Flood, S. N.; Foster, P. J.

Motivation: The sensitivity and resolution of magnetic particle imaging (MPI) depend on the choice of tracer and specific imaging parameters. For cell tracking applications with MPI, both the superparamagnetic iron oxide (SPIO) tracer and the cell labeling efficiency have a significant impact on MPI sensitivity and vary for different tracers. Objective: This study compares three commercially available SPIO tracers (VivoTrax, Synomag-D and ProMag) and SPIO-labeled cells using magnetic particle relaxometry (MPR) and imaging. Further, the effect of imaging parameters (gradient field strength and drive field amplitude) on MPI signal strength, resolution, and cell detection limits, was evaluated. Results: Of the three SPIO tracers, the maximum MPI signal measured by MPR was highest for Synomag-D in solution, however, the signal was significantly lower after intracellular incorporation of Synomag-D. The peak signal for ProMag was not different for free and intracellular particles. The cellular iron loading was higher for ProMag compared to Synomag-D. The total MPI signal measured from images of free and intracellular SPIOs was highest for ProMag. Varying imaging parameters confirmed that a lower gradient field strength and higher drive field amplitude improve tracer and cellular sensitivity. Conclusion: These results show that the evaluation of tracers by relaxometry is not sufficient to predict the performance of all SPIO tracers; in particular not for larger, polymer-encapsulated iron particles such as ProMag, or for SPIO particles internalized in cells. Improvements in MPI sensitivity through lower gradient field strength and higher drive field amplitudes are associated with a trade-off in image resolution.