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.