Friday, January 18, 2013
The US National Oncologic PET Registry lists 2,272 facilities that are approved to conduct PET scans for cancer screening or monitoring. Thus, the use of these machines in the US, while perhaps not exactly routine, is not rare or exotic. But there is something exotic about PET scans--they depend upon matter-antimatter annihilation to produce their images.
Our everyday world is made up of matter and energy (which are convertible). Atoms are a basic unit of matter comprised of subatomic particles called protons, neutrons, and electrons (except the most common form of the element hydrogen has one proton, one electron, and no neutron). Likewise, antimatter is comprised of antiparticles which have the opposite charge and quantum spin of ordinary particles (there are no known naturally-occuring antimatter atoms).
Antimatter particles ("antiparticles") are extremely rare. According to physicist Brian Odom: "In molecular gas clouds in our galaxy, there is less than 1 antiproton for every 1015 protons." Nevertheless, antimatter is produced naturally by cosmic rays colliding with the Earth's atmosphere, lightning, and the beta decay of natural occurring radioisotopes, such as the Potassium-40 found in bananas.
Any way, let's get back to the PET scanner. The figure below illustrates how it works using an artifically-produced radioisotope, fluorine-18 (18F) as an example. If it is intravenuously administered in the form of the glucose ("blood sugar") analog 18-fluorodeoxyglucose then the 18F will be concentrated differentially based upon different rates of glucose metabolism. For example, cancer tumors typically have higher rates of metabolism than surrounding tissue.
18F has a half-life of 109.8 minutes. So, after 109.8 minutes half of the 18F atoms will have decayed into the rare, but stable, oxygen-18 isotope, usually (96.73% of the time) by emitting a positron (an antiparticle also called a "positive electron"). That positron will immediately collide with an electron (a negatively-charged matter particle). When it does the positron and electron will annihilate each other--mass is converted to energy--yielding two 511 keV gamma photons or gamma rays. These pass through the patient's body and are detected by the gamma ray detectors.