Relaxation profiles of USPIO nanoparticles with varying concentration. (a) T1-weighted and T2-weighted images of USPIO nanoparticles in saline solution (0.9% sodium chloride) with different iron concentrations from 0.1 mM to 1.5 mM. (b) The change of mean T1- and T2-weighted signal intensity as the iron concentration. (c) The relationship of longitudinal relaxation rate and iron concentration and the derived r1 relaxivity 0.556 mM−1·s−1. (d) The relationship of transversal relaxation rate and iron concentration and the derived r2 relaxivity 7.55 mM−1·s−1.