A 38-year-old patient of average habitus needs an erect PA chest radiograph at 110 kVp using a three-cell AEC system, with no implants and symmetric lung expansion on the scout light. The radiographer centers at T7 with the outer cells under the lungs. Which AEC detector selection should the radiographer activate for this PA chest exposure?
Select an answer to reveal the explanation.
Short Explanation
You're metering the airy fields, not the dense middle. Outer cells listen to lung air like microphones over the crowd.
Full Explanation
AEC terminates exposure when the selected ion chambers receive preset charge, so for PA chest the mechanism requires sampling aerated lung behind both outer detectors with accurate anatomic alignment, upright positioning, and appropriate kVp, using normal density and a backup mAs to prevent overexposure if AEC fails. Selecting only the center cell samples dense spine and mediastinum, prolonging exposure and overexposing lungs while added density compounds the error. Activating all three cells averages lung with mediastinum and, combined with large negative density, risks underexposure and noisy lungs despite correct centering. Rotating the patient misaligns anatomy over the cells so ribs or mediastinum dominate the measurement, violating the alignment requirement and producing inconsistent termination regardless of correct cell choice.