PET scans and antimatter

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How PET scans utilise antimatter

In 1982, Paul Dirac discovered new equations to describe how particles such as electrons behave when travelling close to the speed of light (shown below). However, upon completion, he found that his equation always had two possible solutions, much as the √4 = ∓2. For his equation, one answer described the electron, however one described something with the same mass and spin as the electron but with an opposite charge: an antielectron, also known as a positron. This is a type of antimatter. Dirac's equation proved true not only for just electrons, but also for other kinds of particles: quarks have antiquarks and because quarks make up protons, antiprotons also exist. Similarly, antiprotons and antielectrons can make up antiatoms and so on.

Positrons can be used for medical purposes in positron emission tomography (PET) scans which construct images to check for things such as checking brain function, metastasis, the body's response to a cancer treatment or examining blood flow. PET works by using a scanning machine to detect photons emitted by a radioisotope, an atom with excess nuclear energy, in the organ being examined. These radioisotope are

made by a particle accelerator called a cyclotron through this process:

1. A negatively charged hydrogen ion is injected into the vaccum chamber of the cyclotron where two D shaped plates are enclosed between the poles of a strong electromagnet

2. An alternating positive and negative voltage is sent through the D shaped plates which attracts and repels the ion in a circular path. Each time the ion passed the gap between the D shaped plates, it accelerates as it gains energy

3. On the outside of the D - shaped plates is extraction foil. When the ion reaches the outside and hits this foil, it is stripped of its electrons, leaving a positively charged proton

4. This proton travels down a beamline towards a target containing atoms. When the proton collides with the nucleus of an atom in the target, a nuclear reaction changes the atoms structure which creates the radioisotope

The radioisotope is then added to a molecule that is easily absorbed by the body such as a sugar, hormone or

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protein. Each of these performs a specific function in the body, allowing doctors to see where in the body that function is happening. Once the biological molecule and radioisotope have been synthesised it is purified and checked it will properly function before being injected into the patient's bloodstream to reach the target organ.

Inside the body, the radioisotope on the newly synthesised molecule emits a positron/ antielectron. Because the positrons and electrons are oppositely charged, when they collide and are both destroyed. Energy is produced in this collision due to the equation 𝐸 = 𝑚𝑐 2 and released as two gamma rays which travel out of the body in opposite directions. Due to the spherical shape of the PET scanner, when it detects two gamma rays on opposite sides of the ring, it is able to locate exactly where in the body the tracer is. By detecting thousands of these events every second, an image can be produced in 3D of the structure of the target organ.

By Alice Grossman

References and further reading: https://www.hopkinsmedicine.org/ health/treatment-tests-and-therapies/ positron-emission-to mographypet#:~:text=How%20does%20PET%2 0work%3F,organ%20or%20tissue%20 being %20examined. https://phys.org/news/2006-10antimatter-chemistry.html https:// www.chemistryworld.com/news/ antimatter-persuaded-to-react-withmatter/3000369.ar ticle https:// iopscience.iop.org/journal/1367-2630/ page/ Focus%20on%20Antimatter%20Physi cs% 20and%20Chemistry https://fedorukcentre.ca/resources/ what-is-a-cyclotron.php

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