FERRIMAGNETIC SUSCEPTIBILITY CONTRAST AGENTS

Bach-Gansmo T. 1993. Ferrimagnetic susceptibility contrast agents. Con trast, agents based on superparamagnetic particles have been in clinic al development for more than 5 years, and the complexity of their effe cts is still not elucidated, The relaxivities are frequently used to g ive an idea of their efficacy, but these parameters can only be used i f they are concentration independent for large superparamagnetic syste ms, the evolution of the transverse magnetization is biexponential, af ter an initial loss of magnetization. Both these characteristics of la rge superparamagnetic systems should lead to prudence in using the rel axivities as indicators of contrast medium efficacy. Susceptibility in duced artefacts have been associated with the use of superparamagnetic contrast agents since the first imaging evaluation took place. The ra nge of concentrations where good contrast effect was achieved without inducing artefacts, as well as blurring and metal artefacts were evalu ated. The influence of motion on the induction of artefacts was studie d, and compared to the artefacts induced by a paramagnetic agent subje ct to motion. With a suitable concentration of a negative contrast age nt, a signal void could be achieved in the region prone to motion, and no artefacts were induced. If the concentration was too high, a displ acement of the region close to the contrast agent was observed. The ar tefacts occurred in a volume surrounding the contrast agent, i.e., als o outside the imaging plane. In comparison a positive, paramagnetic co ntrast agent induced heavy artefacts in the phase encoding direction, appearing as both high intensity regions and black holes, in a mosaic pattern. Clinical trials of the oral contrast agent OMP for abdominal MR imaging showed this agent to be safe and efficacious. OMP increased the diagnostic efficacy of abdominal MR imaging in 2 of 3 cases exami ned, with a significant decrease in motion artefacts. Susceptibility c ontrast agents may also be of use in the evaluation of small lesions i n the liver. Particulate material injected i.v. will be targeted to th e liver and spleen by way of the mononuclear phagocyte system (MPS). S mall particles, without specific receptor affinities were targeted to the hepatocytes and the MPS. The distribution correlated with a high e fficiency as a contrast agent, whereas no correlation to in vitro rela xation rates and relaxivities could be found. Superparamagnetic partic les have important possibilities as contrast agents. The identificatio n of in vitro properties of these agents may help the comparison of va rious agents before in vivo imaging.