The Cyclotron and PET

The most difficult and sophisticated part of a PET installation is the cyclotron. It is a machine used to produce the radioisotopes (radioactive chemical elements) which are used to synthesize the radiopharmaceuticals (the actual substances which are used to make the functional images of the body).

The cyclotron is an accelerator of subatomic particles. It produces a large quantity of protons (heavy particles with an electrical positive charge) and get them moving at an accelerated rate along a circular orbit, inside a chamber controlled by powerful alternating electromagnetic fields. Thus, the particles gain energy and are smashed against a target at nearly the speed of light. The atoms of a substance placed in this target are transformed by this bombardment into radioactive, unstable isotopes, by means of a nuclear reaction.

There are many radioactive isotopes that can be produced in the cyclotron. In order to use them in a PET apparatus for imaging the body, they must:

• be capable of emitting positrons when they decay radioactively, i.e, when they transform from an unstable isotope into another, stable one. The positron is the anti-particle of an electron, that is, we can think of it as being a kind of electron with a positive electric charge, instead of a negative one.
• have a short period of unstability, i.e, it wil take a few minutes for any given atom of this isotope to emit a positron and become a stable element. The average measure of this instability is called half-life, which is the time it takes to decrease by half the number of radioactive isotopes of that element. The most frequently used radioisotopes in PET are:
• Carbon-11, with a half-life of 20 min
• Nitrogen-13, with a half-life of 10 min
• Oxygen-15, with a half-life of 2 min
• Fluorine-18, with a half-life of 110 min
• be readily incorporated into an useful radiopharmaceutical, by chemical synthesis. Since carbon, nitrogen and oxygen exist in practically all naturally ocurring substances of the body, many different molecules can be synthesized and used for specific imaging tasks.

This is why most of the PET installations in the world have the cyclotrons just by the side of the PET machine. It is truly a race against the clock, once the radioactive isotope is produced, to synthesize the radiopharmaceutical and get injected into the patient, so the PET and the cyclotron should be a few minutes away from each other.

The RDS cyclotron, manufactured by Siemens is one of the most frequently used in PET installations around the world. It is shown in the picture above. It incorporates a computer terminal to control the flow of production, and a biosynthesizer unit, which has several kinds of specialized apparatuses to carry out the chemical synthesis of radiopharmaceuticals.

Image credits: The Crump Institute for Biological Imaging, Department of Pharmacology, University of California at Los Angeles. The Center for Positron Emission Tomography, State University of New York at Buffalo, USA

From: The PET Scan: A New Window Into the Brain
By: Renato M.E. Sabbatini, PhD
In: Brain & Mind Magazine, March 1997.