X-rays are a form of electromagnetic radiation. They strike an object and, depending on the density of the object, some of the X-rays will be absorbed. X-rays that aren’t absorbed pass through the item being X-rayed, and they hit a photographic plate to give an image (although other methods can also be used).

X-rays are very good at showing up bones. Soft tissues allow the X-rays to pass through them and when the X-ray hits the photographic plate it turns it black. Dense tissues like bone absorb X-rays so these show as white on the photographic plate.

They can be used for some soft tissue investigations like looking for lung disease. Sometimes other substances like iodine can be injected to help visualise parts of the body like arteries. X-rays are not very useful when trying to look at muscle or the brain.

X-rays are also used by dentists to diagnose and treat a wide variety of problems.

X-rays are a form of ionising radiation – radiation that has enough energy to shift electrons out of atoms and so produce ions. (This can cause damage to biological organisms like humans and can cause DNA to mutate, so having a lot of X-rays is not considered a good idea.) X-rays (for medical imaging use) are produced by accelerating electrons at metal targets. The type of metal involved depends on what application the X-ray is being used for. When an electron hits the metal, it slows down rapidly, and the energy produced causes an electron to be knocked out from the metal atom and this releases X-rays.

Today we can take an X-ray in about 1/50th of a second, but the first X-rays took about 15 minutes of exposure.

How long have we known about X-rays?

In the nineteenth century, if you were sick or injured, doctors had no way of seeing inside your body without operating. They had to rely on touch, for example, feeling around a broken bone to try and set it, or listening to a chest with a stethoscope, to find out what was happening inside.

One of the most important advances in medical science in the late nineteenth century was made not in a hospital but in a German university physics lab by Wilhelm Roentgen, and as with many of our greatest discoveries, X-rays were discovered completely by accident.

Doctors immediately saw the possibilities of X-rays for diagnosis, and the technology quickly spread around the world. People were both fascinated and alarmed by the news – many were concerned that X-rays might be misused to see underneath women’s clothes. On the other hand the equipment was easy to obtain and setup by those with an inclination for things electrical. (There is one case in North Otago where a farmer imported the equipment and used it to diagnose the injuries of his friends, family and dogs. In other many cases they were used as little more than a sideshow attraction.)

Then people started getting sick from the use of X-ray equipment. They developed burns from being too close to the equipment and many of the early radiographers died of cancer. When it was understood that these illnesses were caused by a mixture of radiation and the misuse of the equipment, steps were taken to control the availability of the equipment, to make sure all radiographers were trained and find out what were the safe levels of exposure.

    Published 23 July 2007