Silicon-based particles can be hyperpolarized via dynamic nuclear polarization to enhance 29Si magnetic resonance signals. Application of this technique to nanoscale silicon particles has been limited because of the low signal enhancements achieved; it is hypothesized that this is due to the low number of endogenous electronic defects inherent to the particles. We introduce a method of incorporating exogenous radicals into silicon nanoparticle suspensions in order to improve the hyperpolarization of 29Si nuclear spins to levels sufficient for in vivo MR imaging. Calibration of radical concentrations and polarization times are reported for a variety of silicon particle sizes (30−200 nm in diameter), with optimal radical concentrations of 30−60 mM. Addition of the radical slightly affects the T1 relaxation of the nanoparticles; however, these losses in T1 are overcome by the overall improvement in 29Si magnetization. With optimal amounts of the added radical, 29Si T1 times are ∼20 min, and MR images in phantoms can be achieved over an hour after hyperpolarization. Co-registered 1H/29Si MR imaging of nanoparticles administered to a mouse model is also presented.