Study of Oversized SPIO Nanoparticles with a Magnetic Particle Spectrometer for Suitability as MPI Tracers*

Magnetic Particle Spectrometry (MPS) is a sensitive analytical method that evaluates the magnetic response of superparamagnetic iron oxide nanoparticles (SPIO) under AC magnetic fields. We developed a custom MPS system for characterizing the physical properties of various SPIO. These SPIO are commonly used in magnetic particle imaging (MPI).

Traditionally, single-core SPIO Vivotrax (Magnetic Insight) has been widely used in the MPI community, but recently, the nanoflower-shaped cluster SPIO Synomag-D 70 nm (Micromod, Germany) has become a popular MPI tracer due to superior performance. Other types of SPIO represent oversized clusters, which can be useful for in-vivo MPI. This study explores the suitability of oversized sphere shell SPIO clusters for MPI, specifically Magnefy and ProMag (BangsLab, IN, USA) with a hydrodynamic diameter ~1 µm. Our MPS technique involves applying 1 kHz AC magnetic fields with an amplitude up to 25 mT to the SPIO and detecting their magnetization responses M(H) in the frequency domain. The resulting M(H) curves demonstrated that Synomag-D and Vivotrax exhibit steep magnetization, corroborating exceptional MPI performance. In contrast, the M(H) curves of the oversized nanoparticles show lower magnetization saturation and a less steep magnetization response. Specifically, ProMag and Magnefy exhibit low third harmonic magnitudes compared to Synomag-D's magnitude at ~1.6 mT (Fig. 2). The Point Spread Functions (PSF) of the particles received by our MPS are also compared to the PSFs recorded using our MPI scanner. While its magnetization amplitude is not as high as Vivotrax and Synomag-D, Promag exhibits a full width half max (FWHM) characteristic that is close to that of Vivotrax. This makes Promag a suitable tracer for MPI with an acceptable image resolution.