Positron emission tomography (PET) is a modern diagnostic method that has a great significance in the medical diagnosis. It allows presenting the function of organs and metabolic processes in the body of a patient while using positron radioisotopes. Radioisotopes change in time which results in their radioactive decay. In the diagnostics with PET cyclotron-produced radioisotopes embedded in a pharmaceutical are used. The most commonly used radiopharmaceutical is fluorodeoxyglucose (FDG). Radioactive fluorine decays by beta plus decay which results in the emission of positron. The process in which the emitted positron and electron from the surrounding area are canceled is called annihilation. In the process of the annihilation two gamma photons are emitted at an angle of 180 degrees in opposite directions. If a proper amount of FDG activity is applied to a patient, annihilation of positron and surrounding electrons occurs in the body. Those photons are detected with circular constructed detectors of the PET device. From the data that has been collected a synogram is made, which serves to reconstruct a picture that is presenting diagnostic information adequate for evaluating. The possibilities of positron emission tomography are mainly used in oncology, cardiology and neurology. In oncology cancer and metastasis can be detected by positron emission tomography. It can be used when selecting the ideal treatment method for the patient. In cardiology positron emission tomography is used in the visualization of myocardial function and myocardial flow, whereas in neurology it is used to show brain activity in different parts of the brain.