How do X-rays see through you?

Ted Ed
How do X-rays see through you?
Lesson by Ge Wang
Translator: amara.org

In 1895, a physicist named Wilhelm Roentgen, was experimenting with cathode tubes, tubes in which a ray of electrons illuminates a fluorescent screen. He wrapped the tube in cardboard, in order to contain the fluorescent light and then… something unexpected happened. A second, outer screen was shining and invisible rays were crossing through the cardboard. Wilhelm had no idea what they were, so he named them “X-rays” and his findings eventually brought him a Nobel Prize.

Nowadays we know what happened. When the high energy electrons from the cathode tube hit a metal, they either slow down and release energy or they remove the electrons from the atoms which they hit. This process forms electrons which then repeat the same. In both cases, the E energy released as X-rays is a type of electromagnetic radiation more powerful than the visible light and less powerful than gamma rays. X-rays are strong enough to penetrate several types of matter as if it is semi-transparent and they are especially useful in Medicine, because they can obtain images of organs, such as bones, without hurting them, however with a chance of producing mutations within the reproductive organs and tissues such as the tiroide. That is why we use lead aprons in order to block the X-rays.
 
When X-rays interact with matter, they hit electrons. Sometimes, X-rays transfer all their matter energy and they are absorbed. Other times, only part of the energy is absorbed and the rest is spread. The frequency of these events depends on how many electrons hit the X-rays. These collisions take place more often if the material is denser or if it comprises a higher number of atoms, which also means more electrons. The bones are denser and full of Calcium, an element with a high atomic number, therefore, they absorb a higher quantity of X-rays. On the other hand, the soft tissue is not as dense and it mainly contains elements with a low atomic number such as Carbon, Hydrogen and Oxygen. Therefore, the quantity of X-rays going through the soft tissues, such as lungs and muscles, is higher, resulting in a darker film.

These 2D images are useful up to a certain point. When X-rays go through the body, they interact with many atoms in their path. What is shown on the film, is the result of those interactions. It is just as you would print a 100 pages novel on one page only. To see what is really happening, we should obtain images of X-rays from multiple angles around the body and use them in order to build an internal image of the body.

There is a procedure called Computer Tomograph – CT, another Nobel Prize invention. Think about CT as follows: with one X-ray you could see the density of a patient’s solid tumour, however, you would not know how deep it is. With more X-rays, from different angles, you should be able to determine the position and the shape of the tumour. A CT scanner sends a X-ray cone through the patient’s body and towards a network of sensors. X-rays are rotated around the patient and, many times, moved towards the lower part of the body, following a spiral trajectory.

CT scans give us information which can be processed diagonally, detailed enough to see anatomic features, tumours, blood clots and infections. CT scans can even show heart conditions in mummies buried thousands of years ago. What started as a happy accident became a medical miracle. Nowaday, hospitals and clinics take 100 milion scans per year, treating diseases and saving lives worldwide.