Patrick Vogel

Traveling Wave Magnetic Particle Imaging

Most 3-D magnetic particle imaging (MPI) scanners currently use permanent magnets to create the strong gradient field required for high resolution MPI. However, using permanent magnets limits the field of view (FOV) due to the large amount of energy required to move the field free point (FFP) from the center of the scanner. To address this issue, an alternative approach called “Traveling Wave MPI” is here presented. This approach employs a novel gradient system, the dynamic linear gradient array, to cover a large FOV while dynamically creating a strong magnetic gradient. The proposed design also enables the use of a so-called line-scanning mode, which simplifies the FFP trajectory to a linear path through the 3-D volume. This results in simplified mathematics, which facilitates the image reconstruction.

MRI Meets MPI: a bimodal MPI-MRI tomograph.

While magnetic particle imaging (MPI) constitutes a novel biomedical imaging technique for tracking superpara magnetic nanoparticles in vivo, unlike magnetic resonance imaging (MRI), it cannot provide anatomical background information. Until now these two modalities have been performed in separate scanners and image co-registration has been hampered by the need to reposition the sample in both systems as similarly as possible. This paper presents a bimodal MPI-MRI-tomograph that combines both modalities in a single system. MPI and MRI images can thus be acquired without moving the sample or replacing any parts in the setup. The images acquired with the presented setup show excellent agreement between the localization of the nano particles in MPI and the MRI background data. A combination of two highly complementary imaging modalities has been achieved.

Slicing Frequency Mixed Traveling Wave for 3D Magnetic Particle Imaging

Magnetic Particle Imaging is based on the nonlinear response of ferro- and superparamagnetic particles to magnetic fields [1]. For imaging, a field free point (FFP) within a string magnetic gradient on the order of 1–5 T/m is moved through the sample. A new gradient system design allows performing dynamic imaging in a linear sampling scheme by using a traveling wave approach [2]. We present an extension for doing 3D imaging using a traveling wave in combination with frequency mixing [3] and a sliced field of view (FoV). This approach provides the possibility of an arbitrarily large FoV in one direction without increasing the specific absorption rate (SAR) and allows the spatial encoding in the additional 2 dimensions.

First in vivo traveling wave magnetic particle imaging of a beating mouse heart

Magnetic particle imaging (MPI) is a non-invasive imaging modality for direct detection of superparamagnetic iron-oxide nanoparticles based on the nonlinear magnetization response of magnetic materials to alternating magnetic fields. This highly sensitive and rapid method allows both a quantitative and a qualitative analysis of the measured signal. Since the first publication of MPI in 2005 several different scanner concepts have been presented and in 2009 the first in vivo imaging results of a beating mouse heart were shown. However, since the field of view (FOV) of the first MPI-scanner only covers a small region several approaches and hardware enhancements were presented to overcome this issue and could increase the FOV on cost of acquisition speed. In 2014 an alternative scanner concept, the traveling wave MPI (TWMPI), was presented, which allows scanning an entire mouse-sized volume at once. In this paper the first in vivo imaging results using the TWMPI system are presented. By optimizing the trajectory the temporal resolution is sufficiently high to resolve the dynamic of a beating mouse heart.

Superspeed Traveling Wave Magnetic Particle Imaging

Since the first publication in 2005, several different scanner types for magnetic particle imaging (MPI) have been presented. One of these scanner concepts is traveling wave MPI (TWMPI). It uses a dynamic linear gradient array, which generates and moves a field free point with a strong gradient, which is necessary for scanning the sample in 3-D. Due to the linear properties of the TWMPI device, very fast 2-D imaging with frame rates higher than 1500 frames/s is possible (superspeed mode). Using the superspeed mode different high speed measurements are conceivable, e.g., fluid-dynamic investigations.

