PET is an abbreviation for positron emission computed tomography, a molecular imaging device for functional metabolic imaging, which uses positronium as a tracer to understand the functional metabolic state of a lesion by the uptake of the tracer at the site of the lesion, thus enabling a correct diagnosis of the disease. However, PET is not as good at resolving anatomical structures as CT. CT stands for computerized tomography, an imaging device that uses X-rays to irradiate selected tomographic levels of the body and obtains tomographic images by measuring the amount of X-rays transmitted. It can clearly display the tomographic images of the human body and accurately describe the size, location, morphology and other anatomical features of lesions (such as tumors); however, there are limitations in diagnosing diseases only by the anatomical features of lesions, and some lesions are difficult to make accurate judgments by CT. The so-called PET-CT is an organic combination of PET and CT equipment, using the same examination bed and the same image processing workstation.PET-CT has the functions of both PET and CT, but it is never a simple superposition of the two functions, as PET and CT have complementary advantages, 1+1>2. PET can display the pathophysiological characteristics of the lesion, and it is easier to find the lesion; CT can PET-CT not only has the respective functions of PET and CT, but its unique fusion image, which fuses PET images with CT images, can reflect the pathophysiological changes and morphological structure of the lesion at the same time, which obviously improves the accuracy of diagnosis. PET-CT can observe the abnormalities of functional metabolism at the early stage of the disease. Early detection and early treatment of diseases are crucial to improve the clinical cure rate. In addition, whole body imaging can be completed in one examination, instead of just observing a certain area, thus avoiding missing lesions. Currently, PET-CT examinations commonly used in clinical practice are focused on three areas: tumor, brain and heart. Among them, PET-CT observes metabolic abnormalities of tumors significantly earlier than other imaging means and has higher detection sensitivity, especially for the benign and malignant identification of isolated nodules in the lung. Due to the high sensitivity of PET-CT, the judgment of radiotherapy and chemotherapy efficacy is more accurate and sensitive, thus guiding doctors to adjust the treatment plan in time. PET-CT also has a unique role in the neurological and psychiatric systems. It can provide information on various aspects such as function, metabolism and blood perfusion, which helps to locate epileptic foci and provides a reference for brain surgery to remove epileptic foci.