DYNAMIC LIVER IMAGING WITH IRON-OXIDE AGENTS - EFFECTS OF SIZE AND BIODISTRIBUTION ON CONTRAST

In vivo effective relaxation rates in normal rat liver were evaluated for four dextran coated iron oxide agents: monocrystalline iron oxide nanocolloid (MION) with a mean particle diameter of 3.9 nm, a polycrys talline agent (PION) with a larger mean diameter of 12 nm, and these t wo agents labeled with the asialofetuin (ASF) protein for high hepatoc ytic receptor binding affinity (MION-ASF and PION-ASF), Using echo pla nar imaging at 2 Tesla, dose response was measured with high temporal resolution for 3 h after injection of agent, and by comparing with rel axivities in vitro and in brain, dominant in vivo contrast phenomena w ere elucidated, While transverse relaxivity for PION-ASF exceeded that for MION-ASF by almost a factor of 2 in solution, relaxation rates in vivo became equivalent. Liver relaxation using non-ASF agents was con sistent with rapid water exchange between vascular and extravascular c ompartments, which dominated relaxation as a result of agent accumulat ion in Kupffer cells.