PET-CT is composed of PET and CT, PET is positron emission computed tomography, which is a molecular imaging device for functional metabolic imaging, to understand the functional metabolic status of a lesion through the uptake of tracer at the lesion site, so as to make a correct diagnosis of the disease. PET-CT is the organic combination of PET and CT, which can reflect the pathological and physiological changes and morphological and structural changes of the lesion at the same time, significantly improving the accuracy of diagnosis. PET-CT can achieve localization, qualitative, quantitative and periodic diagnosis in imaging for diseases of various parts. For the current high prevalence of tumor diseases, it can be commonly used in the following cases: 1) early diagnosis of tumors; 2) differential diagnosis of benign and malignant tumors; 3) staging and grading of malignant tumors; 4) finding the primary lesion of tumors; 5) assessment of efficacy and prognosis judgment after tumor treatment; 6) monitoring of tumor recurrence or metastasis; 7) reasonable screening of high-risk groups.