American Society of Radiologic Technologist (ASRT) Practice Exam

The main function of the cathode in an x-ray tube is to?

1. Provide the positive charge for electron collection

2. Emits electrons when heated

3. Limit the x-ray beam

4. Regulate exposure time

The correct answer is: Emits electrons when heated

The primary role of the cathode in an x-ray tube is to emit electrons when heated. This process occurs through a mechanism known as thermionic emission, where the filament within the cathode is heated to a high temperature, causing it to release electrons. These freed electrons are then attracted towards the anode, where they are involved in the production of x-rays upon striking the anode material. Understanding this function is crucial because the flow of electrons from the cathode to the anode is fundamental to the generation of x-rays. Without the cathode effectively producing a stream of electrons, the x-ray tube would be unable to produce the radiation needed for imaging. Other options relate to functions that are not performed by the cathode. For example, the positive charge necessary for electron collection is actually provided by the anode. Additionally, limiting the x-ray beam pertains to collimation, which is achieved through devices separate from the cathode. Exposure time regulation, meanwhile, relates more to the control systems and electronics that govern the operation of the x-ray tube rather than the cathode’s role. Understanding the specific and critical function of the cathode aids in grasping the overall operation of x-ray technology.