> What is an X-Ray?
An X-Ray is a specific type of electromagnetic radiation (EMR). Other familiar forms of EMR include visible light, radio waves, and microwaves. All electromagnetic radiation is comprised of oscillating electrical and magnetic fields that propagate through space. In a vacuum, all forms of EMR travel at the speed of light. X-Rays are a form of EMR that have a very high frequency (the number of oscillations per second) and very low wavelength (the distance between crests of the waves.) By comparison, radio waves have a lower frequency and higher wavelength; EMR waves demonstrate an inverse relationship between frequency and wavelength: as one goes up, the other goes down. Today, X-Rays are used mainly in medical imaging; over 5 billion X-Rays are performed annually.
> How do they work?
X-Rays, as a form of EMR, occur naturally. Half of the exposure to X-Rays that humans experience comes from outer space and the earth itself. A variety of devices, including particle accelerators, can create them as well. Their main source, however, are specialized X-Ray tubes which are a kind of cathode vacuum tube. A vacuum tube is a sealed container, usually made of glass, which has at its two ends specialized equipment: a cathode and an anode. The cathode emits electrons across the vacuum toward the anode, which collects them. If the voltage between the cathode and anode is high enough, the stream of electrons becomes so charged that the tube gives off X-Rays. Because the wavelengths of X-Rays are small, they are able to pass straight through a wide variety of materials, including human tissue. Bones, which are denser than soft tissues, absorb most of the X-Rays. When a photographic plate is placed behind a part of the body, say the arm, the X-Rays travel through the tissue and leave a darkened image on the plate. Because the bone absorbs the other X-Rays, it doesn’t allow them to form an image on the plate, leaving a white shadow behind. This creates the familiar black and white image of a medical X-Ray.
> How are they measured?
X-Rays can be measured in a variety of ways, including their ability to ionize neutral atoms into charged particles (ions). For human tissue, however, X-Rays are best measured not by the charge they create, but rather by the amount of energy that is absorbed by the body. In the International System of Units (the modern form of the metric system), the name of the unit for absorbed radiation is the gray (Gy), named after Louis Gray, a physicist who pioneered research into the effects of radiation on living things. A gray is equivalent to one joule of energy divided by one kilogram of matter.
> When were they discovered?
The history of X-Rays is a little murky. There were many independent experiments involving cathode vacuum tubes that resulted in the creation of X-Rays in the late 19th century. Initial experimenters, however, were either not aware that X-Rays had been created (they are invisible in most conditions) or observed their effects but did not know what caused them. Early scientists that observed the effects of X-Rays included William Crookes, Heinrich Hertz, Nikola Tesla, and Thomas Edison. Wilhelm Röntgen, a German physicist, is widely attributed as the “true” discoverer of X-Rays because he studied them in-depth beginning in 1895 and gave them their name. Because what he observed was then an unknown form of radiation, he initially called it “X” radiation, “X” standing for “unknown”. The initial name has remained since, although they are also sometimes referred to as Röntgen rays in honor of his contributions.
Exact accounts of his discovery conflict, but most agree that he was working with a cathode tube that he had covered in black cardboard. A screen that had been treated with a fluorescent paint stood a few feet away, and as the cathode tube began emitting a highly charged electron stream, Röntgen saw that the screen began to glow lightly. Because all visible light had been blocked by the cardboard, Röntgen realized something else, although invisible, had traveled through the glass tube and cardboard to make the screen glow: X-Rays.
> What early problems occurred?
Because early pioneers and researchers were not aware of the harmful effects that prolonged exposure to radiation caused to the body, there were quite a few incidents of radiation poisoning and cancer among them. One of the more notable victims included John-Hall Edwards, who took the first X-Ray used during a surgery and later had to amputate his left arm due to skin cancer caused by X-Rays (X-Ray dermatitis). An assistant to Thomas Edison, Clarence Madison Dally, also regularly exposed his hands and face to X-Rays during the course of experimentation. He developed malignant cancer and even after amputating both of his arms still succumbed to it, dying in 1904. The death so surprised and shocked Edison that he abandoned research in X-Rays altogether.
> How are they used today?
X-Rays are used today almost exclusively in medicine and science. In medicine, the penetrative properties of X-Rays are used to create radiographs, the familiar black and white images that X-Rays create. Radiographs allow one to see inside a patient’s body without invasive surgery. X-Rays are commonly used to look at the bone structures, chests, and abdomens of patients. The cardiovascular system is visible in a radiograph if contrasting agents like iodine or barium are injected into the bloodstream. If the iodine or barium is ingested, the gastrointestinal tract also shows up on a radiograph. Medical personnel use radiographs to confirm a fracture or break in a bone or the existence of cancer or other anomalies in certain organs of the body. Because they are mainly soft-tissue, which X-Rays pass through, muscle and brain tissue do not show up very well on traditional radiographs. They are imaged by using other techniques such as CAT scans or MRI. Dentists also use radiography to image the teeth and jaws. Another application of X-Rays is the X-Ray microscope, which uses radiation to generate images of microscopic objects. In addition, the Chandra X-Ray Observatory, a NASA satellite, detects the faintest of X-Rays from astronomical objects including suns, white dwarfs, supernovas, and black holes. The satellite has collected data which advances the field of astrophysics, including the confirmation of the Hubble Constant, the rate at which galaxies in space move away from one another as the universe expands.
