Automatic Exposure Controls (AECs): Function, Setup, and Troubleshooting in Radiography

Anatomically Programmed Radiography (APR)

Computerized system that selects exposure factors based on anatomy and projection.

Purpose of AEC

Controls exposure time automatically.

Radiographer Control

The radiographer still controls kVp, mA (if manually selected), SID, Positioning, and Collimation.

Typical Setup of AEC

Most AEC systems use 3 ionization chambers: Left, Right, Center.

Chamber Placement

Center chamber = center of image receptor; Left and right chambers = slightly higher.

Determining Chamber Location

Manufacturers may provide chamber projection inserts and light field indicators.

Automatic Exposure Control (AEC)

Device that automatically terminates the exposure when enough radiation reaches the image receptor.

Ionization Chamber

Detector that measures radiation reaching the image receptor.

Minimum Response Time

The shortest time required for the AEC to respond and terminate the exposure. Modern systems: approximately 0.001 second.

Backup Time

Safety feature that terminates exposure if the AEC fails. Usually set at 150% of anticipated manual exposure time.

Chamber Selection

Radiographer chooses which chamber(s) are active.

Important Rule for Chamber Selection

If multiple chambers are selected, the chamber receiving the most radiation has the greatest influence on exposure.

Operational Amplifier Function

Receives signals from chambers, adds the voltages, divides by number of activated chambers, and terminates exposure when correct voltage is reached.

Density Controls Purpose

Adjust receptor exposure when necessary.

Normal Setting for Density Controls

0 or N.

Increase Density

Positive settings.

Decrease Density

Negative settings.

Exposure Technique Charts

AEC charts include kVp, mA, SID, and chamber selection but do NOT include exposure time.

Positioning Problems

AEC produces a proper exposure for whatever is over the active chamber.

Subject Density & Contrast Problems

Problems occur when unexpected density exists or expected density is missing.

Tight Collimation Problem

If collimation cuts off an active chamber, AEC thinks tissue is very dense, resulting in overexposure.

Wide Collimation Problem

Produces scatter radiation that may reach the detector without passing through the patient, resulting in underexposure.

Timing Problems

Minimum Response Time is the fastest time AEC can terminate exposure.

Backup Time Purpose

Protects patient from excessive exposure.

APR Functionality

APR automatically selects kVp, mA, and active chambers based on anatomy and projection.

Creative Positioning

Experienced radiographers may creatively use AEC, but manual techniques are more repeatable.

Registry / Exam Must-Knows

1. AEC controls exposure time only. 2. Using AEC = losing control of time and mAs. 3. Most AEC errors are positioning errors.

Principal Function of AEC

Automatically terminates exposure when enough radiation reaches the image receptor.

Typical Number and Configuration of Ionization Chambers

Three chambers: center, right, and left.

Purpose of Operational Amplifier

Sums chamber signals, averages them, and terminates exposure.

Effect of Subject Density on AEC

Increased density lengthens exposure; decreased density shortens exposure.

Collimation Effects on AEC

Tight collimation may overexpose; excessive scatter from wide collimation may underexpose.

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