Described are Chemical Exchange-dependent Saturation Transfer (CEST) contrast agents for Magnetic Resonance Imaging (MRI) comprising a polymersome provided with a paramagnetic agent. The polymersome preferably comprises a polymeric shell enclosing a cavity, wherein the cavity comprises a pool of proton analytes, and wherein the shell allows diffusion of the proton analytes. The polymersome-based CEST MRI contrast agents are suitable as drug carriers useful in MRI-guided drug release.
