AUTOMATIC EXPOSURE CONTROL

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AUTOMATIC EXPOSURE CONTROL

• DMI 63

3/10/2012 online
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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

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Purpose of AEC

• To deliver consistent, reproducible exposures
  across a wide range of
• Anatomical thicknesses
• Types of body parts and pts.
• Equipment
• Techs
• Rooms

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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)
• All three of 3 types of AEC units use some type
  of radiation detector device
• Photodiodes (photocell)
• Ionization chamber
• And a backup timer

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1. Phototiming System

• Earliest still widely used
• Uses photodiode detector
• Note Photodiode is after image receptor

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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!
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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

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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)

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Radiation Detecting Devices
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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
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Ionization Chamber System
Most common type! Chamber is between pt and
image receptor (as opposed to photodiode system
which is after IR) Radiolucent so doesnt show
up on image
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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

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Compare Position of IR
Ionization Chamber
Phototiming
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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
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What cells would you select for-

• PA Chest ?
• Lateral Chest?
• Pelvis?
• Pelvis with Left prothesis?
• AP Lumbar spine?

Image receptor detector positions
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AEC is used in Mammography
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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

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• 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?

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Important to remember!

• Never put contrast filled anatomy over AEC cell!
• Or breast implant
• Or metal prosthesis
• Or shielding
• All rooms arent calibrated the same
• Rooms change over time
• Service engineer calibrates initially using
  phantom then must periodically recheck

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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
• from visit to visit
• tech to tech
• room to room
• hospital to hospital

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Downside of using AEC!

• Technologists come to depend upon system and when
  it crashes, cant 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
• Cant use on portables or in OR
• AEC from room to room can be out- of -sync
• Could wind up doing many repeats!

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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 doesnt matter
  (techs will use too high techniques
  intentionally!)
• Should be used together!