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Positron emission tomography (PET) is an advanced imaging method in the field of nuclear medicine. It uses radioactive materials to produce positrons in the body and interact with other electrons in the body, enabling us to scan and image the body. This is particularly useful for the diagnosis of early diseases, such as the early detection of tumors.

Before introducing its principle in detail, we must first understand the source of positron. Positrons come from radioactive materials, which are generally needed by the human body, such as glucose, protein and so on. One of these elements is then replaced by short-lived radioisotopes such as carbon-11, nitrogen-13, oxygen-15, fluorine-18, etc.

Today, I will focus on the more commonly used substance: fluorodeoxyglucose (FDG). It is essentially a glucose analogue. compared with glucose, an oxygen atom has been removed and replaced by the radioisotope fluorine-18. Apart from this, all chemical behaviors, like glucose, are used by cells to generate energy.

Next, we look at the specific radioisotope: fluorine-18, which has a mass of 18 and a proton number of 9, so it has nine neutrons. This form of fluorine is radioactive and decays into oxygen, so FDG becomes normal glucose.

As shown in the figure, we can see that the mass number of oxygen remains the same, while the number of protons decreases to 8, so it decays in a way that one proton becomes a neutron and releases a positron. Positrons are antimatter counterparts of electrons, and they all have the same properties except for different charges. The important thing is that positrons cannot coexist with electrons, and they will annihilate when they come into contact with each other.

Therefore, the positron does not travel very far in the body, it basically collides with the electrons in the body where it is produced, annihilates each other and converts the mass into energy, which is special in that it produces two gamma radiation photons. More importantly, these two photons propagate outward at 180 degrees. On the periphery of the human body is a circle of gamma detectors, the two photons arrive at the relative position of the detector almost at the same time, and then because the two photons pass through different human tissues, their attenuation will be different. The computer can determine the position through a series of complex calculations. However, to accurately calculate where FDG is absorbed and image, the computer needs to do as many as 1 million analyses.

However, if you only do PET, the resolution of the resulting image is very low. As a result, it is often combined with other techniques, such as CT, to better show the exact location of some organs. So, with the combination of the two, we can locate the exact location of metabolism and distinguish the intensity of metabolism by brightness. In general, cancer cells need more sugar, which can lead to abnormal brightness in some areas. Therefore, PET is very suitable for the detection of early cancer.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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