Intracellular SPIO labeling of microglia: high field considerations and limitations for MR microscopy.

The purpose of this study is to investigate MRI contrast as a function of magnetic field strength for microglia labeled with superparamagnetic iron oxide (SPIO) nanoparticles. A rat microglia cell line, Bv2, was incubated with SPIOs for 6 h. In two separate experiments conducted at 11.75 and 21.1 T, the impact of SPIO loading and cell count on T(1) , T(2) and T(2) * contrast were evaluated: (a) cells were incubated with 1, 2 or 5 µl of Feridex; and (b) cells incubated with 5 µl of Feridex were used to form layers of 25 000, 50 000, 100 000 or 200 000 cells. Intracellular iron was analyzed with ICP-MS and histological staining while cell viability was evaluated by Trypan blue dye exclusion. Bv2 cells displayed increases in intracellular iron concentration with SPIO exposure, with the highest labeling yielding 0.83 pg of Fe per cell. Although no differences were identified for T(1) mechanisms, both fields displayed trends toward increasing T(2) and T(2) * contrast with increasing SPIO loading or cell count, with few differences evident between fields. Bv2 cells can be labeled readily with commercially available SPIOs, with the potential of increasing the intracellular iron content over short incubation times without impacting viability. This phagocytotic cell line not only provides direct SPIO uptake but also plays a critical role in inflammation after brain injury, providing a possible neurodegeneration biomarker. With few differences between field strengths and limited ability to quantify intracellular iron content and cell count, this study demonstrates only a slight benefit of SPIO-based contrast agent at high fields based on susceptibility-based contrast and detection, necessitating unique agents for such applications.