Magnetorelaxometry for localization and quantification of magnetic nanoparticles for thermal ablation studies.

In magnetic heating treatments, intratumorally injected superparamagnetic iron oxide nanoparticles (MNP) exposed to an externally applied alternating magnetic field generate heat, specifically at the tumor region. This inactivates cancer cells with minimal side effects to the normal tissue. Therefore, the quantity of MNP needs to be thoroughly controlled to govern adequate heat production. Here, we demonstrate the capability of magnetorelaxometry (MRX) for the non-invasive quantification and localization of MNP accumulation in small animal models. The results of our MRX measurements using a multichannel vector magnetometer system with 304 SQUIDs (superconductive quantum interference device) on three mice hosting different carcinoma models (9L/lacZ and MD-AMB-435) are presented. The position and magnitude of the magnetic moment are reconstructed from measured spatial magnetic field distributions by a magnetic dipole model fit applying a Levenberg-Marquadt algorithm. Therewith, the center of gravity and the total amount of MNP accumulation in the mice are determined. Additionally, for a fourth mouse the distribution of MNP over individual organs and the tumor is analyzed by single-channel SQUID measurements, obtaining a sensitive spatial quantification. This study shows that magnetorelaxometry is well suited to monitor MNP accumulation before cancer therapy, with magnetic heating being an important precondition for treatment success.