PET/CT is a new technology that combines the two diagnostic imaging techniques of PET and CT. PET its full name is: Positron Emission Tomography (Positron Emission Tomography, referred to as PET). CT’s full name is: Computed Tomography (Computed Tomography, referred to as CT). PET is a functional imaging technique, which can diagnose early and subtle lesions in organs and tissues, but the physiological and anatomical location of the lesion area and the shape of the lesion area are not diagnosed accurately enough. CT is an anatomical imaging technique, which can diagnose the precise anatomical location and shape of the lesion area, but cannot diagnose the early stage and subtle lesions of the tissue. PET/CT is a combination of these two technologies, where both CT and PET scans are performed on a single machine, and the images from the CT scan and the PET scan are fused together with special software. In this way, images with detailed information about the diseased tissue (benign, malignant, location and shape of the diseased area) can be obtained in a single scan. Compared to the method of diagnosing tumors by scanning with PET and CT individually and then superimposing the resulting images, due to the change in position of the patient during the two scans, the two images cannot be tightly and linearly superimposed, and the accuracy of the diagnostic information will be greatly reduced. PET/CT with the same frame solves this problem and the accuracy of diagnostic information is greatly improved. PET production is a kind of “nuclide tracer impact technology”. The principle is that the substances needed for human metabolism, such as glucose, proteins, nucleic acids, fatty acids, etc., are labeled with short-lived radionuclides (e.g., 18F) and made into a visualizing agent (e.g., fluorodeoxyglucose (FDG)) that is injected into the body and then scanned and imaged. As the molecular structure of the material made into the imaging agent is destroyed, it cannot be decomposed normally in the cell, and the nuclide in the imaging agent is retained in the cell. Coupled with the metabolic state of the body’s tissues and organs, the metabolic state of diseased tissues and normal tissues are different, and the distribution of radionuclides in different tissues and organs, normal and abnormal tissues and organs are also different. PET equipment can display the distribution of nuclides in the human body by capturing γ photons, and then process the received information through the computer to identify whether there are lesions in the human body tissues and organs, and what tissues have lesions. Because PET production technology utilizes physiological and biochemical activity mechanism, PET production technology is also called biochemical imaging or functional molecular imaging technology, which is currently the only imaging technology that can complete biological imaging at the level of living molecules. Cancer cells divide rapidly and have a high metabolism, consuming larger amounts of glucose than normal cells. As an energy source for division, glucose accumulates in the tissues of cells with a particularly high metabolism. Using this principle, PET scanning uses a tiny amount of fluorinated deoxyglucose (FDG for short) with positrons to be injected into the body: it is transmitted to the body’s tissues and organs through the blood circulation, and the gamma rays emitted by positrons when they decay are utilized to detect the presence of the rays with the PET scanner, and through the supercomputer’s calculation and combination of effects on the province’s various layers, the cancer cells with particularly vigorous metabolism can be seen, providing physicians with a diagnosis and judgment. High sensitivity. PET is a kind of imaging technology reflecting molecular metabolism, when the disease is in the early stage of molecular level changes, the morphology and structure of the lesion area have not yet shown abnormalities, and MRI and CT examination can not be a clear diagnosis, PET examination can find out where the lesion is and obtain three-dimensional images, and can also be quantitatively analyzed to achieve early diagnosis, which is currently incomparable to other imaging examinations. High specificity: It is difficult to judge whether a tumor is benign or malignant when it is detected by MRI or CT, but PET can make a diagnosis based on the high metabolism of the tumor. Whole-body imaging: PET can obtain images of all areas of the body in a one-time whole-body imaging examination. Good safety: Although the nuclides used in PET examination have a certain degree of radioactivity, the amount of nuclides used is very small, and the half-life is very short (2 minutes – 110 minutes), and after