Beryllium is a chemical element in the periodic table that has the symbol Be and atomic number 4. A toxic bivalent element, beryllium is a steel grey, strong, light-weight yet brittle, alkaline earth metal. Beryllium is an ubiquitous element in the environment, and it has many commercial applications. Beryllium is used in alloys with copper or nickel to make gyroscopes, springs, electrical contacts, spot-welding electrodes and non-sparking tools. Mixing beryllium with these metals increases their electrical and thermal conductivity. Other beryllium alloys are used as structural materials for high-speed aircraft, missiles, spacecraft and communication satellites. Beryllium is relatively transparent to X-rays so ultra-thin beryllium foil is finding use in X-ray lithography. Beryllium is also used in nuclear reactors as a reflector or moderator of neutrons. Beryllium and its compounds are toxic and carcinogenic. If beryllium dust or fumes are inhaled, it can lead to an incurable inflammation of the lungs called berylliosis. Because it is strong, stable, can handle elevated levels of heat resistance and is highly transparent to x-rays, beryllium, in thin foil form, has long been critical to the operation of medical and scientific x-ray equipment. Beryllium foil provides the window through which tissue-penetrating x-rays are focused, while maintaining the vacuum inside the x-ray tube generator. Beryllium foil remains indispensible for high-resolution medical radiography, including CT scanning and mammography. Beryllium in newer generation mammography equipment enables a lower radiation dose scan with significantly finer tumor resolution, enabling breast cancer detection at its early, most treatable stages. Beryllium is also used in components of the analytical equipment used to analyze blood for HIV and other diseases, offering the precision and reliability that doctors and patients demand.