X-rays

by Ryan Moorcroft

X-rays are a type of ionising radiation that can be very harmful to living organisms. They can damage cells and DNA, and even cause cancer. Yet, before the 20th century, you would have had to cut a person open to find a broken bone or a tumour. Then the discovery of the x-ray revolutionised medicine and changed everything. Surprisingly though, x-rays were discovered completely by accident!

In 1895 Wilhelm Roentgen, a professor of physics in Germany, was experimenting with a cathode ray (streaming electrons within a vacuum tube onto a metal target). The tube was completely covered in a heavy black sheet. He was shocked to discover an unrelated group of fluorescent crystals on a nearby table would begin to glow simultaneously when the cathode ray was switched on. Roentgen concluded that an undiscovered ray was being emitted from the tube. A ray that could pass through substances. He named them x-rays (meaning unknown rays), as the physicist had no idea what they were. Several weeks after his discovery he took the first x-ray image of a human, his wife's hand. When she saw her skeletal hand she said, "I have seen my death!"

An early x-ray of Roentgen's wife's hand.

Examples of X-Rays

Hot objects in space naturally emit x-rays, such as pulsars, supernovae remnants, or even the accretion disk that forms around a black hole! As x-rays are a form of electromagnetic radiation, they travel at the speed of light in a vacuum and can eventually after many years reach Earth. Thankfully, our atmosphere blocks virtually all of these harmful rays, so they can't hurt us on the Earth's surface.

However, there are natural sources of x-ray radiation on Earth too. Low levels of radioactive material can be found in the soil and deep underground. Examples of radioactive material include radon gas, uranium, and thorium. These x-ray sources help contribute to background radiation.

Tycho's Supernova Remnant, bright in X-ray radiation.

One way to artificially produce x-ray waves is to accelerate electrons across a high voltage into a metal target. When the streaming electrons impact the metal they slow down and release their lost kinetic energy as x-rays. This is essentially a recreation of Wilhelm Roentgen's original experiment. Most medical and industrial x-ray machines use this technique to generate x-rays, and then apply them in a variety of different applications.

Uses of X-Ray in Medicine

X-Rays: Internal Imaging

Roentgen quickly discovered the potential for medical uses of x-rays. Organs with soft tissues such as the lungs, heart, bladder, or skin would partially absorb x-rays at different rates depending on their density, while the bones would nearly completely absorb all the incoming x-ray radiation. Bones contain calcium which has a higher atomic number than atoms in other tissues in the body, such as hydrogen, oxygen or carbon, which is a large contributor to radiological density.

X-Ray procedure

A patient is placed between an x-ray generator and a photographic plate. The radiographer operating the machinery will stand behind a thick lead plate to protect them, as they would otherwise be exposed to harmful x-rays multiple times per day. The patient has to remain completely still as motion can cause blurry images. The photographic plate will darken as it absorbs the x-rays from the generator, while white shadows will appear where the bones absorb the x-rays before they can reach the plate. This high contrast will generate a radiograph (x-ray image).

Direct imaging of a bone using x-ray, adapted from image by Wikimedia Commons

X-Ray Risks

It is the higher penetrating power of x-rays that allows us to take detailed internal scans of the human body or other objects. However, the ionising radiation of an x-ray scan can cause catastrophic damage to our DNA. The molecules that make up our genes can mutate, which might eventually lead to cancer! Before the harmful effects of x-rays were exposed, pregnant women were x-rayed to check the health and development of the foetus. Some shoe stores even used to offer a free x-ray scan of your foot with the purchase of a pair of shoes! Nowadays, health professionals always judge if an injury is worth the health risk of an x-ray.

X-Rays: Radiation dose

We can quantifiably measure how much radiation a living organism has been exposed to using the standard unit, sievert (Sv). However, the millisievert is more often used as humans in the UK are typically only exposed to about 2.7 millisieverts per year, so it is a much more convenient measurement.

$1000\mathrm{mSv}=1\mathrm{Sv}$

Types of X-Ray

Many different types of x-ray are needed for a variety of different reasons when checking the health of a patient. Sometimes multiple x-rays need to be taken to develop a cross-sectional image that can later be combined into a three-dimensional image, this is known as a Computerised Tomography (CT) scan.

Common examples of x-rays used in conventional radiography include:

• Abdominal x-rays produce images of the organs and structures within the abdomen, which include the spleen, large and small intestines, and the stomach.
• Chest x-rays are used to inspect the heart, arteries, diaphragm, lungs, and ribs.
• Dental x-rays form an image of the teeth and the mouth to check oral health. Structures such as fillings will appear white on the x-ray, while teeth, tissue, and fluids will appear in various shades of grey.
• Sinus x-rays can create an image of the sinuses, which are the spaces in front of your skull that are filled with air. This x-ray can detect any blockages in your sinuses such as inflammation or a buildup of fluid.
• Skeletal x-rays are used to detect broken or fractured bones in the body. Only the specific bones that are damaged are scanned - it is rare and usually unnecessary to x-ray an entire skeleton at once.

Other x-ray techniques used in medicine include:

X-Rays: Radiation Therapy to Treat Cancer

You might wonder how x-rays and other types of high-energy radiation can be used in cancer treatment, given we just discussed how radiation can actually cause cancer. High-energy radiation is used to kill cancer cells by damaging their DNA, just like our healthy cells. The doses of radiation used in cancer treatments are magnitudes higher than when used for imaging, so can be harmful and sometimes have serious side effects, including but not limited to hair loss, fatigue, and diarrhoea.

However, there are a couple of clever methods to target cancerous cells that minimise damage to other cells in the body. With external beam radiation therapy, we employ a machine that aims x-ray radiation at your cancer using a narrow beam from the outside, which targets only a small and specific part of the body. Alternatively, we can use internal radiation therapy, where a source of radiation is positioned inside the body near the cancerous cells or tumour. This can be done by ingestion, injection into the bloodstream, or even by surgical placement.

A child undergoing early external beam radiation therapy.