This example illustrates how much we can miss when forced to perform a non-contrast CT in someone with renal failure or a contrast allergy. The image on the left, from a triphasic CT liver, has been performed without contrast; the liver looks a little bit nodular (the patient had cirrhosis) but otherwise fine. On the right is an image from the arterial phase of the CT, taken at the same position - it shows multifocal hepatocellular carcinoma (arrows).