankle…
62 KVP and 5 mAs should work for 11cm ankle

47
Q

Phantoms

A

Used to simulate human anatomy, positioning practice, technique experimentation, exploration of technique charts, info gained without radiating humans

Vary in size/shape/anatomy

48
Q

MAS Calculation Formula

A

mA x s = mAs

49
Q

Grid conversion formula

A

mAs1/mAs2= GCF1/GCF2

As grid ratio increases the amount of X-ray required increases

50
Q

No grid

A

1

51
Q

5:1 grid

A

2

52
Q

6:1 grid

A

3

53
Q

8:1 grid

A

4

54
Q

12:1 grid

A

5

55
Q

16:1 grid

A

6

56
Q

Density maintenance formula

A

MAS1/MAS2=D1sq/D2sq

As SID increases the amount of X-ray required increases

57
Q

15% Rule (15/50 Rule)

A

A 15% increase in KVP = a 50% reduction in MAS

A 15% decrease in KVP = doubling (2x) the MAS

58
Q

Film/screen speed conversion formula

A

MAS1/MAS2=RS2/RS1

As film screen increases the amount of X-ray required decreases

59
Q

Magnification factor formula

A

SID/SOD

60
Q

Factors impacting density

A

Primary - MAS

Secondary - almost all other factors (anything effecting energy to IR)

61
Q

Factors impacting contract

A

Primary - KVP

Secondary - grids, OID (anything that affects scatter reaching IR)

62
Q

Factors impacting detail

A

SID, OID, focal spot size

63
Q

Factors impacting distortion

A

SID, OID, CR angle, motion, film/screen speed

SEM II - Exposure (6 decks)

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