What is an X-ray?

You may be familiar with what an x-ray looks like, perhaps even viewing one of your own broken bones. Many people believe an x-ray is similar to a photograph. While the development of an x-ray film is quite similar, the technology behind x-rays is quite different.

X-rays were discovered by accident in 1895. A physicist by the name of W.C. Roentgen was experimenting with cathode ray tubes (similar to modern fluorescent light bulbs). By filling a cathode ray tube with gas and passing electric voltage through it, he discovered the tube glowed fluorescently. After wrapping the tube with black paper, he then found green light still emanated from the tube. This light, or ray, Roentgen discovered was able to pass through many substances and materials of light atoms, such as wood. The beam would then cast shadows on film.

The fluorescence that Roentgen was producing was not from the cathode ray tube, because light could not penetrate the tube. Roentgen discovered he was producing an unknown form of radiation. The ray was named “X” due to the mathematical unknown quantity “X”. The x-ray, Roentgen discovered, was also able to pass through human tissue, making bone visible.  Because cathode tubes were already utilized, Roentgen’s x-rays were able to be duplicated. Within a year x-rays were being used throughout the United States to diagnose gun shots and bone fractures.  

Roentgen’s discovery of x-rays, and subsequent work, took seventeen years to confirm. Many scientists simply thought that Roentgen had discovered was a type of ultraviolet light. It wasn’t until 1912 that experiments by M.v. Laue confirmed x-rays were indeed electromagnetic waves.

X-rays are similar to visible light, a form of energy. Light is made of electromagnetic particles, a form of electromagnetic radiation. Like light, x-rays are a form of electromagnetic radiation (along with radio waves, gamma rays and infrared waves) but x-rays have a shorter wavelength and higher energy than light does. This allows them to pass through matter such as the flesh and muscle of the human body.

To understand how x-rays are produced you must begin with an X-ray tube. At one end of the tube, there is an evacuated chamber with a tungsten filament. This is called a cathode.  At the other end of the tube there is a metal target, called an anode. Electrical current is run through the tungsten filament. This causes it to glow and emit electrons. The voltage, between tens or hundreds of kilovolts, that is applied to the tube forms an electrical field between the cathode and the anode. This causes ionization of gas molecules. Being bombarded by positive ions, the cathode emits cathode rays (electrons) that race towards the anode. Because the cathode surface is spherical, it causes the cathode rays that accelerate towards the anode to be concentrated. When the rays hit the anode, the electrons cause the focal spot to emit x-rays.

X-ray films are obtained when these beams are passed first through several filters, and then through your body. They are absorbed differently as they pass through different densities, such as flesh, metal or bone. X-rays are not able to pass through very dense materials, such as bone. However, when you break a bone and create a fracture, that fracture will have a different density than solid bone does, making fractures visible on x-ray pictures.

On the other side of the patient, a camera records the patterns of x-ray light that pass through their body. Radiographic film, a film that is coated with sensitive material, is used. As in a photograph, film technology is the same; however, instead of visible light the x-ray sets off a chemical reaction. Your body’s shadows, both light and dark, are then projected onto the film. X-rays that pass through flesh leave a latent image on the film.

The developed film shows the parts of the image that correspond to higher x-ray exposures are dark. This leaves white shadows of bone on the film. The results show dark or gray where your flesh is, because fat and soft tissues absorb less. Muscle appears a different shade of gray. Bone, because it contains calcium and is dense, appears white while the air in your lungs appears black. Metal and swallowed objects, because they absorb more x-rays, appear white and are clearly visible. The very first x-ray was taken of Roentgen’s wife’s hand, showing her finger bones as well as clearly outlining her wedding band.

In 2010 alone, 5 billion imaging studies were performed worldwide.

 Since its accidental discovery in 1895, X-ray technology has developed to include many life-saving diagnostic tests such as fluoroscopy, radiotherapy and computed axial tomography, or CAT scans.  X-ray technology is used in hospitals, diagnostic centers, specialty hospitals such as cancer treatment centers, as well as dental offices.

X-ray technology is also used in the areas of Science, Industry, Security and Consumer Goods.  A recent discovery has been made of a camera, using x-ray technology, that can take an amazing 4.5 million frames per second (compared to High Definition, which captures only 24 frames per second). This discovery may change what we know about matter.



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