This study reports the T 1 and T 2 relaxation rates of rhodamine-labeled anionic magnetic nanoparticles determined at 7, 11.7, and 17.6 T both in solution and after cellular internalization. Therefore cells were incubated with rhodamine-labeled anionic magnetic nanoparticles and were prepared at decreasing concentrations. Additionally, rhodamine-labeled anionic magnetic nanoparticles in solution were used for extracellular measurements. T 1 and T 2 were determined at 7, 11.7, and 17.6 T. T 1 times were determined with an inversionrecovery snapshot-flash sequence. T 2 times were obtained from a multispin-echo sequence. Inductively coupled plasmamass spectrometry was used to determine the iron content in all samples, and r 1 and r 2 were subsequently calculated. The results were then compared with cells labeled with AMI-25 and VSOP C-200. In solution, the r 1 and r 2 of rhodamine-labeled anionic magnetic nanoparticles were 4.78/379 (7 T), 3.28/389 (11.7 T), and 2.00/354 (17.6 T). In cells, the r 1 and r 2 were 0.21/56 (7 T), 0.19/37 (11.7 T), and 0.1/23 (17.6 T). This corresponded to an 11-to 23-fold decrease in r 1 and an 8-to 15-fold decrease in r 2 . A decrease in r 1 was observed for AMI-25 and VSOP C-200. AMI-25 and VSOP exhibited a 2-to 8-fold decrease in r 2 . In conclusion, cellular internalization of iron oxide nanoparticles strongly decreased their T 1 and T 2 potency.