Probing particle-matrix interactions during magnetic particle spectroscopy

Abstract

Abstract Interactions between magnetic nanoparticles (MNPs) and their surrounding matrix were investigated during temperature-controlled magnetic particle spectroscopy (MPS). MNPs are used as tracers in emerging biomedical imaging technologies, including magnetic particle imaging (MPI), which employ spatially inhomogeneous time-varying magnetic fields to generate imaging signals. For safe and reliable imaging procedures, it is important to understand the thermal and mechanical interactions between MNPs and the surrounding biological matrix. We performed in-vitro investigations using magnetic field hyperthermia (MFH) and temperature-controlled MPS to elucidate the excitation-field dependent MNP-matrix interaction. MFH measurements of ferucarbotran (FCT) MNPs were conducted using a low-frequency, high-amplitude, set-up designed to emulate the typical field conditions employed in MPI. First, it was tested whether concentrated FCT samples produce measurable heating in MFH characterization. A strong bulk heating effect (ΔT = 40 K) was observed, showing that the individual MNPs dissipate a significant quantity of heat. However, no measurable bulk temperature increase occurred during MFH of dilute FCT preparations in liquid or gelatin suspensions. Then, temperature-controlled MPS measurements were performed on dilute FCT immobilized in gelatin. The melting transition of the dilute gelatin preparation produced a clear change in the temperature-dependent MPS spectra. The melting transition was reproducible between measurements using the same field excitation, with a standard deviation of σ = 0.2 K. The melting transition temperature was found to depend upon the amplitude of the MPS excitation field. A reduction in the melting transition onset temperature of 2.3 K was observed when the excitation amplitude was increased from 6 mT to 25 mT. This shift is attributed to increasing mechanical and thermal particle-matrix interactions at higher excitation fields. The results show that within the studied system, individual MNPs interact with their local environment in a non-negligible way during MPS measurement, even when bulk heating of the sample is not detected. Our findings hold significance for the potential impact of MPI and related technologies on living tissues, a subject deserving of further study.