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Almost every hospital, dentist office and well-established private practice in North America houses an X-ray machine; the ability to see the internal structure of a patient without risking surgery is absolutely invaluable. And while there are some risks associated with X-ray machines, they've been saving lives, improving health and benefiting mankind for many, many years now. But how do they work?

X-ray Machines use a specific wavelength of light known as "X-rays" (hence the unimaginative name), generally considered to be between 10-0.1 nanometers (that's 1x10^-9 meters, or a millionth of a meter). Visible light is between 680-450nm (Red to Blue), so an X-ray is invisible and very high energy.

Since X-rays are so high energy, they tend to penetrate material very easily; the small wavelength means that the X-ray can pass in between the gaps between atoms that would reflect or absorb longer wavelengths of light. However, they're also big enough to be stopped when needed and to be detected. Lead, for example, is dense enough to deflect or absorb the vast majority of X-ray energy, and photographic paper is generally sufficient to detect them with enough time.

To create X-rays, a small piece of metal is bombarded with electrons. The electrons smash into the atoms of the metal, exciting them into higher energy levels, but the atoms quickly relax back into their ground-state. In order for them to return to their ground state, the energy they absorbed from the collision with the electrons must be emitted in some fashion, and is normally released as X-ray radiation (the speed of the colliding electrons is specifically set so that the energy released is preferentially X-ray instead of microwave, thermal, or visible).

These X-rays are aimed at the patient, who is placed between the X-ray machine and the detector. Historically, the "detector" was a sheet of photographic paper, but more and more X-ray machines are using CCD (Coupled Charge Detectors), phototubes, or other electronic detecting methods as they are faster, cheaper, and increasingly more accurate.

As the X-rays speed their way through a patient, some of them interact with the materials in the patient... things like bone and muscle, being dense materials, absorb or deflect a significant number of electrons, while substances such as blood, air (in the lungs, for example), and fat don't absorb much at all. The detector displays the regions of high X-rays (those regions where the X-rays weren't blocked by dense material) and contrasts it against the regions of low X-rays (where they *were* absorbed or deflected). Normally these are displayed as black-and-white images, the white regions representing the low X-ray areas (so bone and muscle are shown as various shades of white). By varying the exposure time the focus can be shifted from softer tissues to harder tissue, depending on the regions of concern.

There is still a great deal of training required to interpret most X-ray images, especially to differentiate between cancerous regions (which can appear as fibrous, densely packed muscle tissue), soft-tissue damage (internal bleeding, lung damage, etc), and anything located close to bone. Also, because the image of an X-ray is 2 dimensional, a great deal of information is overlapped into one photo. This can be helped by proper placement of the target, but a great deal of skill is still needed for anything but the most basic of images.

Learn more about this author, Marc Quaglia.
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