Positron Emission Computed Tomography (PET) is an imaging technique that uses positron-emission-tomography (PET), or PET for short, to obtain images of the distribution of tracers in the human body as tracers of substances that are biologically active in the body, such as sugars, amino acids, fats, nucleic acids, ligands or antibodies, in order to reflect information about the body’s tissue function and metabolism. PET/CT is a new device of modern medical molecular imaging technology that integrates two imaging technologies, PET and CT, and can obtain functional metabolic images of PET, anatomical images of CT and fusion images of PET and CT in one imaging session. The whole body imaging time is about 20 minutes, and after the multi-dimensional image shows the patient as if a transparent person, various lesions can be seen by the doctor at a glance. PET imaging uses positron radionuclides produced by cyclotron, and the commonly used positronuclides are 11C, 13N, 15O and 18F, etc. The half-lives of these nuclides are extremely short, 20 minutes, 10 minutes, 2 minutes and 110 minutes, respectively, due to their short half-lives, the radiation dose for each examination is low, and most of these nuclides are important basic elements of the human body, which do not interfere with the balance of human tissue metabolism and internal environment, its systemic resolution is 4-6 mm, and it allows quantitative determination of lesions or organs. Clinical Applications of PET/CT At present, PET/CT is mainly used in three major clinical fields: oncology research (65%-85%), neurology (15%-35%), and cardiovascular diseases (15%-25%). PET is mainly used in oncology diagnosis and research for benign and malignant identification, early diagnosis and identification of tumor staging, typing, recurrence and metastasis, detection and monitoring of anti-treatment phenomena, selection of treatment protocols and monitoring of efficacy, and observation and basic research of malignant process. The most widely used positronuclide is fluorine (18F) labeled glucose (18FDG). It is well known that glucose is the main energy source of cells, and 18FDG is an analog of glucose, and the metabolic imaging of tumors can be achieved by measuring the utilization of glucose with 18FDG, and the primary tumors can be graded in vivo through the correlation between glucose utilization and tumor cell proliferation and differentiation . Since the difference of 18FDG aggregation in benign and malignant tumor lesions can be up to 90%, 18FDGPET monitors the occurrence of tumor malignancy and determines the malignancy of tumors by reflecting the degree of tumor glucose metabolism. In addition, 18FDG imaging is an accurate and reliable method to identify the nature of a single lung mass and the stage of lung cancer, and its accuracy rate is higher than that of CT or MRI. 18FDG has a clear value for the diagnosis of neuroblastoma, and has a sensitivity of 85% for the diagnosis of pancreatic cancer, while the compliance rate for chronic pancreatitis can reach 84%. 18FDG is also used for the diagnosis of many kinds of tumors, such as liver cancer, colon cancer, breast cancer, prostate cancer PET is also used to diagnose a variety of tumors such as liver cancer, colon cancer, breast cancer, prostate cancer, melanoma, bone tumors and other soft tissue tumors. For tumor imaging, PET/CT will influence tumor diagnosis and treatment decisions, right through to radiation therapy planning and monitoring of treatment efficacy. In the diagnosis and research of neurological diseases, PET imaging can be applied to cerebrovascular diseases, epilepsy, Alzheimer’s disease, Parkinson’s disease, neurodegenerative diseases, neuropsychiatric drug research and brain function research. Cerebrovascular diseases are the most common neurological diseases in clinical practice. The application of 18FDG and 15O2 dual nuclide imaging for the study of ischemia and infarction related parameters, such as local cerebral blood flow (rCBF), local cerebral oxygen metabolic rate (rCMRO2), local oxygen uptake fraction (rOEF), local cerebral blood flow volume (rCBV), has been widely used in clinical practice for the diagnosis, localization and evaluation of prognosis of early acute cerebral infarction. PET tests have revealed significant differences in blood flow changes in the early locally damaged areas during stroke, and it has been observed that the changes in functional impairment are significantly greater than morphological changes in patients with cerebral infarction when comparing 18FDG imaging with CT alone, and PET can determine whether active neural tissue can be recovered after the occurrence of cerebral infarction. Quantitative measurements can determine that brain cell death is inevitable and interventional therapy is ineffective if rCBF is below 12 ml/(100 g?min) and rCMRO2 is below 65 μmol/(100 g?min) threshold in brain tissue at the infarct site. Epilepsy is one of the common clinical diseases, and PET is extremely valuable for the localization and diagnosis of epileptic foci. 18FDG examination in epileptic patients during seizures are hypermetabolic, and interictal period shows hypometabolic areas, and PET shows localized 18FDG increase or decrease performance, which has been confirmed as epileptic foci by postoperative pathological results. According to PET localization guidance for seizure foci removal, the disease can be controlled in about 93% of patients after surgery. With the aging of our population, neurological diseases that occur in middle and old age, such as dementia and Parkinson’s disease, PET is extremely helpful in the diagnosis of these diseases. 