receptors (as an agonist or antagonist), make that molecule radioac-
tive by replacing one of the carbons in the molecule with carbon-11,
then inject it into the person’s bloodstream. If the molecule crosses
the blood-brain barrier and enters the brain, it will preferentially
accumulate on dopamine receptors because of its affinity for these
receptors. The distribution of the receptors in the brain can then be
determined by measuring the locations of radioactive decay with PET.
The positron-emitting radioactive isotopes tend to have rather
short half-lives, ranging from 2 minutes to 2 hours. How is it possible
to do experiments with such unstable substances? The only way is
for the positron-emitting isotopes to be made on the spot and then
quickly incorporated into the desired molecular form and adminis-
tered to the subject in the PET measurement. The device used to make
these radioactive atoms is called a cyclotron. It is a gadget that was
first constructed by Ernest Lawrence (1901-1958) at the University of
California, Berkeley, around 1930. A cyclotron can accelerate charged
particles (like atomic nuclei) to very high velocities by using oscillat-
ing electromagnetic fields to impart energy to the charged particles
as they move round and round in a circular path. Such speeding par-
ticles can be smashed into other atoms, and sometimes, under certain
conditions, they will stick together and new atoms will be formed.
Cyclotrons are used to produce the various radioactive isotopes used
in PET.
Flashback to the periodic table of elements: Around 1940 physi-
cists at UC Berkeley discovered that cyclotrons could be used to
produce chemical elements heavier than any ever before seen
in nature. At that time, uranium, with atomic number 92, was
the heaviest chemical element known to exist on Earth. By bom-
barding heavy elements like uranium with protons, deuterons,