Temporal evolutions of the longitudinal relaxation rate after USPIO injection in Experiment B. Each graph corresponds to one ROI, and, in each graph, each time course was obtained from a single animal (four time courses per graph: 2 C6 (open symbols) and 2 RG2 (filled symbols)). Dashed lines and double arrows indicate the time at which MR acquisitions and Gd injection were performed during Experiment A, Session 2, prior and after USPIO injection (time= 0 min was defined as the beginning of the USPIO injection).