The liver has a unique dual blood supply, which makes helical computed tomography (CT) a highly suitable technique for hepatic imaging. Helical CT allows single breath-hold scanning without motion artifacts. Because of rapid image acquisition, two-phase (hepatic arterial phase and portal venous phase) evaluation of the hepatic parenchyma is possible, improving tumor detection and tumor characterization in a single CT study. The arterial and portal venous supplies to the liver are not independent systems. There are several communications between the vessels, including transsinusoidal, transvasal, and transplexal routes. When vascular compromise occurs, there are often changes in the volume of blood flow in individual vessels and even in the direction of blood flow. These perfusion disorders can be detected with helical CT and are generally seen as an area of high attenuation on hepatic arterial phase images that returns to normal on portal venous phase images; this finding reflects increased arterial blood flow and arterioportal shunting in most cases. Familiarity with the helical CT appearances of these perfusion disorders will result in more accurate diagnosis. By recognizing these perfusion disorders, false-positive diagnosis (hypervascular tumors) or overestimation of the size of liver tumors (eg, hepatocellular carcinoma) can be avoided.