Christian Kuhlmann

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.

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.

Magnetic Characterization of Clustered Core Magnetic Nanoparticles for MPI

Diagnostic imaging is of utmost importance for clinical routine and research and is also gaining more and more relevance in preclinical research. Magnetic particle imaging (MPI) as a new imaging modality may open new perspectives in the field. Iron oxide magnetic nanoparticles (MNPs) are already used as MRI contrast agents are of general interest for various other biomedical applications, and are also promising as MPI tracers. However, to provide the desired performance for MPI, it is still necessary to optimize the particles signal efficacy. Optimization remains an unexpected challenge and, as a consequence, considerable importance is being given to the research of MNPs, where efforts have been placed to understand the relation between particle structure and particle magnetic properties in more detail. In the present study, two clustered core MNPs exhibiting similar structures, but significant differences in their magnetic particle spectra, are investigated. Their static and dynamic magnetic properties are analyzed to understand the reasons for such differences as well as to gain a better insight of their relation to the particle structure.

A 3D MPI system for biological studies on MICE

By imaging plastic or bio-compatible phantoms, we were studying the properties of the MPI system function and its dependence on components of the imaging system and particle dynamics. In progress towards three-dimensional imaging, depicts a two-dimensional image of an “E” phantom obtained with the scanner, applying the hybrid-matrix approach.

Dynamic Magnetic Properties of Optimized Magnetic Nanoparticles for Magnetic Particle Imaging

Magnetic particle imaging (MPI) requires optimized tracers with monodisperse magnetic nanoparticle (MNP) cores. Organic phase routes to core synthesis are most promising, but require a subsequent phase transfer step to add a biocompatible, water-soluble coating. This transfer process can potentially produce dimers or clusters of magnetic cores that are not resolved by routine hydrodynamic size measurements. Here, we present systematic dynamic magnetic investigations on nominally single-core MNPs with respect to their suitability as tracers for MPI. Two MNP suspensions with similar core and hydrodynamic diameters show differences in their ac susceptibility (ACS) and magnetic particle spectroscopy (MPS) data. The maximum in the imaginary part of the ACS at ~1 kHz for both samples is caused by the Brownian relaxation of multicore (estimated to be dimers) particles with hydrodynamic diameters of more than 70 nm, whereas the maximum at ~150 kHz is caused by the Néel relaxation of single-core particles. The different relative portions of both particle types cause a different frequency dependence of the harmonic spectra.

Optimization of MNPs by size fractionation for MPI application

Magnetic iron oxide nanoparticles are numbered among the most suitable tracer materials for Magnetic Particle Imaging (MPI) since they feature the potential of high signal intensities while simultaneously being well tolerated in vivo. To tap the full potential of this innovative imaging modality, magnetic nanoparticles (MNPs) with tailored magnetic properties are required. To achieve such tailoring, an understanding of the relation between particle structure and the resulting MPI performance is most desirable. In this work, we present the effect of the MPI efficacy of in-house synthesized MNPs in dependence of their hydrodynamic diameter.

Estimating particle mobility in MPI

Numerical simulations and initial measurements with our MPI scanner [3] show that “Mobility”-MPI (mMPI) is a capable extension of the magnetic particle imaging technique which enables binding-state detection.

Particle Mobility in Magnetic Particle Imaging

Methods For magnetic particle imaging a time-varying magnetic field in the range of a few tens of a kHz is applied and the nonlinear response of the particles is observed. Under constraints of an additional gradient field an image is reconstructed from the resulting spectrum of higher harmonics. In existing models, it is assumed that tracer properties are constant and homogenous in the volume of interest. If the sample contains particles with different mobility, e.g. by binding to analytes, MPI images acquired at different imaging frequencies can be used to estimate their mobility in addition to their spatial distribution.