With the progress of modern technology, more and more high-end instruments have been invented and applied to clinical medicine. PET-CT is an obvious example, in recent years, with the rise of tumor, many people know how to “early detection, early treatment”. However, although it is good to kill a chicken with a bull’s-eye knife, it is not good to use a big tool but a small one, but it hurts the body. PET-CT works mainly based on tracers to selectively reflect the metabolism of tissues and organs, reflecting the physiological, pathological, biochemical and metabolic changes of human tissues at the molecular level, especially suitable for the study of human physiological functions. These tracers are like paparazzi, following the movements of the “stars”, but the anatomical structure of the images is not clear; while the CT function uses X-rays to correct the attenuation of PET images, which greatly shortens the data acquisition time and improves the image resolution, and uses CT images for the anatomical localization and differential diagnosis of PET image lesions. anatomical localization and differential diagnosis. Simply put, PET-CT is the use of PET in combination with CT, and PET mainly works through 18-F generation of deoxyglucose in the body, and tumors have a high glucose metabolism, and there will be nuclei aggregation in the tumor site. This is then combined with CT to improve the accuracy of diagnosis and to achieve complementary functional and anatomical image information. Situations where PET-CT is not required I. Routine physical examination Many units organize annual physical examination for their employees, but many people are still uneasy and hope to have PET-CT done during the physical examination with a view to detecting tumors as early as possible; while some are relatively rich at home and blindly superstitious about the role of new technology, hoping that PET-CT can achieve a once-and-for-all effect. In fact also, about nearly one third of PET-CT is used for medical checkups in Japan, Korea and China. Unbeknownst to us, it hurts our money and is not good for our health. According to experts, the radiation dose of PET/CT comes from radionuclides and CT, and the amount of radiation received from a whole-body PET/CT is currently at least 10 millisieverts or more. According to Chinese radiation protection norms, a medical staff working in hospital radiology department, the received dose of radiation should not exceed 20 mSv per year, while for the general public, the received amount should not exceed 1 mSv per year, and in special cases, the maximum effective dose is 5 mSv in a single year. The cancer risk of these radiations should not be underestimated, and the younger the age, the greater the danger. For this reason, it is recommended not to do PET-CT during routine medical examinations, especially for young people. Many people think that since PET-CT can detect tumors at an early stage, it is always possible to do this test if you suspect a tumor, right? For example, for the diagnosis of gastrointestinal tumors, we mainly use direct observation by gastroscopy and colonoscopy, and take pathological biopsy. For breast cancer screening, ultrasound and mammography are also preferred, and mammography is only recommended once a year for women over 35 years old. Therefore, PET-CT is not necessary whenever a tumor is suspected. If other low-risk tests can serve the purpose, do not use “bull’s-eye”. False positive 18-F deoxyglucose is not a specific imaging agent, and all lesions with increased glucose metabolism can cause false positive 18-FDG-PET. Common ones are tuberculosis, inflammation, infection, trauma, nodular disease, sarcoidosis, parasitic diseases and autoimmune diseases. Therefore, the false positive rate of PET-CD is significantly higher in patients with a history of these diseases. Therefore, PET-CT is not the most appropriate and sensitive option when the patient is accompanied by the above mentioned medical history and should be used sparingly. In this case, it is also important not to blindly trust PCT-ET results in order to avoid being misled. IV. False negatives With the presence of false positives, there are few false negatives. False negative means that there is indeed the presence of tumor while the PET-CT result is negative, delaying the diagnosis. Some low-grade malignant tumors or microscopic lesions have insignificantly increased 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 cell carcinomas and fine bronchoalveolar carcinomas, carcinoid tumors, neuroendocrine tumors, or small lesions <7 mm can show false-negative results. Glucose is mainly excreted through the renal system, so primary tumors of the urinary tract are easily masked, which also leads to a high false-negative rate, with limited bladder irrigation and diuretic application. In addition, the energy of normal brain tissue is derived from glucose, resulting in a high background of glucose in normal brain tissue, which also makes the detection rate of lesions low. Therefore, PET-CT is not a panacea, and this bull's-eye can sometimes fail. Conclusion: PCT-CT, as a new imaging tool, does have its own unique applications. Here, we are not denying the advantages of PET-CT, but we need to understand its limitations and avoid blind faith, otherwise it will not only waste money but also cause 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 to cause as little damage as possible.