PET-CT basic knowledge and clinical application of the significance of PET that is, Positron Emission Computed Tomography (Positron Emission Computed Tomography), and spiral CT fusion to form the integration of the performance of the most advanced PET-CT scanner, referred to as PET-CT. The device is the most high-grade PET scanner and advanced spiral CT equipment function integration perfect fusion, but also the current configuration of the most high-grade, most advanced performance PET-CT scanner. This device is the perfect integration of the functions of the most high-grade PET scanner and advanced spiral CT equipment, and it is also the most advanced PET-CT scanner with the highest configuration and the most advanced performance, which is mainly used in the early detection and diagnosis of major diseases in the fields of tumors, brain and heart, etc. PET-CT is able to diagnose tumors and other diseases in an early stage. Due to the active metabolism of tumor cells, their ability to take up the imaging agent is 2-10 times higher than that of normal cells, forming obvious “points of light” on the image, so that hidden tiny foci (greater than 5mm) can be detected in the early stage of the tumor before anatomical structural changes are produced. （Positron emission tomography (PET) is a relatively advanced clinical imaging technique in the field of nuclear medicine. The general method is to label a substance, generally necessary for the metabolism of living beings, such as glucose, protein, nucleic acid, and fatty acid, with a short-lived radionuclide (e.g., F18, Carbon 11, etc.), and then inject it into the human body to reflect the metabolic activity of life through the aggregation of the substance in metabolism, thus achieving the purpose of diagnosis. Recently, the main substance used in hospitals is fluorodeoxyglucose (FDG), the mechanism of which is that the metabolic state of different tissues of the human body is different, and glucose metabolism is exuberant and accumulates more in highly metabolized malignant tumor tissues, and these characteristics can be reflected in the images, which enables the diagnosis and analysis of the lesions. The basic principle of CT (short for computerized X-ray tomography) is image reconstruction. According to the characteristic of unequal absorption of X-rays by various tissues of the human body (including normal and abnormal tissues), a selected level of the human body is divided into a number of cubic blocks (also called voxels). After X-rays have passed through the voxels, the measured density or gray value is called a pixel. The X-ray attenuation value of each voxel is determined by an iterative method and the image is reconstructed to obtain a black and white image of the different densities of tissue at that level. Spiral CT breaks through the design of traditional CT, using slip ring technology, continuous uniform rotation along the long axis of the human body, the scanning bed is synchronized with uniform progress (traditional CT scanning bed is stationary during scanning), the scanning trajectory is spiraling forward, and the volumetric scan can be completed quickly and uninterruptedly. Multi-layer spiral CT is characterized by a multi-layer arrangement of detectors. It is the best combination of high speed and high spatial resolution. The wide detector of multilayer spiral CT is made of high-efficiency solid rare earth ceramic material. Each unit is only 0.5, 1 or 1.25 mm thick, up to 5 mm thick thin layer scanning detector photoelectric conversion efficiency of up to 99% can continuously receive X-ray signals. The afterglow is extremely short and stable. Multi-slice spiral CT can complete a large range of volumetric scanning at high speed, good image quality, fast imaging speed, with high longitudinal resolution and good time resolution. It greatly broadens the scope of CT. PET/CT is mainly used for malignant tumors, and in our country malignant tumors have become the main killer of people’s health, and it is not uncommon for families to be impoverished because of their loved ones suffering from tumors. The actual situation is: although PET/CT, a high-end equipment, is very helpful to the diagnosis and treatment of patients, a considerable number of patients have to give up the use of it due to their inability to pay the expensive examination fees. For this reason, it is difficult for one of the 54 centers mentioned above, with some exceptions, to have more than 1,500 examinations in a year, so that most of the PET/CT and gas pedals are not functioning. It can be expected that with the gradual maturation of the application of PET/CT, the clinical value of PET/CT will definitely be recognized, and once the examination fees of some diseases (such as certain malignant tumors) are included in medical insurance, the demand for PET/CT examination will rise substantially and play a greater role in clinical practice. The combination of PET and CT two different imaging principles of equipment with the same machine is not a simple addition of their functions. Rather, image fusion is carried out on this basis, and the fused image has both fine anatomical structure and rich physiological and biochemical information. The fused image has both fine anatomical structure and rich physiological and biochemical function information, which can provide the basis for determining and finding the precise location of tumor and other lesions for quantitative and qualitative diagnosis. And X-ray can be used for attenuation correction of nuclear medicine images. The core of PET-CT is fusion, image fusion refers to the same or different imaging modalities of the image after a certain transformation process to make their spatial location and spatial coordinates to match, the image fusion processing system using the characteristics of their respective imaging modalities for spatial alignment and combination of the two images, will be registered after the image data is synthesized into a single image.PET-CT homogeneous fusion (also known as PET-CT homogeneous fusion) is a method of fusion. PET-CT co-location fusion (also called hardware fusion, non-image alignment) has the same positioning coordinate system, so the patient does not have to change position during scanning to perform PET-CT co-location acquisition, avoiding errors caused by patient displacement. After acquisition, the two images do not need to be aligned, converted and calibrated, and the computer image fusion software can conveniently carry out the precise fusion of 2D and 3D, and the fused image