Superparamagnetic iron oxide nanoparticles (SPIONs) are used as imaging probes to provide contrast in magnetic resonance images. Successful use of SPIONs in targeted applications greatly depends on their ability to generate contrast， even at low levels of accumulation， in the tissue of interest. In the present study， we report that SPION nanoclusters packaged to a controlled size by a hyperbranched polyglycerol (HPG) can target tissue defects and have a high relaxivity of 719?mM(-1)?s(-1)， which was close to their theoretical maximal limit. The resulting nanoclusters were able to identify regions of defective vasculature in an ischemic murine hindlimb using MRI with iron doses that were 5-10 fold lower than those typically used in preclinical studies. Such high relaxivity was attributed to the molecular architecture of HPG， which mimics that of the water retentive polysaccharide， glycogen. The results of this study will be broadly useful in sensitive imaging applications.