The fundamental principles underlying PET
The method of diagnosis is based on the fact that some substances characteristic of the metabolism of a person as a whole and of cancer cells in particular are labeled with a radioactive label and then introduced into the human body. Such a compound is called radiopharmaceutical – RFP. Subsequent detection of decay products allows the construction of a three-dimensional map of the label distribution in the body to determine absorption regions that are uncharacteristic for a healthy person. An important feature of the PET technique is that the dominant decay mechanism is beta-plus decay, i.e. Decay with the formation of a positron.
PET / CT (positron emission, combined with a computer) tomograph GE Discovery 610. Image taken from the official website of GE Healthcare. Note A vertical stance at the patient's feet is a breath control system.
It is worthwhile to make an advance toward quantum mechanics. The annihilation of a positron and an electron does not occur instantaneously. A positron emitted by a radioactive label, when it meets an electron, forms a bound state – "positronium." Both the electron and positron are fermions, so the total spin of the bound state can be zero (para-positronium) or unity (ortho-positronium). The lifetime of para-positronium is of the order of 0.1 ns, whereas ortho-positronium is 3 orders of magnitude larger. Para-positronium can decay only into an even number of gamma quanta, ortho-positronium, on the contrary, only to an odd number of gamma quanta. This behavior stems from the conservation laws of quantum-mechanical parities and symmetries. In view of the low positron energies in the case of PET, we can assume that only 2-photon and 3-photon decays are possible. In addition, the positron in the composition of ortho-positronium, due to a much longer lifetime, can react with other electrons of the medium with the transition from ortho to para-state. In fact, the dominant decay mechanism is the decay with the formation of 2 gamma quanta, although from the quantum-mechanical point of view the formation of ortho-positronium is 3 times more likely. The above is true only for dense media, which is the human body. It is important that the emitted gamma quanta have the same 511-keV energy and are scattered in strictly opposite directions. Within the framework of quantum mechanics, this statement can be proved rigorously, within the framework of the mechanics of the macro-world, it is possible to imagine this: as long as the energy of positronium exceeds 1022 keV (total energy of electron and positron rest), positronium "lives and moves", losing energy in interaction with matter. As soon as the positronium energy drops to 1022 keV, i.e. It "stops", annihilation occurs with the emission of 2 gamma rays at 180 degrees with the same energy. 19459008 19459006 19459011 19459008 19459006 19459009 Diagrams of the decay of para-positronium and ortho-positronium
The registration of emitted gamma quanta allows the decay point to be determined with high accuracy. An event is the simultaneous recording of 2 gamma quanta on opposite sides of a ring detector
All isotopes used for PET are short-lived. The half-lives of the most widely used isotopes are 18F (fluorine-18) 109 minutes, 11C (carbon-11) 20 minutes, 13N (nitrogen-13) 10 minutes. One of the shortest-lived, used in PET-15O (oxygen-15) with a half-life of 122 seconds. In view of this fact, the only way to obtain isotopes for PET, with the exception of fluorine, is synthesis in situ at the cyclotron. At the word "cyclotron", the LHC is immediately recalled, fortunately, medical cyclotrons for PET are much more compact. The characteristic size is 3 m, the characteristic energy of the protons is up to 30 MeV.
Cyclotron GE PETtrace 800. Image from the official brochure GE Healthcare
After operating in the cyclotron, the isotope enters the specialized laboratory where the synthesis takes place Necessary RFP. The received RFP is subject to mandatory testing in the quality control laboratory to confirm that the substance obtained is the required RFP, does not contain toxins and is safe for administration to the patient. After receiving confirmation from the laboratory of quality control, RFP is administered to the patient and a CT scan (PET / CT or PET / MRI) is performed.
One of the most common (if not the most common) RFP for PET is 18F-FDG (fluorodeoxyglucose), in fact – a glucose molecule labeled with a fluorine-18 atom. When dividing, cancer cells absorb glucose very actively, respectively, if the picture shows a region with a large amount of glucose, uncharacteristic for healthy metabolism, then the growth of a cancerous tumor is likely in this area.
Molecule 18F-FDG. Instead of one of the OH groups, an atom of 18F is attached
It is important to note that PET is a functional method, whereas CT or MRI is anatomical. Those. If there is a tumor in very early stages, then it will not stand out against a healthy organ on CT or MRI, while PET will already "glow". Accordingly, in order to obtain the complete picture, it is necessary to combine the two methods – PET sees the tumor, and CT or MRI gives an exact anatomical attachment to the organ.
Consecutive images of CT, PET and PET / CT . Image from the Internet
PS: Seldom, where it is mentioned, but the PET method is used not only to diagnose cancer, but also to study the functions of internal organs. For example, the method has found wide application in cardiology when examining the functions of the heart.