SPION-labeled macrophages have long retention times in the brain of a stroke mouse. SPION-labeled macrophages have long retention times in the brain of a stroke mouse. We administered 1–2 × 106 nanoparticle (VivoTrax; Magnetic Insight)-labeled mouse macrophages (Raw 264.7; American Type Culture Collection, Manassas, Virginia) to BALB/c mice through tail veins 24 hours after stroke. At 48, 72, and 96 hours poststroke, 2D-MPI was performed using a MOMENTUM MPI system. The MPI protocol used the parameters of FOV = 4 cm, 55 projections, best image quality, and default scan mode. Anatomic images were collected on the eXplore CT-120 microCT. Data analysis and image registration were performed using the Horos and VivoQuant software. In vivo iron oxide quantification was performed by imaging a fiducial containing a known concentration of tracer positioned beside the animal. The MPI signal was detected in the brain at 48, 72, and 96 hours poststroke; the accumulation of iron-labeled cells was the highest at 48 hours and reduced with time. L.C. Wu et al. AJNR Am J Neuroradiol 2019;40:206-212 ©2019 by American Society of Neuroradiology