AEC/Technique charts/Exposure Systems

Question 0

AEC Function

Answer 0

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

Question 1

AEC

Answer 1

Automatic Exposure Control

Question 2

2 Types of AEC

Answer 2

Photo timer type

Ionization chamber type

Question 3

Ionization Chamber AEC

Answer 3

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.

Question 4

Capacitor in AEC

Answer 4

An electrical component that temporarily stores a charge.

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

Question 5

3 Chambers

Answer 5

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.

Question 6

True photo timer

Answer 6

Exposure timed due to visible light emission.

Question 7

Basic design of a photo timer AEC

Answer 7

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

Question 8

Located behind the Bucky

Answer 8

The photo timer

Question 9

Factors influencing AEC accuracy

Answer 9

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

Question 10

Positioning w/AEC

Answer 10

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.

Question 11

Detector selection

Answer 11

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.

Question 12

Density setting

Answer 12

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.

Question 13

Minimum response time

Answer 13

The shortest exposure possible for AEC function.

Question 14

AEC processing time

Answer 14

Very fast but does take time - milliseconds.

Question 15

Backup timer

Answer 15

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.

Question 16

What is the maximum exposure allowed by law?

Answer 16

600 mAs

Question 17

Recommended backup timer setting

Answer 17

150% of the expected mAs

Question 18

Backup timer can be reached by

Answer 18

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

Question 19

Is AEC a substitute for technical skills?

Answer 19

NO!

Question 20

Limitations of AEC

Answer 20

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.

Question 21

Prosthetics with AEC

Answer 21

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

Question 22

AEC and collimation

Answer 22

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

Question 23

AEC and fast screen speed

Answer 23

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

Question 24

AEC and slow screen speed

Answer 24

Under exposure if slower than normal screen speed used.

Question 25

APR

Answer 25

Automatically programmed radiography - exposures preset when specific anatomy selected - can be overridden by tech

Question 26

AEC Error with wrong Bucky

Answer 26

Make sure correct Bucky selected | - patient on table whereas wall Bucky selected

Question 27

AEC error with incorrect chamber selected

Answer 27

Results in the wrong density for desired anatomy

Question 28

AEC error with over collimation

Answer 28

Coming off selected chambers

Question 29

AEC error with undercollimation

Answer 29

Undercutting the anatomy of interest

Question 30

AEC with too low of an Ma station

Answer 30

The longer exposure time will result in unwanted motion

Question 31

AEC error with too high of an mA station

Answer 31

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

Question 32

Function of technique charts

Answer 32

Keep image quality consistent Minimize repeat exposures Minimize patient exposure

Question 33

Are technique charts required by law?

Answer 33

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

Question 34

What are technique charts based upon?

Answer 34

Patient part/thickness as measured in centimeters w/calipers

Question 35

Where do you measure for patient/part thickness?

Answer 35

Along the path of the central ray.

Question 36

What should technique charts include?

Answer 36

``` Body part Part thickness KVP MA S ```

Question 37

Fixed KVP technique charts

Answer 37

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

Question 38

Advantages of fixed KVP technique charts

Answer 38

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

Question 39

Disadvantage of fixed KVP technique chart

Answer 39

Produce more scatter

Question 40

Variable KVP Technique Charts

Answer 40

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

Question 41

Advantages with variable KVP technique charts

Answer 41

Higher contrast images (pretty to look at) | Increased perception of resolution

Question 42

Disadvantages with variable KVP technique charts

Answer 42

Changes in KVP result in changes on contrast | Lower exposure latitude

Question 43

Extrapolating techniques

Answer 43

Phantom testing

Question 44

Anthropomorphic phantom testing

Answer 44

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

Question 45

Extrapolating technique with fixed KVP system

Answer 45

+/- 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

Question 46

Extrapolating technique with variable KVP system

Answer 46

+/- 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

Question 47

Phantoms

Answer 47

Used to simulate human anatomy, positioning practice, technique experimentation, exploration of technique charts, info gained without radiating humans Vary in size/shape/anatomy

Question 48

MAS Calculation Formula

Answer 48

mA x s = mAs

Question 49

Grid conversion formula

Answer 49

mAs1/mAs2= GCF1/GCF2 As grid ratio increases the amount of X-ray required increases

Question 50

No grid

Answer 50

1

Question 51

5:1 grid

Answer 51

2

Question 52

6:1 grid

Answer 52

3

Question 53

8:1 grid

Answer 53

4

Question 54

12:1 grid

Answer 54

5

Question 55

16:1 grid

Answer 55

6

Question 56

Density maintenance formula

Answer 56

MAS1/MAS2=D1sq/D2sq As SID increases the amount of X-ray required increases

Question 57

15% Rule (15/50 Rule)

Answer 57

A 15% increase in KVP = a 50% reduction in MAS A 15% decrease in KVP = doubling (2x) the MAS

Question 58

Film/screen speed conversion formula

Answer 58

MAS1/MAS2=RS2/RS1 As film screen increases the amount of X-ray required decreases

Question 59

Magnification factor formula

Answer 59

SID/SOD

Question 60

Factors impacting density

Answer 60

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

Question 61

Factors impacting contract

Answer 61

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

Question 62

Factors impacting detail

Answer 62

SID, OID, focal spot size

Question 63

Factors impacting distortion

Answer 63

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