Schematic illustrating the transmigration of ultrasmall superparamagnetic iron oxide (USPIO) into the lymphatics and the uptake of USPIO by macrophages at a site of inflammation. (a) Intravenously injected USPIO are able to gain access to the interstitium and the lymphatic vessels as a consequence of their small size and long circulation times. This allows for imaging of the nodal architecture and detection of disturbances in nodal flow as a result of metastases. (b) At sites of inflammation, SPIO accumulate within the interstitium, due to an increase in the porosity of the endothelial wall, and are engulfed by macrophages.