Positron Emission Tomography scanning
Including all Physics components
This tutorial covers mainly the Physics parts of Positron Emission Tomography. The radioactive tracer and biological molecule parts are more Chemistry and Molecular Biology- which are fields I don’t have as much knowledge in. I will however try to be comprehensive in explaining as much as I can, assuming very little prior knowledge.
Side note I — What makes things radioactive?
Alpha particle — not relevant to this article:
The nucleus of an atom contains positively charged protons and neutral neutrons. Charged particles experience electrostatic repulsion to particles with the same charge — so its interesting that the atom is so together.
The reason the atom is together is because of a force called the strong nuclear force. The strong nuclear force pushes against the atom from all directions* and overcomes the forces of repulsion between the protons — except while it is very strong it works at very short distances. What this means is, if the radius of the nucleus exceeds a certain amount, the strong nuclear force is no longer able to keep things together and the atom becomes radioactive. This is also why radioactive isotopes exist and why nuclear fission happens.
Isotopes of an atom have different numbers of neutrons, this makes some radioactive because if there are many more neutrons and the radius of the atom increases considerably — as I just explained — the atom becomes radioactive.
This isn’t really relevant to this article itself but nuclear fission involves bombarding a radioactive material with neutrons at such a high velocity that the strong nuclear force is rapidly overcome — as opposed to gradual radioactive decay. When the strong nuclear force is overcome rapidly, the energy that went into it has to go somewhere and as a result the material heats up and this is where the energy comes from.
Finally, if you want to know more about the strong nuclear force, I’d like to begin by telling you it has a special name called the “Gluon” and is part of a family called the “gauge bosons” and is a force carrying particle — which I will talk more about with Beta radiation.
In beta radiation, which happens when there are too many neutrons in the atomic nucleus, a neutron turns into a proton and an electron, plus an anti-neutrino are emitted.
This goes quite a bit more into the fundamental nature of Physics, but in case you didn’t know, protons and neutrons can be divided further into quarks.
Step 1: the Cyclotron
This part is to make a biological molecule called a tracer. This is the key to PET scans. This needs to be manufactured using a very interesting machine called a Cyclotron. The Cyclotron is essentially a miniature medical-specialised particle accelerator (a particle accelerator, as it suggests, accelerates particles for various uses, for example the Large Hadron Collider at CERN). The Cyclotron is made up of a disc sliced in half through the middle to form a positive and negatively charged end. The particle begins at the centre of the gap between the two discs. All moving charged particles (such as protons, electrons and as you’ll learn POSITRONS). The charge of the plates creates an electromagnetic attraction to the oppositely charged plate, for example if it is a positively charged molecule, it will be attracted to the negatively charged end. But, when the particle reaches that specific disc-half, the polarity switches, the negative half-disc becomes positive and vice versa. This continues at a very, very high rate and results in a highly accelerated particle. The acceleration causes radioactivity because when a particle is accelerated
it gains radioactivity since the nucleus gets excited due to the transfer of energy from the incoming proton resulting in the formation of a radionuclide.
This results in a positron emitting radionuclide particle which is quickly injected into the patient (through an intravenous vein), to prevent radioactive decay. The patient is then rolled into the PET scanning machine.
Step 2: Positron emission and detection
The radioactive tracer emits positrons. Positrons are essentially antielectrons. In Physics, antiparticles destroy their corresponding particles and release ionising gamma radiation on all sides in doing so.
The PET scanning machine is essentially a circularly arranged set of devices called CCDs or Charge-Coupled-Devices, which are essentially semiconductor devices that convert incoming photons to a charge.
They work based on a quantum effect similar to the photoelectric effect. When photons hit the valence electron, it causes the electron to become excited and exit the shell which causes the current to flow. Note that no ionization happens, because electrons return to the shell immediately since it is a closed circuit. A greater current flow means that the incoming photons have a greater energy density which essentially means that there are more of them. This allows doctors to get a high quality, 3D view.
*This is a simplified explanation, but on a deeper level, protons and neutrons contains quarks — and these quarks are bound together with the strong interaction (a quantization of the strong interaction is known as the gluon, the same way the quantization of the electromagnetic force is known as a photon). Some of the force can, so to say, “leak out” and interact with particles surrounding the nucleon [proton or neutron]. This is called the residual strong force, and this works at very short distances.