PET-CT is a combined positron emission tomography (PET) and X-ray computed tomography (CT) imaging system that can be widely used for health screening and tumor diagnosis, efficacy evaluation and monitoring, and has been regarded as the best tool for health screening and tumor diagnosis in developed countries in Europe and the United States. PET-CT is a multi-modality imaging system combining PET and CT, which is the world’s most advanced medical imaging equipment and an imaging technology that can image at the molecular level. PET images provide molecular information such as function and metabolism, while CT provides fine anatomical and pathological information. Through fusion technology, pathophysiological changes and morphological changes of diseases can be obtained in one image, achieving the effect of “1+1>2”. PET-CT examination can show the morphological changes and functional metabolic characteristics of lesions, and has high diagnostic sensitivity and accuracy, but it is impossible for any imaging method, including this method, to achieve 100% accuracy in the diagnosis of all diseases, and it needs to be combined with clinical and other examinations for comprehensive analysis. PET-CT examination features: 1. Positron emission tomography-X-ray computed tomography combined system (PET-CT) can diagnose tumor and other diseases at an early stage. Since tumor cells are metabolically active and proliferate vigorously, their ability to take up imaging agents 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 about the lesions. PET-CT can not only provide the size, scope and density of the lesion, 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 the lesion more deeply; “periodic” means to determine the development stage of the disease. 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 agent used is the basic element of human life, so the examination is very safe, without side effects such as toxicity and allergy.