Which exposure factor is controlled by the automatic exposure control (AEC) system?

The automatic exposure control (AEC) system is designed to automatically terminate the exposure time based on the amount of radiation that reaches the image receptor. This means that the exposure factor that AEC directly controls is time. By adjusting the exposure time, the AEC ensures that the image is obtained with optimal density and contrast, based on the exposure needs for the specific anatomical part being radiographed.

While kilovoltage peak (kVp) affects the quality and penetrative power of the X-ray beam, it is a setting that the technologist usually determines before the exposure based on the procedure requirements. Milliamperage (mA) controls the quantity of X-rays produced and can influence the overall image brightness, but it does not change during the exposure once set. Source-to-image distance (SID) is a fixed measurement that is determined based on the specific imaging technique being used; it impacts the magnification and resolution of the image but is not controlled by AEC.

Thus, the focus of the AEC system is primarily on adjusting the exposure time to achieve the desired radiation dose for optimal imaging, making time the correct answer.