To determine whether the amount of intracellular superparamagnetic iron oxide (SPIO) in macrophages influences MR signal intensity during in vivo celluar tracking.

Peritoneal macrophages harvested from thioglycolate-treated mice were labeled with SPIO using concentrations of 112, 56, and 28 µgFe/ml, and different incubation times of 3h, 6h, 12h, 24h and 48 h, respectively. The iron concentration was quantified with the use of absorption spectrophotometry. Each group of macrophages labeled with different concentrations of SPIO was intravenously injected into 18 mice, after inoculation with S. aureus to the thigh. The relative signal intensity (SI) of the abscess wall (SI of the abscess wall/SI of muscle) was measured on MR and was analyzed by the use of the Kruskal-Wallis test.

A higher concentration of SPIO in the labeling solution and a longer incubation time resulted in a higher concentration of SPIO in the macrophages. The relative SI of the abscess wall (0.63 for 112 µgFe/mL; 0.67 for 56 µgFe/ml; 0.89 for 28 µgFe/mL) significantly decreased with an increase of SPIO concentration (k²=10.53, p < 0.005).

The amount of intracellular SPIO influences the MR signal intensity by the susceptibility effect, and it is recommended to use sufficient iron-oxide label as long as it does not affect cellular function and viability.