Name: 粒子加速器在医院做什么（What’s a Particle Accelerator Doing in a Hospital?）
Uploaded: 2021-07-02T03:07:05.000Z
Duration: 5 min 25 s
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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,

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.

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

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 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.

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

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.

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

So, if one of these particles loses or gains a proton,

it can become an entirely different element.

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

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

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,

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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