Increased contrast in magnetic resonance imaging (MRI) is accomplished using polymeric micelles loaded with superparamagnetic iron oxide (SPIO) nanoparticles encapsulated in biocompatible, biodegradable poly(F-caprolactone) -b-poly(ethylene glycol) (PCL-b-PEG) copolymers (see Figure). The loaded micelles show significantly improved T-2 relaxivities and remarkable MRI detection sensitivity.