physical attenuation and biological metabolism, the time of retention in the body of the examinee is very short. The radiation exposure dose of a PET whole-body examination is much smaller than that of a conventional CT examination of a part of the body, and is therefore safe and reliable. Using PET scanner’s high resolution and high sensitivity for the scanning of the whole province, due to the rapid division of cancer cells, active metabolism, the uptake of fluorinated glucose up to two to ten times the characteristics of normal cells, resulting in the appearance of obvious “points of light” on the scanning image do not have to wait until the tumor tissue to produce structural changes, that is, in the early stage of the cancer cells, tiny, hidden cancer foci can be discovered. PET/CT can detect hidden cancer foci at the early stage of cancer cells without waiting for structural changes in the tumor tissue, and the diagnostic rate is as high as 92%~95%. PET/CT is able to complete localization, qualitative, quantitative and periodic diagnosis, and 80% of its contribution lies in the diagnosis and treatment of tumors, and its unique clinical value has the following points: 1. Early diagnosis of tumors: Since lesions can be detected when the metabolism of tissues is abnormal, PET/CT can detect tumors much earlier than other imaging methods. Early diagnosis of tumor: PET/CT can detect lesions when the tissue metabolism is abnormal. Differential diagnosis of benign and malignant tumors: Since the positron nuclides used are mostly radionuclides that constitute the basic elements of the human body, their metabolic pathways are similar to that of glucose in vivo, except that they are not converted into CO2 and water, but are kept in the tissues. This facilitates the differential diagnosis of benign and malignant tumors because proliferating cancer cells have a higher metabolic uptake rate of glucose than normal cells. Its diagnostic accuracy can reach 95%-100%, which can change the scope of surgical resection or treatment plan and reduce the pain and trauma of patients. Staging and grading of malignant tumors: A whole-body PET/CT examination can detect whether there are metastases in distant lymph nodes or other organs, and stage and grade malignant tumors. 4. Finding the primary tumor foci: clinicians often meet some patients who only have the characteristics of metastatic foci and the primary tissue and organ foci can not be determined, which brings blindness to the doctors’ choice of treatment plan, at this time, 18F-FDG PET/CT whole body scanning can be done, which can effectively find the primary foci, and the positivity rate reaches more than 90%, which provides objective bases for the clinicians to choose the therapeutic drugs, radiotherapy plan, whether to operate and other therapeutic means. The positive rate is more than 90%, which provides an objective basis for clinicians to choose treatment drugs, radiotherapy programs, surgery and other treatment means. 5. Evaluation of therapeutic effect and prognosis judgment: After effective radiotherapy, chemotherapy, thermotherapy or other interventional treatments, the original high glycolysis property of the tumor will be decreased very quickly, so PET/CT functional metabolism imaging can be carried out on the lesions in time after the treatment to understand the therapeutic effect and its prognosis judgment. It can help doctors adjust the treatment program in time to achieve the effective treatment purpose. 6, cardiovascular disease: such as the early diagnosis of coronary heart disease, myocardial infarction after the judgment of the surviving myocardium, if the same patient at the same time to do 13N-NH3 and 18F-FDG scanning, PET/CT can be in the region of poor cardiac perfusion to distinguish between surviving and non-surviving tissue. This is useful for screening patients for indications for coronary artery bypass surgery. 7.Neurological diseases: diagnosis and evaluation of Alzheimer’s disease, detection and localization of epileptic foci. 8, the development of radiation therapy plan: PET / CT is the most important value embodied in the participation of tumor radiation therapy, it can be metabolism, blood flow, tissue proliferation, hypoxia, tumor-specific receptors, angiogenesis, apoptosis, the positioning of biological targets. In radiotherapy, the radiotherapy irradiation field can be increased according to the volume of margin involvement, and the irradiation dose can be elevated while avoiding normal tissues, so as to improve the therapeutic efficacy, and the radiotherapy plan can be continued, changed, or terminated according to the response of the treatment. 9. Clinical imaging and scientific research of various nuclides.