CT and MRI are divided into plain scan and enhancement scan. The significance of enhancement scan is as follows: 1) to find more lesions, more diseases can not be detected because of the inconspicuous contrast, such as renal tumors; 2) to judge the nature of lesions, such as liver diseases, which need to be diagnosed through the enhancement and intensification way. More low-density lesions can be diagnosed by enhancement scanning; hepatocellular carcinoma can be clearly diagnosed by obvious enhancement in arterial phase and faster withdrawal in venous phase, i.e. fast-in-fast-out enhancement, combined with history of cirrhosis and laboratory examination. There are benign and malignant tumors, which are indistinguishable in plain scanning and can be differentiated by enhancement scanning.