The purpose of contrast enhancement is to enhance the contrast between the lesion and the blood vessels and the surrounding tissue to facilitate easier detection of the lesion or to show the extent of the lesion more clearly. CT contrast agents (non-ionic iodine contrast agents) are completely different from MR contrast agents (paramagnetic gadolinium-based contrast agents). CT iodine contrast agents have certain adverse reactions, such as gastrointestinal reactions, vomiting, measles, shock and even death, etc. Hospitals generally perform allergy tests for iodine contrast agents, but as a product that has been used for decades, it is safe for clinical use, and the proportion of serious adverse reactions, such as shock, is about 1 in 1,000, and only 1 in 10,000 cases of death due to shock. Professor Li Kuncheng, a famous radiologist, believes that strictly speaking, the adverse reactions of CT contrast agents are not allergic reactions, but “allergic-like reactions”. Because allergic reactions have an “allergen”, CT contrast agent-like allergic reactions do not have an allergen, but modern medicine cannot clearly explain the mechanism and principle of its occurrence. Some people are fine when injecting small doses of CT contrast agents, but have adverse reactions when injecting large doses; some people have reactions when injecting small doses of contrast agents, but are fine when injecting large doses. People with different body types react differently, and this is not a true allergic reaction. In addition, people with renal insufficiency should not have CT-enhanced examinations. The adverse reactions of magnetic resonance contrast agents are significantly lower than those of CT iodine contrast agents, and the chance of allergic reactions is extremely low, so clinically there is no need to perform allergy tests and scan directly after injection.