An x-ray is a form of electro magnetic radiation, with a wavelength of 10 to 0.001 nanometers (shorter than ultraviolet light). The letter 'X' comes from their initial discovery, when they were an unknown form of radiation and thus given the moniker 'X' to indicate 'unknown.'
The discovery of x-rays
In 1895, German physics professor Wilhelm Röntgen accidentally discovered X-rays. His initial experiment involved a tube that emitted light, also known as a Crookes tube. Röntgen noticed that a fluorescent screen near the tube was glowing, even though the tube was insulated with cardboard. Since no light was passing through to make the screen glow, some other previously unknown form of radiation or "X-Ray" must have been responsible. Röntgen noted that the ray passed through other materials as well, including human flesh! In fact, in discovering x-radiation, Röntgen also discovered its major application: imaging. The first x-ray ever produced is of Röntgen's wife's hand.
How does an x-ray work?
An x-ray is very similar to the light rays that our eye can perceive, except that they have much higher energy. This higher energy corresponds with their shorter wavelength. To generate an x-ray, a cathode is heated to a high temperature. The heat causes electrons to break away from the cathode. The anode across the vacuum tube has a high potential difference, so it attracts the electrons at great velocity. The electron's collision with the anode (typically made of tungsten) causes an x-ray photon to be released. The entire tube is shielded except for a narrow opening, which allows the rays to escape in a concentrated beam.
The beam travels across space until it comes in contact with tissue. In our body, soft tissue cannot absorb the high energy ray and it passes straight through. The high dense material, like bone made of calcium, do absorb the radiation. The rays that pass through to the film detector, which works much like plain camera film. The black areas are the exposed areas, representing the rays that passed through the soft tissue, while the white areas are the unexposed areas, where the rays were absorbed by tissue.
How are x-rays acquired?
The patient is positioned between the x-ray machine and the film. It is the x ray technician's job to properly align the patient in space in order to capture the appropriate 2-D plane. Areas not of interest should be appropriately shielded. After the beam is activated, the film is shown to the radiologist. With PACS systems, digital film is often used so that the image can be viewed directly on a computer monitor without true film being utilized.