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Center For Positron Emission Tomography (CPET) |
A positron is an anti-matter electron (yes, there is such a thing as anti-matter!). It is identical to the electron in mass, but has an opposite charge of +1 (the electron is defined to have a charge of -1)
Positrons can come from a number of sources. However, in PET they are all produced by nuclear decay. Basically unstable nuclei are produced in a cyclotron by bombarding target material with protons. A typical reaction is to have a bombarding proton enter the nucleus of the target material and kick a neutron out in the process. For example, bombarding 18-O (an isotope of oxygen that has two extra neutrons relative to the familiar 16-O) results in the proton being captured and a neutron being ejected from the target nucleus. Changing the number of protons in the nucleus changes the atomic species and in this case the atom is changed from oxygen to fluorine. You can represent this as [18-O + proton => 18-F + neutron]. Other reactions are also possible, e.g., nitrogen to carbon with the participants being [14-N + proton => 11-C + alpha] (an alpha particle is composed of two protons and two neutrons). The new nucleus created in this manner is unstable and eventually decays into a more stable form. The time it takes for this decay to occur depends on the particular species created, and can range from the almost instantaneous to thousands of years.
In PET the target material is chosen so that the product of the bombardment decays to a more stable state isotope by emitting a positron. Taking 18-F above, the nucleus has too many protons in it to remain stable so one of these protons decays into a neutron and, in the process, emits a positron and a neutrino. This can be represented as [proton (+1 charge) => neutron (0 charge) + positron (+1 charge) + neutrino (0 charge)]. After the decay, we're left with 18-O again (the original target material), a positron, and a neutrino. The neutrino is an odd little particle which has no mass, no charge, and travels at near the speed of light. It can easily pass through a planet without interacting with anything so for all intents and purposes once created, it is out of the picture. The positron on the other hand, being the anti-matter electron that it is, has a different fate in store for it.
Left over energy from the nuclear decay process is shared between the positron and the departing neutrino. This energy takes the form of kinetic energy i.e. motion. Because of the need to conserve energy and momentum (fundamental conservation laws we can thank physics for providing us with) the positron is forced to hang around in this world - colliding with other particles and loosing kinetic energy - until it almost comes to rest. Once the positron has lost most of its kinetic energy it is ready to participate in the annihilation reaction that Star Trek has prepared us to expect of mater/anti-mater contact. The positron will encounter an electron and, now free to obey the momentum/energy conservation laws, the two completely annihilate each other - converting all their mass into energy. The result of the annihilation process is two photons (light). Conservation of energy says that the system before annihilation (positron rest mass energy, electron rest mass energy, small bit of remaining kinetic energy) must be the same as the energy of the system following annihilation (two photons). Conservation of momentum dictates that the momentum of the system before annihilation - which is basically zero since the positron and electron are almost at rest - be the same as the momentum of they system following annihilation. The only final system state which is allowed under these conditions is one in which the two photons travel off in opposite directions (so the net momentum of the system is zero), and each has an energy corresponding to half the energy of the initial system. Since the rest mass of the positron and electron are identical (511keV), each photon has 511keV of energy - remember, thanks to Einstein we know mass and energy are essentially equivalent things so we can speak of the mass of a particle in terms of energy units. Also note: you may have noticed some hedging words used, like "basically". This is because there is a small bit of kinetic energy and net momentum in the initial system when annihilation occurs, and this has to make its way into the final system. This is done by having the photons fly off at not quite 180 degrees from each other, and have energies a little off the ideal 511keV rest state annihilation value. However, for all intents and purposes these deviations are small and can be ignored.
The positron is now no more, and we've got two 511 keV photons flying off at 180 degrees from each other. The next step in PET is to detect these photons with a PET camera. Detecting the photons allows us to determine where they came from (the site of the annihilation). Assuming that site is in close proximity to the nucleus which released the positron in the first place, we will have spatially located the site where that nucleus was when it decayed, and finding out where this nucleus goes in the body (riding the chemical compound we want to follow) is what the whole game is about.