DMI 63
AUTOMATIC EXPOSURE
CONTROL
3/10/2012 online
What Is Automatic Exposure Control?
(AEC)
Any device that measures quantity of radiation
either passing through Pt or image receptor -then
Automatically terminates exposure when predetermined
optimal density is reached
You give up control of exposure density to a machine!
Most technologists refer to AEC as “phototiming”
Purpose of AEC
To deliver consistent, reproducible
exposures across a wide range of:
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Anatomical thicknesses
Types of body parts and pts.
Equipment
Techs
Rooms
DIFFERENT TYPES OF AEC
1. Phototiming: (no computer involved)
2. Programmed Exposure: (computer controlled)
3. Anatomically Programmed Radiography (APR) (similar to Programmed
Exposure but with little pictures of anatomy)
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All three of 3 types of AEC units use some type of radiation detector device
– Photodiodes (photocell)
– Ionization chamber
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And a backup timer
1. Phototiming System
Earliest – still
widely used
Uses photodiode
detector
Note: Photodiode is after
image receptor
Technique Selection is mostly done by Tech
mA
kVp -based on body part thickness
Level of density (N, 1/2, 1 1/4 etc)
(each increment changes by approx. 30% per step)
Backup time- generally, 2 second or 1.5 times expected
exposure should be used in case AEC fails
Machine selects time!
2. Programmed Exposure System
• Uses microprocessor (computer)
• Microprocessor allows tech to digitally select any
kVp or mAs
– then microprocessor automatically chooses mA
station and time
– Backup times automatically programmed in
3. Anatomically Programmed Radiography System
(APR)
Similar to “Programmed Exposure System”
But uses touch screen with picture of anatomic
part instead of words or numbers
– (essentially a computerized technique chart)
Radiation Detecting Devices
How a photodiode System works
• Radiation goes through Pt and
image receptor
• To photo diode detector
• Diode lights up when hit by
radiation
• Converts light to electric signal
• Exposure cuts off when signal
reaches a predetermined
intensity
photcell
Photodiode detector
Ionization Chamber System
Most common type!
Chamber is between pt
and image receptor (as
opposed to photodiode system
which is after IR)
Radiolucent so doesn’t
show up on image
How Ionization Chambers Work
• Chambers contain cells
filled with air
• During radiation
exposure, air is ionized
• When charge reaches
preset level in cell,
exposure terminates
• Location of chambers
shown by small
rectangles on image
receptor
Compare Position of IR
Phototiming
Ionization
Chamber
Proper Cell Selection
Generally 2 or 3 cells
Tech must select cells
appropriate to area of
anatomical interest
Using 2 cells or even 3
creates a signal that is
averaged from for more
uniform density
Image receptor
What cells would you select forPA Chest ?
Lateral Chest?
Pelvis?
Pelvis with Left prothesis?
AP Lumbar spine?
Image receptor detector positions
AEC is used in Mammography
AEC does not relieve tech of following
obligations:
Skill in positioning!!!!
Technique selection: still need to select mA and kVp
and backup time (newer models build it in)
Anatomic recognition: different parts require
different settings
Awareness of Idiosyncrasies of equipment
Positioning accuracy is critical!
• Anatomy must be placed directly over correct
detector
• Certain anatomy works well (abdomen)
• Certain anatomy does not (shoulder)
• Why might it not work well on children?
Important to remember!
Never put contrast filled anatomy over AEC
cell!
Or breast implant
Or metal prosthesis
Or shielding
All rooms aren’t calibrated the same
Rooms change over time
Service engineer calibrates initially using phantom then must
periodically recheck
Upside of using AEC
– Takes less time to set up technique -speeds up exam
– Improves exposure accuracy, as long as proper
positioning is used
• Lowers repeat rate
• Saves pt exposure
– More consistency in density
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from visit to visit
tech to tech
room to room
hospital to hospital
Downside of using AEC!
Technologists come to depend upon system and when it crashes,
can’t remember manual techniques!
Over-confidence in system may cause technologist to become
neglectful and commit errors
If you are not centered directly over area of interest, exposure may
not be correct
Can’t use on portables or in OR
AEC from room to room can be out- of -sync
Could wind up doing many repeats!
Has Digital Radiography made AEC Obsolete?
No!
While digital equipment will override image density
produced by AEC,
AEC controls pt exposure- good for pt!
whereas digital radiography only controls appearance of
image- pt. dosage doesn’t matter (techs will use too high
techniques intentionally!)
Should be used together!