What feature does a grid-controlled tube include?

A grid-controlled tube is specifically designed to enhance the efficiency of x-ray production and control the exposure of images during fluoroscopy. This type of tube includes a focusing cup that allows for rapid switching of the tube current. The focusing cup works with the grid mechanism to precisely control the flow of electrons in the tube, enabling quick adjustments in exposure without lag time. This feature is essential for achieving high-quality imaging during real-time procedures where timing is critical.

In comparing the other options, automatic exposure control refers to a method of automatically adjusting the exposure based on the density of the tissue being imaged, which is not a specific feature of grid-controlled tubes. A static anode may provide certain benefits related to heat distribution, but it does not represent the defining characteristic of how a grid-controlled tube operates. Digital readouts for dose provide important information during imaging but are not inherent to the grid-controlled mechanism itself. Thus, the presence of a focusing cup for fast switching of the tube current correctly identifies the distinctive feature of grid-controlled tubes.