The x-ray tube serves the function of creating x-ray photons from electric energy supplied by the x-ray generator. The process of creating the x-ray beam is very inefficient, with only 1% of the electric energy converted to x-ray photons and the remaining 99% converted to heat in the x-ray tube assembly. Thus, to produce sufficient x-ray output for diagnostic imaging, the x-ray tube must withstand and dissipate a substantial heat load, a requirement that affects the design and composition of the x-ray tube. The major x-ray tube components are the cathode and anode assemblies, the tube envelope, the rotor and stator (for rotating anode systems), and the tube housing. The design of the x-ray tube determines the basic characteristics of the x-ray beam such as focal spot size, x-ray field uniformity, and the x-ray energy spectrum. These x-ray beam characteristics are important because they affect radiologic parameters such as spatial resolution, image contrast, and patient dose.