Liposomes in diagnostic radiology.

Most imaging agents can be seen as frameworks that support and carry imaging elements. The imaging element provides radiopacity or radioactivity, or induces changes in echogenicity or magnetic relaxation times in the tissues. In medicine liposomes are known as potential carriers for therapeutic agents. The utilization of liposomes as vehicles for contrast agents has been proposed and studied by many (McDougall et al., 1975; Caride et al., 1976; Richardson et al., 1978; Caride, 1981), and is the subject of the present paper. There are important differences between therapeutic and diagnostic uses of liposomes. For example, the targeting in imaging is not mandatory. Although absolute targeting, i.e. delivery of the cargo, only to the target site, is very important for therapy, partial targeting is an acceptable compromise for imaging. In many occasions the delivery of the imaging agent to areas adjacent to the 'target area' will be as effective as the delivery of the agent exclusively to the target (Caride & Sostman, 1983). The images are generated by acquiring information from a given area of the body; therefore the requirements for targeting are less demanding, since contrast enhancement results from the relative regional distribution of the tracer between the target and the non-target tissues. When this ratio is high the target can be made visible, even if only a small fraction of the administered dose is present in the area under scrutiny. In short, liposomes can provide an additional protective structure for the imaging agent, alter its pharmacokinetics with potential selective organ or lesion enhancement, amplify the contrast-enhancement effect, transport multiple imaging agents at once and decrease toxicity. There are two basic approaches: to use liposomes as vehicles for conventional imaging agents or to transform liposomes into the framework that carries the imaging element. In the last instance radioactive, radiopaque, sonoreflective or paramagnetic elements form part of the lipid vesicles. In this paper we refer to radiolabelled and radiopaque liposomes.