When does the back-up timer terminate the x-ray exposure?

The back-up timer is a safety feature in radiographic equipment designed to prevent excessive exposure time and radiation dose to the patient. It automatically terminates the x-ray exposure after a predetermined amount of time or mAs (milliampere-seconds) if the intended exposure has not been reached.

In this context, the correct answer relates to the maximum limit that the back-up timer is typically set to terminate the exposure. A standard value often used in practice is 600 mAs. This is based on ensuring that both patient safety and image quality are maintained, preventing prolonged exposure that could lead to unnecessary radiation doses.

The use of 600 mAs as a back-up timer setting is common, as it is considered sufficient for a variety of imaging procedures while still providing an adequate cushion for exposure adjustments in scenarios where the automatic system might fail to terminate the exposure in a timely manner. This setting allows radiologic technologists to ensure safety while accommodating for variations in patient size and anatomy that can affect exposure needs.

Higher values, such as 1,200 mAs, should generally not be typical for back-up timer settings, as they could lead to unintentional excessive radiation exposure, contrary to the principles of radiation safety.