Magnetic nanoparticles are attracting extensive interest for their ability to enhance the magnetic resonance contrast effect.[1,2] Spherical superparamagnetic iron oxide(SPIO)nanoparticles(e.g.,Feridex and Resovist)have been developed as T2 negative contrast agents for magnetic resonance imaging(MRI)in clinical use due to their biocompatibility and ease of synthesis.However,the relatively low transverse relaxivity(r2)and poor performance of spherical SPIO nanoparticles as T2 contrast agents has hampered their clinical applications.Herein,we report the rational design strategies to achieve high MR contrast sensitivity by controlling the composition,size,surface,and morphology,including gadolinium-embedded iron oxide(GdIO)nanoparticles,octapod SPIO nanoparticles and(111)exposed SPIO nanoplates.[3-8] Compared to conventional SPIO nanoparticles,these high-performance iron oxide based nanoparticles are much more effective contrast agents for in vivo MRI and small tumor detection and analysis,which holds great promise for highly sensitive,early stage and accurate detection of cancer in the clinic.