One of the most amazing and enigmatic discoveries in quantum physics was the prediction and later discovery of antimatter, that consists of particles with properties opposite to those of matter, as consequence, when they meet they annihilate, transforming into energy in the form of gamma radiation, according to the famous Einstein’s formula: E = mc 2 . This property of annihilation and consequent production of gamma radiation, with specific properties, is used in the medical imaging technique called “Positron Emission Tomography”, abbreviated as PET. Nowadays several laboratories and clinical centers possess this kind of facilities and daily perform tents of exams and medical images making use of antimatter, quantum physics and relativity. In this article we review shortly the physical foundations of PET technique as well as some of the main working characteristics and some applications in medical diagnostics.