X-rays are created by bombarding a tungsten target with electrons inside a device known as the x-ray tube. To generate this stream of electrons inside the x-ray tube, a powerful x-ray generator first takes the regular alternating current (AC) electricity from the power line at about 120 to 480 volts and transforms it into power in the range of 35 to 150 kilo volts (kV or thousands of volts). When this very high voltage potential is applied to the x-ray tube, a tight beam of electrons is fired out of a small wire (called the cathode) and strikes a metal disk (called the anode). When this stream of electrons hits the special metal compound of the anode (often tungsten or alloys including tungsten), it causes x-ray energy to be released from the metal's atomic structure. These x-rays are often filtered and collimated (or focused) as they leave the x-ray tube. The rays pass through the body part of interest in a straight line and are then recorded onto film or captured by an image intensifier and TV system to make the final image.
X-ray tubes are precision designed and manufactured and have evolved tremendously over the past 100 years. X-ray tubes are often the most expensive component in an x-ray system and can cost more than $50,000 each (for the ones used in high speed CT scanners or cardiac catheterization labs). The x-ray tube is a glass or metal envelope with a vacuum seal inside. X-ray tubes create tremendous heat while the beam of electrons is bombarding the cathode to produce the x-ray. Like a light bulb, an x-ray tube requires replacement up to a few times per year, depending on use.
