Thilo Wawrzik

Optimization of Magnetic Nanoparticles for Magnetic Particle Imaging

Systematic studies of the magnetic properties of magnetic nanoparticles from FeraSpin R and the FeraSpin Series with respect to their suitability as tracers for magnetic particle imaging are presented. Magnetic particle spectroscopy measurements indicate that FeraSpin R exhibits a harmonic spectrum very similar to that of Resovist, whereas FeraSpin L, XL, and XXL show a 2.5-fold increase of harmonic amplitudes compared to FeraSpin R. To understand the differences between the various samples of the FeraSpin Series, representing size-selected, narrowly distributed particles of identical composition extracted from FeraSpin R, measurements of the ac susceptibility (ACS), magnetorelaxometry (MRX), and static M-H curves were performed on suspended and immobilized particle samples. ACS and MRX measurements on immobilized samples indicate a wide distribution of anisotropy energies despite the narrow distribution of hydrodynamic sizes. Static magnetization measurements show that all samples exhibit a bimodal distribution of magnetic moments: The fraction of larger moments corresponds to the contribution from the overall particle core, whereas the smaller is attributed to the contribution from the crystallites comprising the core.

Characterization of magnetic nanoparticle systems with respect to their magnetic particle imaging performance

Abstract The optimization of magnetic nanoparticles (MNPs) as markers for magnetic particle imaging (MPI) requires an understanding of the relationship between the harmonics spectrum and the structural and magnetic properties of the MNPs. Although magnetic particle spectroscopy (MPS) – carried out at the same excitation frequency as the given MPI system – represents a straightforward technique to study MNPs for their suitability for MPI, a complete understanding of the mechanisms and differences between different tracer materials requires additional measurements of the static and dynamic magnetic behavior covering additional field and time ranges. Furthermore, theoretical models are needed, which correctly account for the static and dynamic magnetic properties of the markers. In this paper, we give an overview of currently used theoretical models for the explanation of amplitude and phase of the harmonics spectra as well as of the various static and dynamic magnetic techniques, which are applied for the comprehensive characterization of MNPs for MPI. We demonstrate on two multicore MNP model systems, Resovist® and FeraSpin™ Series, how a detailed picture of the MPI performance can be obtained by combining various static and dynamic magnetic measurements.

Characterization of Resovist® Nanoparticles for Magnetic Particle Imaging

This study investigates the dynamic magnetic properties of Resovist® for magnetic particle imaging (MPI) utilizing static M-H, ac susceptibility (ACS) and magnetic particle spectroscopy (MPS) measurements on a Resovist® suspension and an immobilized sample. Investigating the magnetic moment and anisotropy energy barrier distributions in the sample as well as the relationship between them, we clarified that the MNPs with large magnetic moment (10− 24~10− 23 Wb·m) and small anisotropy energy barrier play an important role in MPI.

New Perspectives for MPI: A Toolbox for Tracer Research

Since its invention, magnetic particle imaging (MPI) has gained increasing attention in academic as well as industrial research and development. It requires the application of an imaging agent, but the structure-efficacy relations are far from being fully understood and no ideal MPI tracer has been found so far. Here, we present a systematic investigation of the size dependence of the MPI spectra of identically composed, but differently sized iron oxide nanoparticles. We furthermore present a small angle X-ray scattering (SAXS) study as a route to assess the particle core structure. To that goal we used FeraSpinTM R and the FeraSpinTM Series (XS to XXL). We show that FeraSpin R offers an equal whereas FeraSpin L to XXL offer an improved MPI signal as compared to the hitherto “gold-standard” Resovist®. Moreover, the FeraSpin Series constitutes a versatile “toolbox” for MPI tracer research.

Drive-Field Frequency Dependent MPI Performance of Single-Core Magnetite Nanoparticle Tracers

The drive-field frequency of magnetic particle imaging (MPI) systems plays an important role for system design, safety requirements, and tracer selection. Because the commonly utilized MPI drive-field frequency of 25 kHz might be increased in future system generations to avoid peripheral nerve stimulation, a performance evaluation of tracers at higher frequencies is desirable. We have studied single-core magnetite nanoparticles that were optimized for MPI applications, utilizing magnetic particle spectrometers (MPS) with drive-field frequencies in the range from 1 to 100 kHz. The particles have core diameters of 25 nm and a hydrodynamic size of 77 nm. Measurements in the frequency range above 5 kHz were carried out with a newly designed MPS system. In addition, to exclude possible particle interaction, samples of different concentrations were characterized and compared.

Magnetic Particle Imaging: Exploring Particle Mobility

Magnetic Particle Imaging (MPI) is a promising new imaging modality, providing 3-dimensional imaging of magnetic nanoparticle tracers with high spatial and temporal resolution. Some recently developed experimental scanners have proven MPI to be feasible for small animal imaging. So far, one assumes that all particles contributing to the MPI signal share the same size distribution. An interesting extension of MPI would be to measure the mobility (or binding affinity) of the particles in the imaging volume. In this scenario, particles in certain regions may be partly immobilized by chemical binding, resulting in a transition from a Brownian to a Neel-dominated magnetization behavior – which is generally assumed for MNP tracers in blood. We propose that using two distinct frequencies, one below and one above the Brownian-Neel transition frequency, the binding state of the particles can be determined and utilized in MPI imaging. In this paper, we describe our MPI system and present simulations of 2-dimensional “Mobility MPI”.

Multivariate Magnetic Particle Spectroscopy for Magnetic Nanoparticle Characterization

An extended Magnetic Particle Spectrometer (MPS) setup comprising a resonating excitation and broadband detection channel, as well as an additional coil for static field exposure is used to measure the generation of higher harmonics depending on field and frequency parameters. Two samples are compared to demonstrate how the harmonic spectrum reflects the dimensional properties of the particles.

Effect of Brownian relaxation in frequency-dependent magnetic particle spectroscopy measurements

Our recent experiments show that the transition from Brownian to Néel relaxation regime strongly depends on the particle properties and the analysis of the spectrum for different frequencies reveals the dynamics of the ferrofluid.

Magnetic particle imaging scanner with 10-kHz drive-field frequency

Abstract Magnetic particle imaging (MPI) can be used as a fast diagnostic method for obtaining three-dimensional images from inside the body of small animals by the use of functionalized magnetic nanoparticles as tracer. Here, we present our scanner setup working at 10-kHz drive-field frequency to sample a field of view of 22×22×15 mm3 with up to 32 volume images per second. A resolution of about 1×2×1 mm3is achieved with iron oxide nanoparticles (Resovist). We discuss the properties of the complete system for application in imaging small animals such as mice.

Debye-Based Frequency-Domain Magnetization Model for Magnetic Nanoparticles in Magnetic Particle Spectroscopy

Signal generation in magnetic particle imaging and magnetic particle spectroscopy is generally described via the Langevin function of superparamagnetism. To include the magnetization dynamics, the Langevin function is augmented with a frequency-dependent term derived from the linear Debye model. Here, we extend that expression to cover the dynamics of higher harmonics, where each harmonic is multiplied with a complex-valued weighting factor, obtaining a frequency-domain model for application in the context of viscosity-/binding-dependent magnetic particle spectroscopy measurements.