With the progress of modern science and technology, more and more high-end instruments have been invented and applied to clinical medicine. However, it is inevitable that these instruments are abused and blindly trusted. PET-CT is an obvious example, in recent years, with the rise of tumors, many people know how to “early detection, early treatment”. But don’t use a bull’s knife to kill a chicken, although the bull’s knife is good, it will hurt you if you use it too much. The working principle of PET-CT PET is mainly based on tracers to selectively reflect the metabolism of tissues and organs, from the molecular level to reflect the physiology of human tissues, pathology, biochemistry and metabolism and other changes, especially suitable for the study of human physiological function. These tracers are just like paparazzi, following the whereabouts of the “stars”, but the anatomical structure of the image is not clear; and CT function is the use of X-ray attenuation correction of PET images, greatly reducing the data acquisition time, improve the image resolution, the use of CT images of PET images of the lesion site for anatomical localization and differential diagnosis. Anatomical localization and differential diagnosis. Simply put, PET-CT is the combination of PET and CT, and PET mainly works through the 18-F deoxyglucose in the body, and the glucose metabolism of the tumor is exuberant, and there will be nuclide aggregation in the tumor site. It is then combined with CT to improve diagnostic accuracy, realizing the complementarity of functional and anatomical image information. Circumstances in which PET-CT is not required I. Routine physical examination Many units organize physical examination for their employees every year, but many people are still uneasy and hope to have PET-CT during physical examination with a view to detecting tumors as early as possible; while some of them are relatively affluent in their families and are blindly superstitious about the role of new technologies, hoping that PET-CT can achieve the effect of once-and-for-all. In fact, about one-third of PET-CT is used for medical checkups in Japan, Korea and China. Unbeknownst to the public, it hurts money and is not good for the body. According to experts, the radiation dose of PET/CT comes from radionuclides and CT, and the amount of radiation received from doing a whole-body PET/CT is currently at least 10 millisieverts. According to China’s radiation protection norms, a medical staff working in the radiology department of the hospital, the annual radiation received dose can not exceed 20 millisieverts, and for the general public, a year should not be more than 1 millisieverts, under special circumstances, the maximum effective dose of 5 millisieverts in a single year. The cancer risk of these radiations should not be underestimated, and the younger you are, the more harmful it is. For this reason, it is recommended not to do PET-CT during routine medical checkups, especially for young people. Diseases that do not require “bull’s-eye” Many people think that since PET-CT can detect tumors at an early stage, then if there is a suspicion of tumors, they can always do this test, right? In fact, it is not true, many tumor treatment guidelines do not include PET-CT in the gold standard, for example, for the diagnosis of gastrointestinal tumors, we mainly through gastroscopy, enteroscopy direct observation, and take pathological biopsy. For breast cancer screening, ultrasound and molybdenum target examination are also preferred, and molybdenum target examination is only recommended once a year for women over 35 years old. Therefore, not as long as the suspicion is a tumor, it is necessary to do PET-CT, if other low-risk examination can achieve the purpose, do not use “bull knife”. Third, false-positive 18-F deoxyglucose is not a specific imaging agent, all lesions with increased glucose metabolism can cause false-positive 18-FDG-PET. Common ones are tuberculosis, inflammation, infection, trauma, tuberculosis, sarcoidosis, parasitic diseases and autoimmune diseases. Therefore, the rate of PET-CD false positives is significantly higher in patients with a history of these conditions. Therefore, when a patient has a history of any of these conditions, PET-CT is not the most appropriate and sensitive option and should be used sparingly. In this case, do not blindly trust the PCT-ET results either, in order to avoid being misled. IV. False Negatives With the presence of false positives, there are fewer false negatives. False negative means that there is indeed the presence of a tumor, while the PET-CT results are negative, delaying the diagnosis. Some low-grade malignant tumors or tiny foci have an insignificant increase in FDG metabolism. According to the literature, about 30% to 50% of highly differentiated hepatocellular carcinomas, 22% to 50% of highly differentiated thyroid carcinomas, 6% to 23% of renal clear cell carcinomas, 33% to 100% of prostate carcinomas, 33% to 60% of bladder carcinomas, 41% of mucinous adenocarcinomas, 38% of gastric indolent carcinomas, and fine bronchial alveolar carcinomas False-negative results can be seen in carcinomas, carcinoid tumors, neuroendocrine tumors, or small lesions <7 mm. Since glucose is excreted mainly through the renal system, primary tumors of the urinary tract can be easily masked, which leads to a high false-negative rate and a limited role for bladder irrigation and diuretics. In addition, normal brain tissue derives its energy from glucose, which leads to a high background of glucose in normal brain tissue, also resulting in a low detection rate of lesions. Therefore, PET-CT is not omnipotent, and this knife may fail sometimes. Conclusion: As a new imaging tool, PCT-CT does have its unique applications. Here, we are not denying the advantages of PET-CT, but we should understand its limitations and avoid blind faith, or not only wasting money, but also causing unnecessary harm to the body. After the examination, it is also important to drink plenty of water to promote the excretion of radioactive substances in the body and minimize damage.