The story of the x-ray machine (CT scanners as well as some radiation therapy equipment also use x-rays) is the story of what is know called medical imaging and that story begins in 1895 with Wilhelm Conrad Rontgen.
Dr. Rontgen was a Prussian born physicist who was doing experiments with cathode ray tubes. (Cathode ray tubes are vacuum tubes that permit the flow of electrons to occur in one direction but not the other.) He noticed that a screen coated with barium platinocyanide fluoresced when the vacuum tube was energized even though the tube had been covered to prevent visible light from escaping. Rontgen spent the next few days documenting his discover which he termed "x-rays", "x" being the mathematical symbol for an unknown variable or quantity. He would eventually receive the Noble Prize for physics for this very valuable discovery.
X-rays are a part of the electromagnetic spectrum, i.e. energy waves that are comprised of photons. Cosmic, gamma, infra red, ultra violet, television, radio, microwaves and visible light are other forms electromagnetic energy. All of these rays are comprised of photons that travel in waves. What makes one form different from the others is the wavelength i.e. the distance between consecutive peaks or valleys of the waveform. Since all electromagnetic energy travels at the same speed, as the wavelength decreases the frequency (cycles per second) increases and (more important to this article) the energery of the wave increases.
X-rays have enough energy to remove electrons from the particles of whatever substance they pass through, a process known as ionization. X-rays also do not normally reflect or refract, as visible light, infra red and microwaves do. They also have enough energy to penetrate many substances with low to medium range atomic numbers without much lost to attenuation. It is interesting to note that it is this attenuation (absorption) that allows "photographs" of internal body structures to be made. Different substances absorb x-radiation in different amounts, e.g. bone absorbs more than water, muscle absorbs more than air. On a radiograph the areas that are the lightest in color or shade are the more dense structures. It is this contrast in shades of gray that allow doctors to visualize anatomic structures and / or disease.
Tomography: tomo, to cut or slice, + graphy is a technique used to visualize the body's internal structures one slice at a time. Conventional (non computerized) is by moving the x-ray tube and the film or photographic plate in opposite directions so as to "blur out" the image of superficial tissue and create a clear image of the inside of a structure, like a tumor or a cast for example. These images are produced on a standard x-ray machine and table. Computed Axial Tomography or Computed Tomography is accomplished by by passing a narrow beam of x-ray energy through a patient as photoreceptors rotate around the patient. This analog information is then analyzed by a computer and a digital image is then made.