Delivery of diagnostic agents in computed tomography

Computed tomography is a reliable, widely available imaging modality with high spatial resolution. Except for trauma detection, in practically all cases contrast agents are administered for improving image quality. The presently available agents are nonspecific compounds with distribution in the extracellular space. Specificity (i.e. high contrast in selected tissues) is only possible when the pharmacokinetic behavior (peak levels at the region of the interest) is followed on a time scale which counts in seconds. If the peak time is missed, a second injection of contrast material is necessary. The objective of extensive research efforts have therefore been the search for tissue-specific contrast agents which should result both in higher and in longer lasting local concentrations. For the tissue targets, liver, spleen, lymph nodes and the blood pool, efficient contrast enhancing compounds have been described utilizing different active transport systems or passive diffusion-limiting molecular sizes. The agents include water-soluble metal chelates, iodinated polymeric compounds and contrast agent-carrying liposomes. The remaining task for the future will be to improve the tolerability of the contrast agents to such an extent that the side-effect incidence and severity is low enough to allow for the use of these agents in patients.