PET or Positron Emission Tomography is a non-invasive imaging technique used in nuclear medicine, which produces images of the interior of the body. In contrast to X-rays techniques, PET provides images that measure the metabolic activity of the organs by analyzing the distribution of a radioactive marker administered via oral or intravenous. The imaging acquisition principle behind PET is the pair production effect. This effect produces a free gamma-ray (or photon) from the annihilation between a positron in the radioactive marker and an electron in the patient's body. Then, detectors get the signal and a PC reconstructs the image.