For the in vivo imaging of macrophages using MRI, the feasibility of labeling macrophages with iron oxide and the number of SPIO-labeled macrophage detected in 1.5 T MR, were assessed.

The MR signal intensity was measured with variable concentrations of iron oxide, ranging from 112 to 2.384×10^-7 µgFe/mL. The macrophages were incubated in SPIO solution (112 µgFe/mL) for 24 hours. The MR signal intensity was measured in variable numbers of SPIO-labeled macrophages.

The MR image signal intensity gradually increased with decreasing SPIO concentration, and reached a plateau at a concentration of 0.219 µgFe/mL. After incubation with iron oxide, the compact uptake of SPIO was detected in the cytoplasm of the macrophages using Prussian blue staining. No susceptibility effect was detected in the tubes of more than 122 macrophages.

The MR signal intensity was dependent on the number of macrophages. No susceptibility effect due to a cluster of SPIO-labeled macrophages was detected in more than 488 cells.