Serial MR detection of the accumulation of MPIO-labeled macrophages to a rejected cardiac allograft. A heart from a Dark-Agouti rat was transplanted in to Brown Norway rat, leading to acute tissue rejection based upon strain differences in immunological markers. MPIO was delivered intravenously 3.5 days after operation and macrophage recruitment to the transplanted heart was monitored by MR imaging at 3.5 days (A), 4.5 days (B) and, (C) 5.5 days. In (C), large numbers of punctate hypointensities are present in the parenchyma of the rejected heart. After comparing MPIO to USPIO for macrophage detection in rejected organs, this study found that single punctate hypointensities, presumably single macrophages, were more frequently detected using MPIO whereas USPIO resulted in larger, contiguous hypointensities. Reproduced, with permission, from Ref. [89].