With the development of genomics, small animals are becoming more and more important in modern molecular biology experiments. High resolution small animal PET appears as an important tool for molecular imaging. In vivo gene imaging, gene therapy and drug research. PET is a kind of functional imaging in vivo, which can be used for tumor metabolic imaging, receptor imaging, gene expression and so on. CT can not only provide accurate attenuation correction and anatomical localization for PET, but also provide morphological diagnosis information. The combination of CT with rich anatomical details and functional imaging of PET is the trend of molecular imaging.