SOLUBLE-TYPE HEPATOBILIARY CONTRAST AGENTS FOR MR IMAGING

CONTRAST AGENT-ENHANCED abdominal magnetic resonance (MR) imaging is likely to improve detection and diagnosis of liver disease. Recent research has shown MR imaging to be potentially useful for quantitation and localization of diffuse liver disease and for noninvasive assessment of hepatic function. However, the primary scope of liver imaging today is the detection of malignant focal lesions, possibly against a background of diffuse liver disease. Such malignant lesions should be distinguishable from nonmalignant ones, such as hemangiomas, focal nodular hyperplasia, and hytadid cysts. Imaging may additionally reveal sites of biliary occlusion and bile stones. Of primary interest is the detection of liver metastases and, in particular, their number. The presence of no more than five enables one to consider a partial hepatectomy. In such cases, the questions crucial for surgery concern their location with respect to liver segments and major blood vessels. Finally, one might be interested in the pathologic nature of the lesion. The lack of specificity of present therapeutic procedures, however, relegates knowledge of the pathologic type of metastasis to mere academic interest. The scope of a contrast agent must be to increase lesion conspicuity. Obviously, increased conspicuity enables detection of smaller lesions, and, in this respect, the smaller the malignant lesions are at the time of their detection the better the chance of successful therapy. Presently, the preferred techniques for detection of liver lesions are contrast-enhanced computed tomography (CT) and ultrasonography. However, these techniques leave much to be desired. MR imaging contrast agents that can distinguish normal liver parenchyma from focal lesions would therefore be desirable. T2-relaxing MR imaging contrast agents consisting of coated superparamagnetic iron oxide particles targeted to either reticuloendothelial cells or hepatocytes are currently being studied as described elsewhere in this issue. The present article concerns only soluble T1-relaxing hepatobiliary contrast agents. To date, no such hepatobiliary contrast agents for MR imaging have reached the market, although two agents are currently undergoing clinical trials. Data from these clinical trials allow a preliminary assessment of the usefulness of these contrast agents. Given that a final clinical assessment is not yet possible, this article focuses instead on the chemical characteristics of the active ingredients and their pharmaceutical formulations, since these properties pertain to imaging efficacy, pharmacotoxicologic safety, and clinical side effects.