AEC/Technique charts/Exposure Systems Flashcards
AEC Function
A tool used to consistently reproduce images of uniform density.
Must be used correctly in order to be effective.
AEC
Automatic Exposure Control
2 Types of AEC
Photo timer type
Ionization chamber type
Ionization Chamber AEC
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.
Capacitor in AEC
An electrical component that temporarily stores a charge.
When capacitor filled, it discharges sending a signal that terminates the exposure.
3 Chambers
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.
True photo timer
Exposure timed due to visible light emission.
Basic design of a photo timer AEC
Fluorescent screen
Lucite tubing
Photomultiplier tube (PMT)
An electric circuit to terminate exposure
Located behind the Bucky
The photo timer
Factors influencing AEC accuracy
Part position Detector selection Density setting Minimum response time Back up timer
Positioning w/AEC
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.
Detector selection
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.
Density setting
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.
Minimum response time
The shortest exposure possible for AEC function.
AEC processing time
Very fast but does take time - milliseconds.
Backup timer
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.
What is the maximum exposure allowed by law?
600 mAs
Recommended backup timer setting
150% of the expected mAs
Backup timer can be reached by
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
Is AEC a substitute for technical skills?
NO!
Limitations of AEC
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.
Prosthetics with AEC
NO - results are overexposure of surrounding tissue and bone but adequate density of the metallic component.
AEC and collimation
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
AEC and fast screen speed
Overexposure if a faster screen speed used than that normally used.
AEC and slow screen speed
Under exposure if slower than normal screen speed used.
APR
Automatically programmed radiography
- exposures preset when specific anatomy selected
- can be overridden by tech
AEC Error with wrong Bucky
Make sure correct Bucky selected
- patient on table whereas wall Bucky selected
AEC error with incorrect chamber selected
Results in the wrong density for desired anatomy
AEC error with over collimation
Coming off selected chambers
AEC error with undercollimation
Undercutting the anatomy of interest
AEC with too low of an Ma station
The longer exposure time will result in unwanted motion
AEC error with too high of an mA station
results in an overexposure reached prior to X-ray shutting off
Function of technique charts
Keep image quality consistent
Minimize repeat exposures
Minimize patient exposure
Are technique charts required by law?
Yes! They must be specific to the individual X-ray room.
What are technique charts based upon?
Patient part/thickness as measured in centimeters w/calipers
Where do you measure for patient/part thickness?
Along the path of the central ray.
What should technique charts include?
Body part Part thickness KVP MA S
Fixed KVP technique charts
KVP remains constant with variation of thickness of patient anatomy.
- changes in mAs for variable patient thickness
Advantages of fixed KVP technique charts
Constant KVP equals constant scale contrast
Decrease patient dose
Increase exposure latitude
Disadvantage of fixed KVP technique chart
Produce more scatter
Variable KVP Technique Charts
KVP is adjusted to compensate for variation of thickness of patient anatomy.
Advantages with variable KVP technique charts
Higher contrast images (pretty to look at)
Increased perception of resolution
Disadvantages with variable KVP technique charts
Changes in KVP result in changes on contrast
Lower exposure latitude
Extrapolating techniques
Phantom testing
Anthropomorphic phantom testing
Produce phantom images
Select optimal phantom image
Allows to extrapolate technique chart info
Extrapolating technique with fixed KVP system
+/- 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
Extrapolating technique with variable KVP system
+/- 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
Phantoms
Used to simulate human anatomy, positioning practice, technique experimentation, exploration of technique charts, info gained without radiating humans
Vary in size/shape/anatomy
MAS Calculation Formula
mA x s = mAs
Grid conversion formula
mAs1/mAs2= GCF1/GCF2
As grid ratio increases the amount of X-ray required increases
No grid
1
5:1 grid
2
6:1 grid
3
8:1 grid
4
12:1 grid
5
16:1 grid
6
Density maintenance formula
MAS1/MAS2=D1sq/D2sq
As SID increases the amount of X-ray required increases
15% Rule (15/50 Rule)
A 15% increase in KVP = a 50% reduction in MAS
A 15% decrease in KVP = doubling (2x) the MAS
Film/screen speed conversion formula
MAS1/MAS2=RS2/RS1
As film screen increases the amount of X-ray required decreases
Magnification factor formula
SID/SOD
Factors impacting density
Primary - MAS
Secondary - almost all other factors (anything effecting energy to IR)
Factors impacting contract
Primary - KVP
Secondary - grids, OID (anything that affects scatter reaching IR)
Factors impacting detail
SID, OID, focal spot size
Factors impacting distortion
SID, OID, CR angle, motion, film/screen speed
