X-RAY TUBES
How X-ray Tubes Work
X-rays are produced when thermal electrons emitted by the cathode (filament) of an X-ray tube, accelerated and focused by a high voltage between the cathode and anode, hit the anode (target).
Most X-ray tubes produce continuous X-rays (white X-rays) and these are useful for general diagnostic imaging, but special X-rays are used for mammography and other soft tissue imaging.
The potential difference between the cathode and the anode is called the "tube voltage" and the electron flow from cathode is called the "tube current".
X-rays are part of the electromagnetic spectrum, and have properties such as wavelength and frequency. Increasing the X-ray tube voltage results in both a higher X-ray intensity and a higher energy distribution. However, increasing the X-ray tube current at a constant X-ray tube voltage increases the X-ray intensity without affecting the energy distribution. Differences in anode structure distinguish the two types of X-ray tubes manufactured for diagnostic purposes: stationary anode X-ray tubes and rotating anode X-ray tubes.
In the field of medical diagnostics, rotating-anode X-ray tubes are more popular due to the advantage of obtaining higher X-ray radiation with a smaller focal spot size.
X-ray production is very inefficient. For diagnostic x-ray production, less than 1% of the kinetic energy of the electrons is converted into x-rays. The remainder of energy is converted into heat causing the X-ray tube to warm up during operation. As illustrated in the diagram, the X-ray tube is enclosed in a housing unit filled with insulating oil. This oil not only provides electric shielding from the tube voltage, but also transmits heat generated in the housing unit to the unit's surface. The exterior of the housing unit is cooled with a fan, and the insulating oil is cooled by passing it through a heat exchanger.
