Signal Intensity versus Spatial Resolution: Divergent MPI Performance of SPIONs in Mesenchymal Stromal Cell Labelling

Magnetic Particle Imaging (MPI) is a highly sensitive modality for non-invasive cell tracking; however, imaging performance is strongly influenced by nanoparticle properties and intracellular behavior following labelling. In this study, four commercially available superparamagnetic iron oxide nanoparticles (SPIONs) - ProMag, VivoTrax, SynoMag-D, and Ferumoxytol - were evaluated for labelling efficiency and cytocompatibility in mouse mesenchymal stem/stromal cells, followed by MPI characterization of biocompatible candidates.

SPION labelling efficiency and cytocompatibility were systematically evaluated across a range of iron concentrations and incubation durations using Prussian blue staining, and ATP-based viability assay respectively. Based on these outcomes, biocompatible candidates were selected for further analysis. Cells labelled with ProMag or VivoTrax for 2-hour underwent MPI characterization to evaluate signal properties, alongside transmission electron microscopy (TEM) to examine intracellular nanoparticle localization and distribution.

ProMag achieved >90% labelling efficiency at 20 µg Fe/mL, whereas VivoTrax required ≥240 µg Fe/mL to reach >75% efficiency. SynoMag-D and Ferumoxytol required transfection agents that induced cytotoxicity and were excluded. MPI analysis demonstrated a linear correlation between signal intensity and cell number for both SPIONs. ProMag-labelled cells exhibited a 2.33-fold increase in total signal intensity, closely matching the ~2.3-fold increase in intracellular iron loading, confirming iron content as the primary determinant of signal strength and detection sensitivity (limit: 7,812 cells). In contrast, VivoTrax-labelled cells produced higher maximum signal per unit iron and significantly improved spatial resolution, indicated by reduced full width at half maximum, suggesting that spatial resolution is primarily governed by particle-specific physicochemical properties and their intracellular behavior following uptake. TEM revealed ProMag predominantly as dispersed particles, whereas VivoTrax formed aggregates within endosomal structures.

Intracellular iron loading dictates MPI signal strength, whereas particle-specific characteristics govern spatial resolution. Optimizing SPION selection is therefore essential to balance sensitivity and image quality for preclinical cell tracking applications.