The principle of nuclear magnetic resonance is the resonance of hydrogen protons within a static magnetic field, excited by a specific radio frequency pulse and absorbing a certain amount of energy. The abnormal signal of MRI usually refers to the change of composition and proportion within tissue cells, or replaced by other tissues. The abnormal signal represents the main content of the usual work of imaging doctors, which may represent inflammation, tumor, ischemia, infarction, hemorrhage, etc. For example, if a patient with sudden onset does cranial MRI and finds high signal when diffusion map, and combined with low signal on T2 and ADC map, it can be clearly determined It is an acute cerebral infarction, and the clinical doctor will carry out thrombolytic therapy to win precious treatment time for patients, improve their prognosis and quality of life. If a tumor is found, the tumor characteristics will be analyzed from different levels and angles, and further enhanced MRI is needed to perform plain scan enhancement contrast to discover the tumor blood supply and determine whether it is a benign or malignant tumor. If malignant tumor, further observation of tumor invasion to surrounding tissues, whether there is lymph node metastasis, whether there is distant metastasis, etc. is needed.