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When doctors need to quickly identify tumors,
diagnose heart diseases and even watch our brains at work,
they use the by-products of something called radioactive decay.
The decay comes from specific radioactive isotopes that emit a certain type
of antimatter particle called a positron, the counterpart to electrons.
Normally, when the positron gets in contact with things like electrons,
they destroy each other.
By carefully sneaking in radioactive molecules that emit positrons,
doctors can see what’s happening inside your body.
But here’s the catch...these molecules need to be made fresh every day,
against the clock, in the basement of a hospital.
Doctors measure the positrons emitted by the decay of a radioactive atom
with a technology called Positron Emission Tomography, or PET.
And it works by sensing gamma rays,
a type of photon emitted when positrons and electrons come into contact.
To accomplish this feat, physicists and chemists must work quickly
to incorporate radioactive isotopes into chemical compounds called radiotracers.
They do this by swapping a normal part of the molecule
for something radioactive that releases positrons.
Now you may be thinking, radioactivity sounds bad,
but it’s commonly used in different things in medicine like X-rays.
And these radioactive molecules only emit very small amounts of radiation for a short
period.
The way the radiotracers work is by accumulating in specific areas of our bodies.
For example, tumors have a faster metabolism compared to normal cells.
So when tumors use chemical compounds, like radiotracers,
instead of normal sugar, they’ll accumulate faster compared to normal cells.
Which is why tumors are more visible on a scan.
When injected, the radiotracer will travel through the entire body
and accumulate in things like tumors.
And the difference of where the tracer is hoarded compared to the rest of the body
will create a contrast in radioactive decay picked up by the PET instrument.
But making radiotracers is a race against the clock
because the very thing that makes them work,
radioactivity, also means they won’t last long.
A common radiotracer used for PET scans is fluorodeoxyglucose, or FDG, for short.
And its half-life is about 110 minutes — just under two hours.
Half-life is the time it takes for half of a radioactive sample to decay.
As soon as FDG is made in a lab,
the clock starts to tick to get the molecule purified from reaction by-products,
tested to ensure its safety, prepared in a solution, and into the patient.
Because in two hours, only half of what was made will be useful.
The journey of these radioactive molecules starts with a
compact particle accelerator called a cyclotron,
usually housed in a well-shielded room in the basement of a hospital
or at a specialized facility nearby.
The main job of the cyclotron is to create a beam of
positively charged hydrogen atoms, also called protons.
Scientists use the magnets inside the cyclotron to speed up and steer
negatively charged hydrogen atoms creating the proton beam.
To make FDG, scientists use oxygen-18,
which has two extra neutrons compared to “normal” oxygen-16.
The process starts by displacing one neutron from oxygen-18 and
adding one proton using the beam from the cyclotron.
By displacing that neutron, scientists create the isotope fluorine-18.
This change is possible because the identity of an atom is defined by
how many protons it has.
So, if one of these particles loses or gains a proton,
it can become an entirely different element.
But fluorine-18 is unstable.
Meaning that eventually,
that extra proton decays into a neutron and emits that extra energy as a positron.
Making the atom back into stable oxygen-18.
But a pile of radioactive fluorine isn’t very useful because it’s so reactive.
So one way to use fluorine-18 as a radiotracer is to incorporate it into molecules like the
sugar FDG.
To make FDG, chemists take the radioactive fluorine from the cyclotron and then
subject the sugar to a series of chemical reactions to substitute radioactive fluorine
on an area of the molecule.
Next, purified and tested FDG has to make its way to the patient,
which can be as simple as taking it upstairs in the hospital
r as complicated as driving it across a state.
Once the FDG is injected into the patient’s body,
the sugar starts to get used by different parts of the body, including tumors,
which doctors can image with the PET scanner after a few minutes.
And the scanner can be used by doctors to diagnose things like cancer,
heart disease, or Alzheimer’s.
So while it might seem strange to have a small particle accelerator in a hospital,
it’s a critical piece in the puzzle to image your body.
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