Nanometer-sized materials offer a wide range of applications in biomedical technologies, particularly imaging and diagnostics. Current scaffolds in the nanometer range predominantly make use of inorganic particles, organic polymers or natural peptide-based macromolecules. In contrast we hereby report a supramolecular approach for the preparation of self-assembled dendritic-like nanoparticles for applications as MRI contrast agents. This strategy combines the benefits from low molecular weight imaging agents with the ones of high molecular weight. Their in vitro properties are confirmed by in vivo measurements: post injection of well-defined and meta-stable nanoparticles allows for high-resolution blood-pool imaging, even at very low Gd(III) doses. These dynamic and modular imaging agents are an important addition to the young field of supramolecular medicine using well-defined nanometer-sized assemblies.