The discovery of immune checkpoint pathways such as CTLA4 and PD1/PDL1, which control T-cell activation and activity, has fuelled interest in their modulation to achieve sustained anti-tumor immunity. This requires sufficient T-cell infiltration and activity in tumors. However, these processes are incompletely understood, in part due to the terminal nature of current analysis techniques. We therefore optimized labeling of activated T-cells with iron oxide nanoparticles, transferred labeled T-cells into tumor bearing hosts and performed serial MRI. Although, hypointense spots could be detected in the tumor rim following T-cell transfer, quantification is complicated by vascular abnormality induced susceptibility changes.