What is a cyclotron?
A cyclotron is an electrically powered machine that accelerates charged particles to high speeds and beams them at a suitable target, producing a nuclear reaction that creates a radioisotope.
Cyclotrons do not use uranium or produce difficult to dispose of fission product wastes. When operating, the cyclotron is surrounded by an intense field of radiation, but this disappears quickly when the machine is switched off.
Cyclotrons belong to a class of machines called particle accelerators. These exist in two varieties, linear and cyclic. Both create charged particles and accelerate them to high velocities to bombard target materials. Linear particle accelerators work, as the name suggests, by accelerating particles in a straight line. Cyclic particle accelerators, such as the cyclotron, make the particles travel many times around a central point, thus achieving higher acceleration than is possible with linear accelerators.
How a cyclotron works?
Charged particles (ions) created from a suitable source material are injected into the centre of the cyclotron. The ions are then forced to travel in a circular path around a central point and repeatedly accelerated by electrical fields. During acceleration, the charged ions are forced by a strong magnetic field to travel in an outward spiral path, in an evacuated gap between magnetic poles. As the speed of the particle beam increases, the spiral path of the particles increases in radius until, when the desired speed is reached, the beam is extracted from the machine at the ion extraction point. The extracted beam is guided by magnets to one of several possible targets. When the target is bombarded by the beam of particles, a nuclear reaction occurs, altering the physical composition of the target material and producing radioactivity. When, for example, nitrogen gas is used as a target, it is converted into radioactive carbon. Commercial medical cyclotrons are predominantly designed to produce a beam of protons.
What is PET?
Positron emission tomography (PET) is a nuclear medicine imaging technique which produces a three-dimensional image or map of functional processes in the body. The system detects pairs of gamma rays emitted indirectly by a positron-emitting radioisotope, which is introduced into the body on a metabolically active molecule. Images of metabolic activity in space are then reconstructed by computer analysis, often in modern scanners aided by results from a CT X-ray scan performed on the patient at the same time, in the same machine.
To conduct the scan, a short-lived radioactive tracer isotope, which decays by emitting a positron, which also has been chemically incorporated into a metabolically active molecule, is injected into the living subject (usually into blood circulation). There is a waiting period while the metabolically active molecule becomes concentrated in tissues of interest; then the research subject or patient is placed in the imaging scanner. The molecule most commonly used for this purpose is fluorodeoxyglucose (FDG), a sugar, for which the waiting period is typically an hour.
As the radioisotope undergoes positron emission decay (also known as positive beta decay), it emits a positron, the antimatter counterpart of an electron. After travelling up to a few millimeters the positron encounters and annihilates with an electron, producing a pair of annihilation (gamma) photons moving in opposite directions. These are detected when they reach a scintillator material in the scanning device, creating a burst of light which is detected by photomultiplier tubes or silicon avalanche photodiodes (Si APD). The technique depends on simultaneous or coincident detection of the pair of photons; photons which do not arrive in pairs (i.e., within a few nanoseconds) are ignored.
