At the big bang, theorists believe the universe was composed of equal amounts of matter and antimatter, which immediately annihilated each other releasing a tremendous amount of energy. However, for some unknown reason a tiny amount of the original matter remained undestroyed, and that matter is what we see as the universe today.
Antimatter is rare in the universe today because it would be annihilated when it came into contact with normal matter, but antimatter particles are constantly produced by high-energy collisions and by the decay of some radioactive elements. Last week I had a positron emission tomography (PET) scan to determine the extent of my cancer. That test is based on the annihilation of electrons and their antimatter equivalent, positrons. Here’s how it works.
The PET scan depends on the decay of an unstable isotope of fluorine, fluorine 18. This isotope is produced in a cyclotron by adding a proton to the atomic nucleus. The fluorine 18 atoms are combined with glucose to form a material that is both radioactive and recognized by the body as glucose.
When I arrived for my test I was taken to a small room with a gurney, and an IV was started. Then, the technician brought in a metal container similar to a small thermos bottle with a syringe sticking out of the top. He removed the syringe from the bottle and immediately inserted it into a metal holder. The holder looked like the clear plastic holders technicians use when taking blood samples using vacuum tubes, but it was made of metal. Then the glucose was injected into the IV.
I then had to lie on the gurney in a darkened room for about an hour as the glucose spread throughout my body. All cells take up glucose for energy, but some cells such as cancer and brain cells use more glucose than others. For example, infants’ brains consume about 87% of their metabolic energy, and even as adults our brains, which represent only about 2% of the mass of the body, consumes about a quarter of the body’s energy. At the end of my wait, the glucose was well distributed throughout my body, and I literally glowed with the gamma radiation from the decaying fluorine atoms. It is this glow in the gamma ray spectrum that makes the test possible.
Then I was taken into the room with the scanner. First a quick CT scan was done with x-rays to provide a 3D model on which to attach the results of the PET scan. Then I was slowly moved through the scanner for about 20 minutes as the machine recorded the effects of the decay of the fluorine atoms.
When a fluorine atom decays, it gives off a positron, which immediately interact with a nearby electron in the same kind of annihilation event that occurred at the big bang. When the two particles react, two gamma photons fly away in opposite directions. When they simultaneously strike detectors in the PET camera, their positions are recorded, and that information is used to calculate the position of the fluorine atom when it decayed. The results from all of the atomic decays are then combined to create an image of the body with those cells containing the most radioactive glucose highlighted. On a negative image, dark areas such as the brain, bladder, and cancer indicate high levels of glucose. You can see examples of PET scans here: http://www.economist.com/node/16349422 and http://en.wikipedia.org/wiki/File:PET-MIPS-anim.gif.
The physics and engineering underlying the PET scan are a marvel to me and make me thankful to the generations of thinkers who devised the philosophical basis for the scientific method, the physicists who uncovered the secrets of the atom, the engineers who designed the scanner, and the medical professionals who use it for the benefit of their patients. On Wednesday, I will get the results of the scan.
David
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