We have developed a new class of metal-loaded nanoparticles that have potential as contrast agents for medical imaging. These polymeric materials have a core-shell morphology where the interior is a functionalized polymer, that provides high affinity for a specific metal, and the shell consists of a porous hydrophobic polymer that modulates access to the core. In the present case, the nanoparticles an loaded with Gd3+ to provide contrast in magnetic resonance (MR) imaging. The Gd3+-loaded nanoparticles described in this paper have a diameter of 120 nm and are, therefore, small enough to pass easily through the vasculature. These particles have been shown to reduce relaxation times in vitro, and provide excellent contrast when used to image the heart and gastrointestinal tract in a rat animal model.