Liver cancer is a highly prevalent cancer in China, and there are many methods for early detection of liver cancer, while the liver is the largest substantive organ of human body, which is responsible for various important metabolic functions of human body. From biochemical tests to imaging tests, at least seven or eight kinds of tests have to be done to diagnose a disease, and many patients cry out in pain and feel that they have spent money unjustly. Many patients in the middle stage do not understand why it is necessary to do PET-CT examination when liver cancer has been diagnosed and metastases are suspected? Since PET-CT examination is relatively expensive, many patients cannot understand how to troubleshoot this situation. Is PET-CT necessary for suspected distant metastasis of liver cancer? Although liver cancer has been diagnosed, usually patients only have CT examination for liver when diagnosed, if the tumor has metastasis in other organs doctors cannot make exact judgment, for the size of metastatic lesions and surrounding conditions, doctors also need to determine with precise instruments before taking follow-up treatment. It is related to the principle of PET-CT treatment. PET-CT principle is based on metabolic imaging and quantitative analysis of PET-CT, which uses substances essential to human metabolism, such as: glucose, fatty acids, nucleic acids and proteins, etc. Short-lived radionuclides labeled with short-lived radionuclides are injected into the body and scanned for imaging. The high metabolism of malignant tumor tissues is in sharp contrast to the low metabolism of normal tumor tissues, and the reaction is reflected to diagnose and analyze the lesion. Experts point out that, especially after injecting the imaging agent, the scan can clearly see the distribution of tumors in the whole body, and where the metabolism is high, it should be highly suspected that there may be metastases. In our country, there are many problems for both patients and doctors. Doctors need to explain to patients as much as possible the reasons for doing various types of tests before they receive them, and it is necessary for patients to learn about them from various sources.