Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) are non-invasive imaging techniques that combine both high sensitivity and spatial resolution. In addition, these techniques enable the visualization of how the body is functioning at the cellular and molecular levels just by administering a tiny amount of radioactive substance (radiotracer or radiopharmaceutical) into the living subject. These techniques have a tremendous impact in our society since they are broadly used every day in the clinical arena for highly sensitive diagnostics of diseases, evaluation of therapy response or to better understand the mechanism of the disease under analysis, among other applications.