Sensitivity and Cellular Labelling Performance of Magnetically Fractionated SPIONs for Multimodal MRI/MPI Imaging

Magnetic Particle Imaging (MPI) detects superparamagnetic nanoparticles, enabling bimodal contrast with MRI. Resovist®/Ferucarbotran, still used in research, has clinical safety compatibility but suboptimal MPI performance due to many small 5 nm SPIO cores. Magnetic fractionation can enrich larger cores, improving MPI signal and supporting cellular imaging applications. To enable bimodal MRI/MPI and assess in vivo extracellular vesicle (EV) labelling, we characterized the imaging sensitivity and cell-labelling performance of VivoTrax, a commercial formulation similar to Resovist®, and VivoTrax Plus, obtained by magnetic fractionation. VivoTrax Plus showed higher MRI transverse relaxivity and MPI sensitivity than VivoTrax, and both formulations displayed low toxicity toward adipose-derived stem cells (ASCs). VivoTrax Plus allowed MRI detection of small cell numbers, around 100 cells, in agarose phantoms with greater sensitivity than VivoTrax. However, EVs of 30–150 nm isolated from ASCs labelled with VivoTrax Plus did not retain SPIONs, whereas EVs from VivoTrax-labelled ASCs did. Preliminary in vivo experiments in SOD-G93A mice showed MRI-detectable signal voids in the brain after intranasal EV administration, suggesting EV migration to lesioned areas. Overall, magnetic fractionation improves SPION imaging sensitivity but may alter relevant biological properties.