PET-CT should be examined twice because each examination only represents the physical condition at the time of the examination, and changes in the condition or re-evaluation require a second examination. PET-CT, known as Positron Emission Tomography – X-ray Computed Tomography Combined System, is a new type of examination equipment that integrates PET scanner and CT equipment together, and is often used for tumor examination, which is a more sensitive type of imaging examination. PET-CT examination is most commonly used in the clinic for tumors of unknown origin, systemic metastasis of tumors, etc., and secondly, it can also be used in the examination of stroke, coronary heart disease, transient ischemia, etc. Like other auxiliary examinations, it can only represent the situation at the time of the examination, especially in the case of malignant tumors, which need to be examined again in case of recurrence, metastasis, and so on. Whether the patient needs the second examination, according to the professional doctor’s opinion and suggestion.