Most people, including plenty of us still in medical training, know the basics of a PET scan on the surface. PET stands for positron emission tomography; patients get injected with a radioactive glucose tracer called fluorodeoxyglucose (FDG), and areas of high metabolic activity light up on the scan. What gets left unsaid is everything underneath it: what a positron actually is, why it exists at all, and how the answer reaches all the way down to particle physics. Physics has a reputation for being dry and hard to care about, but there’s something genuinely fun about realizing that a scan ordered every day in hospitals is, underneath the surface, a demonstration of some of the strangest rules in the universe.
Building the tracer: a cyclotron and a borrowed neutron
Before a PET scan can happen, someone has to manufacture the radioactive atom that makes it possible: fluorine-18. This isn’t sitting on a shelf ready to use. It has a half-life of under two hours, so it has to be made on-site, just hours before a patient is injected, inside a machine called a cyclotron.
The process starts with a capsule of water, but not ordinary water. It’s made using oxygen-18 instead of the far more common oxygen-16. The cyclotron uses alternating electromagnetic fields to accelerate protons (stripped hydrogen nuclei) in a tightening spiral until they’re moving at a meaningful fraction of the speed of light, then steers them directly into the oxygen-18 target.
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When a proton collides with an oxygen-18 nucleus, which normally holds 8 protons and 10 neutrons, the proton gets absorbed and the collision energy knocks a neutron loose in exchange. What’s left behind has 9 protons and 9 neutrons. That’s fluorine-18.
Ordinary, stable fluorine is fluorine-19, with 9 protons and 10 neutrons (one extra neutron than protons). Fluorine-18 is short one neutron compared to that stable ratio, and that imbalance is exactly why it’s radioactive. The nucleus is unstable and looking for a way to correct itself. Automated chemistry modules attach the newly made fluorine-18 to a glucose molecule to produce FDG, the tracer that eventually gets injected into a patient.
The decay: why a positron exists at all
To understand how fluorine-18 fixes its imbalance, it helps to know what protons and neutrons actually are. Neither one is a truly fundamental particle. Both are composite particles built out of smaller elementary particles called quarks. Protons and neutrons are each made of three quarks, specifically up quarks and down quarks in different combinations. A proton is two up quarks and one down quark. A neutron is one up quark and two down quarks.
Fluorine-18 corrects its neutron shortage by converting one of its protons into a neutron. Inside that proton, one of the up quarks converts into a down quark. That single quark-level change is enough to turn the whole proton into a neutron, since the particle’s identity is defined by its quark composition.
That conversion releases energy, and for a fleeting instant, that energy takes the form of a W+ boson, one of the universe’s fundamental force-carrying particles. It exists only briefly, borrowing its energy from the vacuum for an instant that quantum physics allows, then decays into two other particles: a positron and an electron neutrino. The neutrino escapes the body without interacting with anything and has no further role in imaging, while the positron interacts with our bodies almost immediately.
A positron is the antimatter version of an electron. It has the same mass and the same basic properties, but the opposite charge, positive instead of negative. Antimatter cannot coexist peacefully with ordinary matter for long.
Once fluorine-18 emits that positron, the atom left behind is oxygen-18. This isn’t the ordinary oxygen most people picture, but it’s a completely stable, naturally occurring isotope, not a radioactive one. Roughly 1 in every 500 oxygen atoms in the air we breathe is already oxygen-18, and our lungs and cells use it the same way they use ordinary oxygen.
Annihilation: turning matter into the signal we actually detect
The positron only travels a fraction of a millimeter through tissue before it runs into an ordinary electron. That electron belongs to one of the patient’s own atoms, part of the everyday carbon, hydrogen, and oxygen that make up cell membranes and fluid. When the positron and electron meet, they annihilate each other completely. Their entire combined mass converts into pure energy, released as two gamma-ray photons traveling in almost exactly opposite directions, each carrying 511 kiloelectronvolts (keV) of energy, a unit equal to the energy an electron gains crossing 1,000 volts.
This is the actual signal a PET scanner detects. The ring of detectors surrounding the patient isn’t looking for the positron itself. It’s looking for two 511 keV photons arriving at nearly the same instant on opposite sides of the ring. That timing lets the scanner reconstruct a line through the body along which the annihilation happened, and thousands of these lines, layered together, become the image that gets read.
Diagnosis today, treatment tomorrow?
If particles this small can already be manipulated precisely enough to find disease, a natural question follows: Could the same principles be used to treat it too, not just find it?
To a limited extent, this already exists in a field called theranostics. The idea is to pair a molecule that targets diseased tissue with a therapeutic radioisotope instead of a diagnostic one. Iodine-131, used in thyroid disease, gets absorbed by thyroid cells the same way normal iodine would, and its decay damages those cells from within. Newer agents like lutetium-177 and actinium-225 work the same way for other cancers, binding to tumor cell receptors and delivering radiation directly while sparing surrounding tissue.
Whether this approach can expand meaningfully beyond cancer and thyroid disease into a wider range of illness is still an open question, not yet settled clinical practice. But it’s hard to look at how a PET scan works and not wonder how far that same idea could eventually go.
Jack Tiller is a medical student.
