Purpose: 1. To improve intrinsic contrast by means of positive or negative contrast agents. 2, Intravenous rapid injection to observe signal changes in lesions or organs at different phases and time points to determine blood supply information. 3.Selective distribution of specific contrast agent to tissues, so as to judge the biological or even molecular level information of lesions. The principle is different from X-ray and CT, iodine directly affects the penetration and absorption of x-rays, and the enhancement is directly linear with the concentration of the local contrast agent; while MRI mostly uses rare earth elements such as gadolinium, Fe, Mn, etc., and the most widely used and safest is gadolinium, which contains a lot of unpaired electrons with strong paramagnetism, which can interfere with the relaxation process of the neighboring water protons, resulting in the shortening of T1 and T2 time. The degree of signal alteration that ultimately results in MRI images is also related to a number of factors including: local contrast concentration, relaxation properties of the contrast agent in vivo, relaxation properties of the tissue, and parameter settings of the MRI sequence. Gd-DTPA is most commonly used, and routine doses of Gd-DTPA cause changes in tissue relaxation rates in vivo that are proportional to their dose. Routine enhancement: CNS and skeletal muscle, focusing on the observation of reinforcement behavior and enhancement effects, while there is no particular requirement for the time course of enhancement. The time dependence is not obvious and can be well observed 20-30 min after injection. The mechanism of enhancement in CNS and skeletal muscle is related to the contrast concentration, vascular structure, local blood perfusion status, and the integrity of the blood-brain barrier. Lesion enhancement may persist for a longer period of time after contrast injection and is not of great significance at different time points in general CNS disease (except pituitary gland). Dynamic enhancement scans: abdominal organs, pituitary, breast disease. Focus on the observation of reinforcement behavior at different time points. Fast T1WI sequences should be selected, scrambled-phase GRE sequences are preferred, and breath-hold scanning should be used for the abdomen. Coronal position is preferred for pituitary. The signal in the central region of K-space is acquired at the peak moment of the contrast, if K-space center preferred acquisition is not used, then we should be the moment of the midpoint of the sequence scan is the same as the moment when the peak of the contrast reaches the vessel of interest.