AEC_SlideDeck

Automatic Exposure Control (AEC)

• Also known as:

  • Automatic Exposure Device

  • Phototimer

• Functionality: Controls the duration of exposure to ensure optimal imaging.

• Components:

  • Detectors (Sensor Cells, Ionization Chambers)

  • Located in front of the Image Receptor (IR)

• Operation: When a predetermined amount of radiation is received, exposure is terminated.

• Exposure Considerations:

  • Larger body parts absorb more radiation, necessitating longer exposure times.

  • Smaller body parts require shorter exposure times.

AEC Ionization Chambers

• Design and Functionality:

  • 3 detectors configured on the upright bucky.

  • Thin and radiolucent properties allow correct sensing.

• Operational Mechanism:

  • Operational amplifier averages the voltage from active detectors to signal the end of exposure.

Positioning and Detector Configuration

• Body part must align directly over the detector selected.

• Detector Configuration for Thoracic Cavity:

  • Center Detector: Primary sensor for central exposures.

  • Outer Detectors: Assist in capturing peripheral anatomy.

• Exposure Selection:

  • Set on ‘normal’ to ensure consistent imaging.

  • Calibration required if settings deviate.

  • Adjustments of +/- 1 result in +/- 25% mAs, +/- 2 results in +/- 50% mAs.

Back Up Time (Back up mAs)

• Definition: Max exposure time/mAs set by the technologist; serves as a safety mechanism.

• Settings: Recommended at 150% of manual mAs technique.

• Importance: Protects both patient and equipment from:

  • Equipment failure

  • User error

• Operational Limits:

  • Units above 50 kV terminate at 600 mAs.

  • Units below 50 kVp terminate at 2000 mAs.

Minimum Response Time

• Definition: The fastest response time of AEC to shut off exposure.

• Implications: Shorter response time may lead to overexposure if required exposure is lower than the minimum.

• Adjustment: May decrease mA to lengthen exposure, crucial for very small body parts.

Importance of Exposure Factors

• mA: Must remain constant according to the body part.

  • Higher mA leads to shorter exposure durations.

• kVp: Should be optimized according to the anatomical part being imaged.

• Back up time is protective against excessive exposure.

• Exposure Selectors: Allow for adjustments based on specific anatomy, alternating exposure values by radiologist preference.

• Limitations:

  • Inadequate coverage of detectors with small anatomy.

  • Requires perfect positioning and centering.

  • Difficult imaging with complex anatomy and contrast.

  • Necessitates collimation and proper photocell combination.

  • Must consider the minimum response time limitations.

Uses and Non-Uses of AEC

• Common Uses:

  • Imaging of trunk, femurs, neck, cranium.

• Additional Use Cases with Digital Imaging:

  • Effective primarily for larger structures.

• Non-Uses:

  • Sinuses, mastoids, mandibles, distal extremities, small or complex anatomy, though some used in Digital Radiography.

Troubleshooting AEC Issues

• Common Issues Encountered:

  • Underexposure or overexposure with various imaging setups (e.g., patient size affecting AP hip imaging)

  • Misalignment of sensor cells with anatomy.

• Creative Positioning Techniques:

  • Adjusting positions for young children or smaller individuals for accurate imaging without overexposure.

Professional Judgement in Radiography

• Importance of Radiologic Technologists:

  • No automation can substitute the critical thinking of trained professionals.

• Action in Doubt:

  • Check the mAs readout.

  • Opt for manual exposure techniques as needed.