There are normal scans and enhanced scans for CT. Normal scan is a direct scan without contrast injection, such as the scan after contrast injection is called enhancement scan. Liver cancer shows a lower density than the surrounding liver tissue in normal scan, which can show the size and shape of liver cancer. However, there are a few hepatocellular carcinoma whose density does not differ much from that of the surrounding liver tissues and cannot be shown in the ordinary scan; because the density of blood does not differ much from that of hepatocellular carcinoma, the cross section of blood vessels may be mistaken for hepatocellular carcinoma, and if hepatocellular carcinoma invades or metastasizes into blood vessels, the ordinary scan cannot show it either. Enhanced scan can easily compensate for the shortage of ordinary scan according to the dynamic changes of density of liver cancer, surrounding liver and blood vessels. For example, in the early stage after contrast injection (arterial stage), liver cancer is mainly supplied by the hepatic artery with significant enhancement and higher density, while the surrounding liver tissues are mainly supplied by the portal vein with insignificant enhancement and relatively low density; in the late stage after contrast injection (venous stage), liver cancer is no longer enhanced and its density decreases, while the surrounding liver tissues have significant enhancement and higher density. Thus, scanning at different times after contrast injection can show liver cancer more clearly. Enhancement scan can clearly show the shape of blood vessels by filling the vessels in the liver with contrast, which can identify the vascular section and hepatocellular carcinoma; if there is a cancer thrombus in the portal vein, the filling defect of contrast will be shown in the portal vein after contrast injection to reveal the cancer thrombus. Therefore, the examination of hepatocellular carcinoma usually involves plain scan and enhanced scan.