Microfluidic formulation and characterization of size-tunable microparticle magnetic particle imaging tracers

Magnetic particle imaging (MPI) is a novel imaging modality capable of quantitatively tracking the distribution of magnetic particles in living subjects. While early work on MPI has focused on magnetic nanoparticles as tracers, recent studies have highlighted the potential of magnetic microparticle tracers in MPI, suggesting higher sensitivity on a per particle basis. In this study, we formulated an MPI-tailored, size-tunable microparticle tracer by encapsulating Synomag-D, a commonly used commercial tracer, in an alginate matrix. We evaluated the magnetic properties and MPI performance of Synomag-D pre- and post-encapsulation. Our results show that microfluidics enable the monodisperse formulation of microparticle tracers ranging from 10 to 80 μm in diameter with consistent magnetization behavior. MPI performance evaluation indicated consistent properties across all microparticle tracers and demonstrated that microparticle signal is not dependent on medium viscosity, unlike Synomag-D, suggesting potential advantages in quantification. Dilution experiments revealed detection limits as low as 50 ng of iron for the smallest (~11 μm) microparticles and the potential to detect as few as four of the largest (80 μm) microparticles. Notably, these microparticle tracers exhibit a distinct signal dependence on excitation field amplitude, compared to the free Synomag-D tracer. These microparticle tracers for MPI possess potential applications in cell tracking, perfusion imaging, and multi-contrast MPI.