an attempt to deliver the greatest iodine concentration with the least toxicity. From a simplistic viewpoint, the intravascular contrast agents can be viewed merely as vehicles for delivering iodine to a blood vessel or structure. These water-soluble intravascular contrast agents can be subdivided into the following categories: (1) ionic, high osmolality, roughly five times the osmolality of blood; (2) nonionic, low osmolality, roughly twice or slightly more than the osmolality of blood; and (3) isotonic agents—nonionic dimers. The basic structures and physicochemical characteristics of available contrast agents are not covered here; these topics are discussed in appropriate specialized publications.1 At the x-ray energies used in CT, the mass attenuation coefficient for iodine is considerably greater than that of surrounding soft tissues and blood. After the intravascular injection of iodinated contrast, initial CT images reveal aortic and major arterial enhancement, followed by a capillary or parenchymal “blush” and eventual venous opacification. The rate of contrast injection and timing of subsequent CT scans determine the structures enhanced on any one image. Compared with earlier scanners, multidetector CT (MDCT) scanners require shorter injection rates because of the short scanning times; as a result, faster injection rates of more concentrated contrast agents are necessary, and a contrast’s viscosity has a dominant role. Various techniques of intravascular contrast agent administration are discussed elsewhere in this text. A number of drugs, especially more acidic ones, are incompatible to mixing with contrast agents, an incompatibility less evident with nonionic agents. Nevertheless, as a general safety precaution, a drug probably should not be mixed with a contrast agent, and a catheter should be flushed if used for both drug and contrast injection.