Novel systems based on colloidal magnetic nanocrystals (NCs), potentially useful as superparamagnetic (SP) contrast agents for magnetic resonance imaging (MRI) have been investigated. The NCs we have studied comprise organic-capped single-crystalline maghemite (g-Fe2O3) cores possessing controlled sizes and shapes. We have comparatively examined spherical and tetrapod-like NCs, the latter being branched particles possessing four arms which depart out at tetrahedral angles from a central point. The NCs have been made soluble in aqueous solution by applying a procedure based on the surface intercalation and coating with an amphiphilic polymer shell. NMR relaxivities R1 and R2 were compared with ENDOREMs, one of the standard commercial SP-MRI contrast agent. We found that the spherical NCs exhibit R1 and R2 relaxivities slightly lower than those of ENDOREMs, over the whole frequency range; on the contrary, tetrapods show relaxivities about one order of magnitude lower.