Molecular imaging is an attractive technology that is widely used in clinical practice and greatly improves the understanding for the assessment of body functions and for the diagnosis and treatment of diseases. It is a multidisciplinary technique that defines in vivo characterization and qualification of biological processes at the molecular and cellular levels. Among all molecular imaging modalities, positron emission tomography (PET) and single photon emission computed tomography (SPECT) occupy a special position that can visualize and measure physiological processes in vivo using high-affinity and highly specific molecular radioactive tracers as imaging probes.