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MEDICINE 101

The Nature of X-Rays

Back in 1895, a German physicist named Wilhelm Roentgen made the first X-ray pictures.   He was working on the property of light in his laboratory with electric current passing through vacuum tubes.   He noted that invisible rays came out of the cathode tube and caused the photographic plate across the room to become exposed.   He put his hand in front of the plate and produced the first X-ray view of the bones of the hand.

Nowadays, X-rays are produced by electrical machines.  They emerge as a beam of light, much like this flashlight beam.   The X-rays then pass through patients and expose a large photographic plate held behind the patient.   Here's the important part.   X-rays are absorbed in different degrees by the various structures in the body that they pass through.   For example, on this film, all the X-rays hitting this coin have been absorbed.   No X-rays get through to expose the photographic plate and so the image remains white.   In the human body there are four basic patterns of absorption of X-rays and these correspond to four basic shades visible on the X-ray plate.   The first is air - air absorbs very little in the way of X-rays, so the plate is very much exposed.   It appears dark, like the lung part of this chest X-ray.   The second is bone.   Bone absorbs quite a bit of radiation so none gets through to the photographic plate.   The X-rayed bone appears white.   The next two are fat and water, both of which are somewhere between in their ability to absorb X-rays.   Look at this chest X-ray.   The ribs appear white.    That's because the bone in the ribs absorbs most of the X-ray coming through.   The lungs appear quite dark.   The outline of the heart is white as well because many X-rays are absorbed by the tissue passing through.   It's the interpretation of these various amounts of penetration of X-rays that make this such a valuable diagnostic tool.

For example, it's easy to see on this film that the bone is broken.

Another way to get more information is to ask the patient to swallow a material that is known to absorb a lot of X-rays such as this barium.   As you can see on this picture the barium outlines the lining of the stomach very well.

With innovations such as CAT scans and contrast radiography, X-rays have come a long way from their chance discovery 106 years ago.   The next time you have an X-ray, you'll have a better understanding of how the doctor will read it and why, when radiographers gather for a toast, they usually say "Here's looking at you, Babe".