OPM magnetorelaxometry in the presence of a DC bias field

Aaron Jaufenthaler,Volkmar Schultze,Ronny Stolz,Theo Scholtes,Christian B Schmidt,Michael Handler,Daniel Baumgarten

doi:10.1140/epjqt/s40507-020-00087-3

Abstract

Spatial quantitative information about magnetic nanoparticle (MNP) distributions is a prerequisite for biomedical applications like magnetic hyperthermia and magnetic drug targeting. This information can be gathered by means of magnetorelaxometry (MRX) imaging, where the relaxation of previously aligned MNP’s magnetic moments is measured by sensitive magnetometers and an inverse problem is solved. To remove or minimize the magnetic shielding in which MRX imaging is carried out today, the knowledge of the influence of background magnetic fields on the MNP’s relaxation is a prerequisite. We show MRX measurements using an intensity-modulated optically pumped magnetometer (OPM) in background magnetic fields of up to 100~upmu mbox{T}. We show that the relaxation parameters alter or may be intentionally altered significantly by applying static fields parallel or antiparallel to the MNP’s alignment direction. Further, not only the relaxation process of the MNP’s magnetic moments could be measured with OPM, but also their alignment due to the MRX excitation field.

Highlights

Magnetic nanoparticles (MNP) offer a variety of applications in biomedicine, e.g. magnetic hyperthermia [1] and magnetic drug targeting [2]
After switching-off the external field, the MNP’s net magnetic moment decays, which can be measured by highly sensitive quantum magnetometers, e.g. superconducting quantum interference devices (SQUID) [3] and optically pumped magnetometers (OPM) [4,5,6], as well as fluxgates [7]
Common visible spikes in the noise spectrum arise at the mains frequency and its harmonics, whereas the spikes occuring at a single background magnetic field only, e.g., the spike at 390 Hz at B0 = 5 μT, arise from lock-in amplifier (LIA) mixing with the ac current driving the OPM heater

Summary

Introduction

Magnetic nanoparticles (MNP) offer a variety of applications in biomedicine, e.g. magnetic hyperthermia [1] and magnetic drug targeting [2]. All these applications require knowledge about the quantitative spatial distribution of MNP and magnetorelaxometry (MRX) is a promising technique for their quantitative imaging. After switching-off the external field, the MNP’s net magnetic moment decays, which can be measured by highly sensitive quantum magnetometers, e.g. superconducting quantum interference devices (SQUID) [3] and optically pumped magnetometers (OPM) [4,5,6], as well as fluxgates [7]. MRX with spatially different excitation fields allows, after solving an appropriate inverse problem, to obtain quantitative spatial MNP information, e.g. with SQUID [10] or commercially available OPM [11]

Methods

Results

Discussion

Conclusion