ONLINE COVER Tracking TAMs. This two-photon microscopy image shows uptake of perfluorocarbon-containing nanoparticles (PFC-NPs; red) by tumor-associated microglia and macrophages (TAMs) in a central nervous system (CNS) tumor. Vasculature is shown in yellow and tumor cells are shown in green. Increased TAM abundance in CNS tumors has been correlated to more aggressive disease; however, monitoring these cells in patients has only been achievable using invasive biopsies. Here, Croci et al. showed that CNS TAMs could be monitored by magnetic resonance imaging (MRI) for PFC-NPs enriched for the stable isotope, fluorine-19 (¹⁹F). The authors found that TAMs could be tracked longitudinally and in response to radiotherapy, highlighting the potential of ¹⁹F MRI as a monitoring strategy for patients with CNS tumors.