Tuning MRI contrast with sub-5 nm magnetic iron oxide nanoparticles. Magnetization of the nanoparticles is size-dependent with reduced magnetic moment and susceptibility in sub-5 nm scale (A, adopted from the reference 26 with permission, Copyright © 2011, American Chemical Society). Reducing the nanoparticle size leads to high r1/r2 ratio (B) and enhanced vasculature and kidney by 3-nm SIO-3 in T1-weighted MRI at the field strength of 3T (C), adopted from the reference 27 with permission. By modulating the surface coating, T1 and T2 dual contrast can be optimized as shown in MRI of subcutaneous tumors (D, adopted from the reference 108 with permission, Copyright © 2017, American Chemical Society).