1. Positron emission tomography-X-ray computed tomography combined system (PET-CT) can diagnose tumors and other diseases at an early stage. Since tumor cells are metabolically active and proliferate vigorously, their ability to take up imaging agent is several times that of normal cells, forming obvious concentration points on the image, so that tiny lesions that are not easily detected by other imaging can be detected before anatomical changes are produced in the early stage of tumor. 2.PET-CT can obtain whole body image quickly in one examination. Other imaging examinations only scan a selected part of the body, which is easy to miss lesions in other parts of the body, while PET-CT takes only about 20 minutes for a single scan, and can obtain PET, CT and fusion of the two in various cross-sections and three-dimensional images of the whole body, which can visually see the parts and distribution of lesions involved in the whole body. 3.PET-CT can achieve the “four definitions” of medical imaging diagnosis. In other words, “localization” refers to the detection of lesions and the clarification of lesion sites; “characterization” refers to the clarification of the pathology and pathophysiology of lesions (benign and malignant differentiation); “quantification” refers to the fact that PET-CT not only provides the size and scope of lesions, but also provides the information on the distribution of lesions. PET-CT can not only provide the size, scope, density and other values of lesions, but more importantly, it can provide functional and metabolic indicators (glucose metabolic rate, local blood flow, oxygen consumption, etc.), which can reflect the nature and extent of lesions more deeply; “periodic” means to determine the stage of disease development. 4.PET-CT examination has good cost performance, although the cost of PET-CT examination is high, but if the indications are strictly grasped, and the results achieved, the final results can save about 20%-30% of the cost for the majority of patients, and more importantly, about 30% of patients can avoid unnecessary surgical treatment. PET or PET-CT has been used for nearly 30 years in developed countries such as the U.S. The radiation produced by radionuclides and X-rays is within the safe range, and the imaging agents used are the basic elements of human life. Therefore, PET-CT can be widely used in health checkups.