Using radioactive drugs to see inside your body Pedro Brugarolas
This syringe contains a radioactive form
of glucose known as FDG.
The doctor will soon inject its contents
into her patient’s arm,
whom she’s testing for cancer
using a PET scanner.
The FDG will quickly circulate
through his body.
If he has a tumor,
cancer cells within it will take up a
significant portion of the FDG,
which will act as a beacon
for the scanner.
PET tracers such as FDG
are among the most remarkable tools
in medical diagnostics,
and their life begins in
a particle accelerator,
just hours earlier.
The particle accelerator in question
is called a cyclotron,
and it’s often housed in a bunker
within hospitals.
It uses electromagnetic fields to propel
charged particles
like protons faster and faster
along a spiraling path.
When the protons reach
their maximum speed,
they shoot out onto a target that contains
a few milliliters of a type of water
with a heavy form of oxygen
called oxygen-18.
When a proton slams into one of these
heavier oxygen atoms,
it kicks out another subatomic particle
called a neutron.
This impact turns oxygen-18
into fluorine-18,
a radioactive isotope that can be
detected on a PET scan.
In a little under two hours,
about half the fluorine will be gone
due to radioactive decay,
so the clock is ticking
to get the scan done.
So how can fluorine-18 be
used to detect diseases?
Radiochemists at the hospital can use
a series of chemical reactions
to attach the radioactive fluorine
to different molecules,
creating radiotracers.
The identity of the tracer depends on what
doctors want to observe.
FDG is a common one because the rate at
which cells consume glucose
can signal the presence of cancer;
the location of an infection;
or the slowing brain function of dementia.
The FDG is now ready for
the patient’s scan.
When a radiolabeled tracer
enters the body,
it travels through the circulatory system
and gets taken up by its target—
whether that’s a protein in the brain,
cancer cells, or otherwise.
Within a few minutes,
a significant amount of the tracer has
found its way to the target area
and the rest has cleared from circulation.
Now the doctors can see
their target using a PET,
or positron emission tomography, scanner.
The radiation that the tracer emits
is what makes this possible.
The isotopes used in PET decay
by positron emission.
Positrons are essentially electrons
with positive charge.
When emitted, a positron collides
with an electron
from another molecule in its surroundings.
This causes a tiny nuclear reaction
in which the mass of the two particles is
converted into two high-energy photons,
similar to X-rays,
that shoot out in opposite directions.
These photons will then impact an array
of paired radiation detectors
in the scanner walls.
The software in the scanner
uses those detectors
to estimate where inside the body
the collision occurred
and create a 3D map of the
tracer’s distribution.
PET scans can detect the spread of cancer
before it can be spotted with
other types of imaging.
They’re also revolutionizing the diagnosis
of Alzheimer’s disease
by allowing doctors to see amyloid,
the telltale protein buildup that
otherwise couldn’t be confirmed
without an autopsy.
Meanwhile, researchers are actively
working to develop new tracers
and expand the possibilities of what
PET scans can be used for.
But with all this talk of radiation and
nuclear reactions inside the body,
are these scans safe?
Even though no amount of ionizing
radiation is completely safe,
the amount of radiation the body receives
during a PET scan is actually quite low.
One scan is comparable to what you’re
exposed to over two or three years
from natural radioactive sources,
like radon gas;
or the amount a pilot would rack up
from cosmic radiation after
20 to 30 transatlantic flights.
Most patients feel that those risks
are acceptable
for the chance to diagnose
and treat their illnesses.