AUTOMATIC EXPOSURE CONTROL DMI 63 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: 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) All three of 3 types of AEC units use some type of radiation detector device –Photodiodes (photocell) –Ionization chamber 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 for- PA 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 from visit to visit tech to tech room to room hospital to hospital

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! Downside of using AEC!

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!