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The Magic behind Medical Images

At some point in your life, you’ll likely find yourself having to get a medical scan to ensure your body is functioning normally and healthily. If it’s a broken bone, pregnancy scan or a serious condition such as cancer, you’re guaranteed to need medical imaging. Techniques such as X-rays are one of the most revolutionary weapons used to diagnose issues within the body.

An X-Ray depicting the chest cavity
An X-Ray depicting the chest cavity

As a first-year Biomedical Engineering student, my core unit (BMET1960) dives into the science of how these fantastic technologies work. Much to my joy and surprise, it turns out that there was more physics involved in medical imaging than I had thought.

For a disease such as cancer, certain molecules like glucose are attracted to the location of a tumour. By understanding how these specific molecules operate within the body, we can exploit these properties to help us form our medical images. If we attach a given radioisotope to these specific molecules, we produce a molecule called a radiotracer. Radioisotopes are variations of radioactive elements by their mass number. Radiotracers, as the name suggests, can be used to perform some sort of “tracking/tracing” of their location in the body.

But how exactly does it work?

A diagram showing two examples of radioactive decay (Beta+ and Beta-)
A diagram showing two examples of radioactive decay (Beta+ and Beta-)

All radioactive materials have a unique property called “radioactive decay,” which causes them to lose mass exponentially over a period of time. There are various forms of nuclear decay processes (alpha, beta, gamma, etc). Radioactive decay is measured by the half-life of a radioactive element. This is the amount of time it would take for half of a given sample to decay. It is important to remember that depending on the type of decay a radioactive element undergoes, it can emit different types of radiation, including some forms of antimatter particles. These are corresponding particles to "ordinary" particles. For example, a positron is the antiparticle of an electron.

Feynman Diagram depicting electron-positron annihilation which produces detectable gamma rays
Feynman Diagram depicting electron-positron annihilation which produces detectable gamma rays

If we utilize a particular sample of a radioisotope for our radiotracer, the natural decay can create a unique signature at the tumour location. This will be identified with a detector. This is a simplified explanation of the nuclear physics behind radiation imaging. This basic concept is used in techniques such as PET (Positron Emission Tomography) scans. In this particular process, a radioisotope undergoes beta plus decay and emits a positron. The positron will then annihilate with an electron (matter-antimatter annihilation) releasing gamma rays that will be picked up by the detector.

As incredible as these machines are, radiation imaging does have disadvantages. Whilst it is powerful and accurate, placing radioactive material in the body can have its respective side effects. Therefore, it is not the safest method available. For this reason, biomedical engineers have utilized a different form of radiation to help us create medical images.

Transmission imaging techniques such as CT and CAT scans measure the amounts of electromagnetic radiation passing through materials. CT (Computational Tomography) scans utilize X-Rays! This form of electromagnetic (EM) radiation is passed through bones and tissues. It does not linger in the body as opposed to the radioisotopes from the PET scans, which remain for longer. The radiation is simply scattered and absorbed in various regions. We call this scattering the process of attenuation. Similar to how old photographic film cameras work, placing a special kind of film behind a CT scan will allow X-Rays to be detected. These rays strike at different locations with varying intensities to generate feature-rich images of our broken bones (like after attempting a daredevil move in sports). If we need a more complex reconstruction of a body part, we can use CAT (Computational Axial Tomography) scans which essentially take CT scans at multiple angles.​

CAT Scan Machine
CAT Scan Machine

With a basic understanding of the electromagnetic spectrum and nuclear physics, biomedical engineers successfully developed these machines with more reliability and safety. Personally, the most intriguing part about all of this is how fundamental science concepts, which might’ve seemed practically useless during high school, have real-life applications. Specifically, in the medical industry where it aids early cancer diagnosis and surgical/ therapeutic treatments. Who knows where you can get with Lady Physics!

By Dillon

Article References/Further Reading:

[1] The Science of Medical Imaging: SPECT and PET

[2] The science of medical imaging: X-rays and CT scans

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