In automatic exposure control (AEC), where must the detectors be positioned, and what can mispositioning cause?

• A. Detectors placed outside the primary beam; mispositioning does not affect density.
• B. Detectors placed in the patient’s field of view; mispositioning causes underexposure.
• C. Detectors placed in the primary beam; mispositioning can cause improper density or over/underexposure.
• D. Detectors placed in the patient’s bones; mispositioning has no effect.

Answer: (C)

In AEC, the detector must sample the x-ray beam that will form the image, so it can signal the system to stop exposure once the desired density is reached. Placing detectors in the primary beam ensures they measure the actual radiation reaching the image receptor, reflecting how much energy has passed through the patient.

If detectors are mispositioned—outside the primary beam or in the wrong area—they won’t sample the correct beam. The exposure can be terminated too late or too early, leading to improper density. In practice this can mean the image is overexposed or underexposed, with the patient receiving unnecessary dose or the diagnostic quality being compromised. Keeping detectors within the primary beam and over the region of interest is essential for consistent, accurate automatic exposure control.