The inflammatory response following traumatic brain injury (TBI) is regulated by phagocytic cells, comprising resident microglia and infiltrating macrophages. The present study was to monitor the early effect of monocytes/phagocytic accumulation and further to explore its kinetics in TBI mice. Localized macrophage population was monitored using USPIO nanoparticles enhanced in vivo serial magnetic resonance imaging (MRI). Flow cytometry based gating study was performed to discriminate between resident microglia (Ly6GˉˉCD11b+CD45low) and infiltrating macrophages (Ly6GˉCD11b+CD45high). Imaging and flow cytometric analysis revealed that maximum macrophage infiltration occurs between 66-72 h post injury (42-48 h post administration of USPIO) at the site of inflammation.