Slice scanning mode for traveling wave MPI

The sample shown in fig. 1B contains a tube with an inner diameter of 860 μm filled with undiluted Resovist® (Bayer-Schering, Germany) embedded in acrylic glass. The acquisition time for one slice is tslice=1/f1, with 4000 averages and f1=920 Hz the overall acquisition time is t=4000/f1=4,35 s. The intrinsic spatial resolution is given by the setup and was calculated to be 2 mm in z- and 4 mm in x-direction. The deconvoluted image (see fig. 1B) shows a good agreement and proves the feasibility of imaging studies, e.g. in pre-clinical context, based on TWMPI.

Numerically efficient estimation of relaxation effects in magnetic particle imaging.

Abstract Current simulations of the signal in magnetic particle imaging (MPI) are either based on the Langevin function or on directly measuring the system function. The former completely ignores the influence of finite relaxation times of magnetic particles, and the latter requires time-consuming reference scans with an existing MPI scanner. Therefore, the resulting system function only applies for a given tracer type and the properties of the applied scanning trajectory. It requires separate reference scans for different trajectories and does not allow simulating theoretical magnetic particle suspensions. The most accessible and accurate way for including relaxation effects in the signal simulation would be using the Langevin equation. However, this is a very time-consuming approach because it calculates the stochastic dynamics of the individual particles and averages over large particle ensembles. In the current article, a numerically efficient way for approximating the averaged Langevin equation is proposed, which is much faster than the approach based on the Langevin equation because it is directly calculating the averaged time evolution of the magnetization. The proposed simulation yields promising results. Except for the case of small orthogonal offset fields, a high agreement with the full but significantly slower simulation could be shown.

Rotating Slice Scanning Mode for Traveling Wave MPI

Since the introduction of magnetic particle imaging in 2005, several different types of scanner were presented. One of them is the traveling wave magnetic particle imaging (TWMPI) scanner. It uses for imaging a dynamic linear gradient array for a dynamic generation of a strong gradient and field free point. An unresolved issue of the TWMPI approach so far is the non-isotropic spatial resolution. As an alternative in this paper, a rotating slice scanning mode for TWMPI is presented to overcome this issue. This approach rotates the scanning-slices around the scanner axis and uses a projection reconstruction method to get a 3-D volume with a high isotropic resolution.

Rotational Drift Spectroscopy for Magnetic Particle Ensembles

Magnetic particles have become a core ingredient for many applications in chemistry, biology, and medical diagnostics, e.g., as a basis for bioanalytical methods or as tracer material for medical imaging. This paper presents a new method called rotational drift spectroscopy (RDS) which uses rotating magnetic fields for measuring the properties of magnetic nanoparticles (MNPs) in liquid suspensions. The RDS signal is based on the nonlinear rotational drift behavior of MNPs in rotating magnetic fields, which is highly dependent on the properties of the MNPs as well as their interaction with the environment. This dependency allows detecting the binding of functionalized MNPs with, e.g., proteins, viruses, or cells with potentially very high sensitivity. This paper presents first experiments demonstrating rotational drift behavior on aggregated magnetic particle ensembles and the corresponding experimental setup.

Bimodal TWMPI-MRI Hybrid Scanner—Coil Setup and Electronics

Magnetic particle imaging (MPI) was first presented in 2005. It is based on the nonlinear response of ferromagnetic material and the fact that the magnetization saturates at sufficiently high magnetic fields. In contrast to magnetic resonance imaging (MRI), MPI directly detects the concentration and distribution of superparamagnetic iron-oxide nanoparticles without any background of any tissue. To overcome this issue, a traveling wave MPI (TWMPI) device was combined with a low field MRI scanner to demonstrate the feasibility of a hybrid scanner, which contains both imaging modalities in a single device. The hardware of both separate approaches should be improved and optimized to reach higher fields and a higher resolution, especially for the MRI measurement. Therefore, the dynamic linear gradient array from the TWMPI scanner was modified in a way to produce also a homogenous magnetic field, which can be used for MRI.