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.
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.
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.
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.
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.
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.
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.
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.
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.