PET, positron emission tomography, is a technique that uses radioactive atoms to release positrons that annihilate with electrons to release gamma photons, which are detected by the scanner. It generates anatomical pictures of the body and is also able to find metabolically active parts, which is good for finding tumors, because they are metabolically very active. In PET a lot of different radioactive isotopes or radioisotopes are used. The most commonly used isotope is fluorine-18, it has An fluorine-18 has 9 protons and only 9 neutrons, while a normal fluorine has 9 protons and 10 neutrons. An isotope is when the number of neutrons differs from the number of the original atom.
An isotope is not very stable and likes to decay. The radioisotope of fluorine decays into oxygen, whereby a proton turns into a neutron and thereby a change in atom occurs and thereby a loss of positive charge occurs. Due to the loss of positive charge, a positron (positively charged electron) comes to life (e+). A positron is the antimatter particle of an electron. When an electron meets a positron, they will annihilate. During annihilation they destroy each other and only energy will be left. This energy is emitted as 2 gamma photons in exactly the opposite direction to each other (180 degrees). These photons have a very high energy and thereby a very high frequency.
The energy of photons is encoded by their frequency. Radioactive isotopes attach to glucose, because it goes to metabolically activate parts in the body and therefore this radioisotope also goes to the metabolically active tissues. There the annihilation takes place. The radioactive isotope of fluorine binds to glucose via the removal of a hydroxyl group (OH) from glucose and then binding to glucose.
This type of glucose is called fluorodeoxyglucose, also called FDG (when the normal fluorine is bound, not the isotope). When the radioactive isotope of fluorine is bound to glucose, the glucose is called 18F-fluorodeoxyglucose. The 18F-FDG is injected into the person that is thought of to have a tumor. The 18F-FDG goes into the bloodstream and transported to metabolically active tissue, cells or tumors. There it goes inside the cell and is transformed into 18-FDG-6-phosphate by enzymes involved in glycolysis by adding a phosphate group to the 6th carbon atom of glucose.
The enzymes that catalyse this reaction are hexokinase (found in skeletal muscle) and glucokinase (found in liver cells and sometimes cancerous cells). When normal glucose is transformed into glucose-6-phosphate, it goes into respiration (glycolytic pathway). However, this does not occur for 18-FDG-6-phosphate. The 18-FDG-6-phosphate accumulates into the cell and the cell cannot get rid of it due to the fact that it due does not undergo respiration. The 18-FDG-6-phosphate is called the radioactively-labeled probe or PET probe. The positron travels 1 or 2 millimeters and then finds an electron to annihilate with. The PET scanner detects the photons with detectors that are all around the patient. The photons do not necessarily come off vertically but can also come diagonally or horizontally.
Due to the detection on 2 sides of the patient, the place of annihilation can be known. By comparing the time until the upper photon reaches the detector and the time until the lower photon reaches its detector, it can be worked out where the annihilation has taken place and so where the 18-FDG-6-phosphate has gone inside the body. Thereby, you can visualize the metabolically active tissues in the body. A tumor will show up as a place from which a lot more photons will arise than other metabolically active parts of the body. Eventually, you can work out a picture from the density of the annihilations throughout the body and thereby you can see how metabolically active each tissue is in the body. PET scans expose you to ionizing radiation.
The dose of radiation that you get is 8 to 25 milliSievert. Sievert tells you how damaging the dose of radiation is. The radiation from the background that a normal person gets in a year is around 3mSv.