Non-invasive imaging of inflammation is crucial for understanding both inflammatory processes and immunotherapy. We propose a non-invasive and non-radioactive approach for longitudinal imaging of activated immune cells. We show that the use of the natural nucleoside cytidine as CEST imaging probe enable the monitoring of specific activated T-cells population with MRI. By using fluorescent analog of cytidine it is shown that CD8 T-cells are in situ labeled, underlying the specificity of nucleosides as potential probes for monitoring activated immune cells. We conclude that by using cytidine as a MRI-based probe one could monitor specific activated T-cells population during inflammation.