18FDG imaging is now also widely used in the diagnosis of patients with dementia, especially for the differentiation of Alzheimer’s disease from multiple cerebral infarct dementia. The former is most commonly characterized by depressed glucose metabolism in the temporal, parietal, and frontal lobes and reduced cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2). In contrast, the latter shows multiple diffuse reductions in cerebral blood perfusion (CBF), reduced glucose metabolism, and reduced CMRO2, mostly focal or wedge-shaped and associated with a specific arterial distribution area.PET has a sensitivity of 94.6% and a specificity of 97% for the diagnosis of Alzheimer’s disease, and PET can show metabolic abnormalities prior to dementia symptoms and structural changes. Parkinson’s disease is another central neurodegenerative disease that occurs in middle and old age. It is essentially caused by abnormalities in dopamine transmission in the substantia nigra-striatal system. In the diagnosis and research of cardiovascular diseases, PET can be used for the diagnosis of occult, high-risk and difficult coronary heart disease, the detection of myocardial survival, pre- and post-interventional monitoring, the diagnosis of cardiac transplantation, cardiomyopathy and treatment follow-up observation, etc. The application of 18FDG imaging to observe myocardial metabolism is the gold standard for the diagnosis of coronary heart disease and is currently the most used item. PET can determine the possible outcomes of myocardial ischemia in coronary heart disease PET can determine whether myocardial damage is reversible or not, and has a positive prediction rate of 85% and a negative prediction rate of 92% for surviving myocardial infarction, so 18FDG-PET imaging plays an invaluable role in the screening and success rate of coronary artery bypass surgery. In conclusion, since PET has been introduced into the clinic, it is currently being explored for a wide range of applications in many medical fields, such as: child development, tissue survival, injury repair, organ transplantation, physiopathological studies, design and development of new drugs, in vivo pharmacokinetic observation and study of drug mechanisms of action at the molecular level, and drug dependence (addictive) diseases. Preparation and precautions before PET/CT: 1) The patient should voluntarily provide the physician with a medical history, including a history of diabetes, pregnancy and breastfeeding, weight, medical condition, and tolerance to the examination, and should also include information on recent imaging studies. Information on recent biopsies and surgical procedures (site, pathology results, etc.), information on radiotherapy, chemotherapy and other treatments, etc. 2) Fasting for more than 4 hours before the examination to lower blood glucose to increase FDG uptake by tumor tissue. During the fasting period most of the drug treatment can be carried out and water can be consumed. Water rinsing before the scan can reduce the FDG content in saliva, which is beneficial to the head and neck examination. 3) Patients should drink sufficient water before and after tumor imaging, 500 to 800 ml or more before and after injection, and continue to drink water after the scan. Drink more water and urinate more often to keep the bladder empty and reduce the absorbed dose from the bladder. 4) Avoid high-intensity exercise to limit muscle uptake of FDG on the day of the test. patients should remain in a quiet, resting state after the injection and during the absorption period, and should not walk around and avoid talking. 5) The FDG test theoretically requires a blood glucose level of ≤150 mg/dl. Therefore, diabetic patients or patients who are intolerant to sugar need to have their blood glucose monitored before the PET examination. For patients with high blood glucose levels it is not necessary to adjust blood glucose before FDG injection. For diabetic patients it is simply a matter of adhering to oral therapeutic medication or regular insulin application to keep their blood glucose levels as normal as possible. If the blood glucose level is very high, a diabetic specialist should be asked to cooperate in controlling the blood glucose before the test and wait until the blood glucose is lowered. (6) Although the half-life of FDG is very short and the dose equivalent of a single PET examination is only equivalent to a single CT examination, it should be used with caution in pregnant and lactating women according to nuclear medicine examination protocols, and the benefits should be weighed against the disadvantages.