A macromolecular contrast agent for magnetic resonance imaging of the vascular system is constructed of a polymeric backbone structure with a plurality of spacer arms bonded to the backbone structure, each spacer arm terminating in at least one paramagnetic complex. The polymeric backbone thus serves as an amplifier by supporting a multitude of paramagnetic complexes, and the spacer arms contribute to the molecular weight. The spacer arms further contribute useful properties to the agent, such as hydrophilicity and the ability to cleave at a relatively rapid rate in blood.