simultaneously shows the human anatomy and metabolic activities of the organs, which greatly simplifies the technical difficulty of the whole image fusion process, avoids the complex labeling methods and a large number of calculations after the acquisition, and solves the problem of temporal and spatial alignment to a certain degree, and greatly improves image reliability. It also solves the problem of temporal and spatial alignment to a certain extent, and greatly improves the reliability of the images. The fusion of calibrated PET images with CT images, after information complementation, will provide more information about the relationship between anatomical structure and physiological function, which is of great clinical significance for the positioning of tumor patients in surgery and radiation therapy. PET-CT can make early diagnosis and differential diagnosis of tumors, identify whether there is recurrence of tumors, stage and re-stage tumors, find primary and metastatic foci of tumors, guide and determine the treatment plan of tumors, and evaluate the efficacy of treatment. Among the tumor patients, a considerable number of them have changed their treatment plan due to clear diagnosis after PET-CT examination; PET-CT can accurately evaluate the efficacy, adjust the treatment plan in time and avoid ineffective treatment. PET-CT can accurately evaluate the efficacy of treatment, adjust the treatment program in time, and avoid ineffective treatment. Overall, it greatly saves medical costs and strives for valuable treatment time. Second, PET-CT can accurately localize epileptic foci, and is also a unique examination method for diagnosing depression, Parkinson’s disease, Alzheimer’s disease and other diseases. The treatment of epilepsy is one of the world’s top ten medical problems, the difficulty lies in the accurate localization of epileptogenic foci, PET-CT makes this medical problem solved. PET-CT guidance, the use of X-knife or γ-knife treatment, received a very good therapeutic effect. Third, PET-CT can identify whether the myocardium is alive or not, and provide objective basis for whether surgery is needed. At present, PET-CT myocardial imaging is recognized as the “gold standard” for the evaluation of myocardial viability, and it is a necessary examination before the treatment of myocardial infarction re-vascularization (hemotransfusion) and provides a basis for the evaluation of radiotherapy.PET-CT is also valuable for the diagnosis of early coronary heart disease. PET-CT is also a means of health checkup, which can complete the whole body test with one time imaging, and can detect tumors and heart and brain diseases that seriously endanger people’s health at an early stage, so as to achieve the purpose of early treatment of diseases and prevention of diseases. Modern medicine believes that most diseases are the result of dysfunctional biochemical processes in the body, and PET-CT can dynamically and quantitatively observe biochemical changes at the molecular level in the body under physiological conditions. With the deciphering of human genes, the generation, development and post-treatment regression of tumors, heart and brain diseases and various hereditary diseases that endanger the health of human beings will be fundamentally understood, and it is also expected to find an effective treatment plan from the root.PET-CT gene imaging is a bridge connecting the clinic and the basic genetic research. Principle of PET-CT Diagnosis of Tumors The principle that PET-CT can examine malignant tumors lies in the fact that normal tissues and malignant tumors do not have the same uptake ability, and malignant tumors have a very strong uptake ability. a tracer that can be uptaken is injected into the PET-CT examination, generally 18F-FDG. 18F-FDG uptake ability of the various parts of the body is examined to determine whether or not it is a malignant tumor. What is SUV? SUV, or Standardized Uptake Value, is a quantitative measure of 18F-FDG tracer uptake by tumor tissue. SUV is the most commonly used semi-quantitative index to measure the amount of 18F-FDG uptake in a lesion, and most scholars consider SUV 2.5 as the limit of benign and malignant, SUV>2.5 is considered as malignant, SUV between 2.0 and 2.5 is the critical range, and SUV<2.0 can be considered as benign lesion. can be considered as benign lesions. For example, 18F-FDG PET SUV>2.5 in lung nodules suggests a higher likelihood of malignancy. Most benign lesions do not take up 18F-FDG or take up 18F-FDG mildly, but a small number of benign lesions (e.g., active tuberculosis, acute inflammation, etc.) show high uptake of 18F-FDG and radiolucency, and careful analysis of the morphology of the lesion is helpful in identifying benign and malignant lesions. The results of one study showed that the SUV of lung cancer tissue was 5.63±2.38, and the SUV of benign lung lesions was 0.56±0.27, and the difference was significant compared with the two (P<0.001). Diagnostic usefulness of 18F-FDG PET SUV>2.5 for malignant tumors Tumor Type Diagnostic Accuracy Lung Cancer 94% Colorectal Cancer 90% Melanoma 100% Lymphoma 95% Breast Cancer 90% Head and Neck Cancer 90% Ovarian Cancer 90% Why is it important to control blood glucose for PET-CT? Blood glucose concentration is an important factor affecting the distribution and uptake of FDG (fluorodeoxyglucose) injected for PET-CT in vivo. Elevated blood glucose decreases the uptake of FDG (fluorodeoxyglucose) in tumor and brain tissues, which results in a decrease in the SUV values of these tissues calculated by different methods. This effect of blood glucose factor is realized through the following two pathways: ① Tumor and brain are non-insulin-sensitive tissues, and the increase of insulin secretion caused by the elevation of blood glucose can’t increase the uptake of glucose and FDG, on the contrary, with the elevation of blood glucose, glucose in the body will compete with FDG to enter the cells of these tissues for the Glut and glucokinase, which will make FDG to enter the cells of these tissues. So that FDG into the above tissue cells to reduce; ② Increased insulin secretion after the rise in blood glucose insulin-sensitive tissues such as myocardium, adipose, skeletal muscle, FDG intake increases, FDG clearance in the blood circulation faster, so that a certain period of time in the blood available for tumors and brain tissue intake of FDG decreased, which also caused a decrease in FDG intake of the above tissues. Therefore, it is required to fast 5-6 hours before the examination.