PET Scan Definition

Positron emission tomography or pet scan, for its acronym in English, is a non-invasive diagnostic technique belonging to nuclear medicine. The images obtained in the PET Scan provide information on the activity and metabolism of certain tissues of the human body.

Antimatter has many possible applications due to the large amount of energy it releases when reacting with ordinary matter. One of the applications that antimatter currently has is obtaining diagnostic images of the human body. The PET Scan uses positrons, which are the antiparticles of electrons, to achieve this goal.

Matter and antimatter reaction

A simple way to understand the difference between matter and antimatter is as follows: The former is made of particles and the latter is made of antiparticles. Well, but what is an antiparticle? It is a type of particle that has the same mass as its analogue, but some of its properties, such as its electric charge, are reversed.

Let us think, for example, of the positron, which is the antiparticle of the electron. A positron has the same mass as an electron, but its electrical charge is positive, even though its magnitude is equal to that of the electron. The electric charge of the electron is -e≈-1.6×10

^-19 C and the electric charge of the positron is +e=+1.6×10^-19.

When a particle and an antiparticle meet, they annihilate each other. In this process, energy and a pair of photons are generated which, when generated, travel in opposite directions.

and^–+e⁺→γ+γ (511 keV)

The energy released in these annihilations is immense, this has made antimatter an ideal candidate for use as a power source in the future. However, producing antimatter is very difficult and very expensive, so we are far from being able to use it as a source of energy. What we can currently do is use particle-antiparticle annihilations to obtain images of the human body.

How does the PET Scan work?

The PET Scan basically takes advantage of the photons released in the annihilations of electrons and positrons to generate images of certain tissues. Fluor-18 is a radioisotope that decays radioactively via β-decay⁺ to give rise to a stable isotope of Oxygen-18. In this type of decay, a proton decays radioactively to produce a neutron, a positron, and an electron neutrino.

p⁺→n+e⁺+ν_and

In the case of Fluor-18 the radioactive decay looks like:

¹⁸F→¹⁸o+e⁺+ν_and

Fluor-18 is capable of binding to a glucose molecule by substituting a hydroxyl group on it. This incorporation of Fluor-18 into glucose results in a compound called Fluorodeoxyglucose (FDG).

A PET Scan study begins by introducing an FDG sample into the patient intravenously. FDG is distributed throughout the body through the bloodstream. Glucose is the main source of energy for our cells, so they begin to metabolize FDG as if it were normal glucose.

Once inside cells, the Fluor-18 radioisotopes incorporated into the FDG molecules decay radioactively and emit positrons. Positrons quickly annihilate with the electrons around them, generating pairs of photons that travel in opposite directions. With the help of some detectors that are placed around the patient, all the pairs of photons resulting from said annihilations are collected and the places where they occurred are mapped.

What information do the images by PET Scan provide us?

The images obtained in the PET Scan show those sites where there was greater degradation of FDG, that is, where there was greater energy consumption by the cells. These a priori images are used to make metabolic evaluations of certain tissues and to be able to determine their functioning. For example, if some tissue that we know to consume a lot of energy appears dimly glowing on a PET Scan image, it could indicate a fault in that tissue.

One of the things that consume the most energy when in the body are cancerous tumors. Cancer cells are cells that divide uncontrollably at a high rate. The process of cell division requires a higher energy consumption, therefore it is to be expected that cancerous tumors use a lot of energy.

The images by PET Scan can give indications of those places where a tumor could exist malignant because they appear brighter indicating that there was a greater consumption of energy in said zones.

Although radioisotope and antimatter are used in the PET Scan study, the dose of radiation received by the patient is very low and the radioactive tracer is eventually removed from the body.