Simulation study to minimize the single-sided FFP MPI scanner

One medical application scenario of a single-sided field free point (FFP) magnetic particle imaging (MPI) scanner is the localization of sentinel lymph nodes of patients with breast cancer. For this clinical application, the minimization of the scanner is essential to enable a comfortable patient access. Existing single-sided MPI scanners are non-flexible systems with limited field of view. This abstract presents a simulation study of a flexible downscaled single-sided FFP MPI scanner with unconfined field of view.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009047, DOI: 10.18416/IJMPI.2020.200904

First images obtained with a rabbit-sized Magnetic Particle Imaging scanner

The subject of this work is a magnetic-particle-imaging system featuring a field generator with an inner bore diameter of 180 mm. The scanner, which is large enough to accommodate small animals, is capable of generating a selection field either with a field free point or with a field free line. In this work, the imaging system is used to acquire two-dimensional data from a particle distribution using a field free point to record a system matrix. Finally, a plain particle phantom is measured to evaluate the imaging data and show the first two-dimensional image that is recorded with this setup.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009033, DOI: 10.18416/IJMPI.2020.200903

Highly symmetric filter for a fully differential receive chain

Differential signaling can provide a higher fidelity of the receive signals in MPI, which is especially important for mobile scanner setups not being operated in a shielding room. A passive, highly symmetrical filter is presented as a part of the fully differential receive chain of the scanner setup. Due to the combined use with a gradiometric receive coil, the requirements for the components in terms of voltage  and current specifications can be lowered which allows for partly building the filter on a PCB with well-matched SMD capacitors and trace lengths. The achieved attenuation for the differential-mode excitation field feedthrough is 77 dB while providing a common-mode rejection of more than 49 dB throughout the receive bandwidth.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009031, DOI: 10.18416/IJMPI.2020.200903

An Algorithm for computing optimal SNR-thresholds of a single-sided FFP MPI device

Due to the topology of a single-sided MPI device, the magnetic field suffers from inhomogeneities which are limiting the penetration depth. Additionally, the sensitivity profile of the receive coils compromises the measured signal. Therefore, the signal spectrum shows a relatively high noise level. To achieve a sufficient reconstruction and reduce the reconstruction time, only the frequency components are used, that have a sufficiently high signal to noise ratio (SNR). The algorithm presented in this paper allows for computing optimal SNR-thresholds which promise reconstructed images with high quality. For now, the algorithm is limited to 2D-reconstruction, but it already promises to enhance the existing reconstruction algorithm for the single-sided field free point MPI device significantly

Investigation of the spatial resolution and penetration depth of a single-sided MPI device in three-dimensional imaging

A one-sided arrangement of the components relevant for magnetic particle imaging leads to improved patient access, as the object size can be unlimited. In this paper, we further investigate the properties of the single-sided MPI system published earlier. A phantom with up to five positions filled with Perimag particles was rotated to evaluate the spatial resolution depending on the orientation of the receive coils. The reconstruction was done with a Kaczmarz algorithm and a Tikhonov regularization with individually calculated regularization parameters for each measurement. The penetration depth and the spatial resolution in the y-,z-plane were evaluated.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009053, DOI: 10.18416/IJMPI.2020.200905

A concept for an MPI scanner with Halbach arrays

One of the major challenges of MPI is the upscaling of scanners. Typically, the magnetic fields are generated electromagnetically with immense power consumption, which becomes a severe issue for larger setups. However, electromagnets provide high flexibility in terms of adjusting the gradient strength of the selection field and providing quickly alternating drive fields. The here proposed scanner concept reduces the power consumption drastically, because both the selection field and the drive field are generated by rotating permanent magnets. In addition, it maintains the flexibility to adjust the gradient strength for different imaging sequences. The selection field consists of two concentric Halbach quadrupoles. Mutual rotation of the nested quadrupoles enables the variation of the gradient strength. The drive field is generated by two reversely rotating Halbach dipoles driving the field free point on a radial trajectory. To show the feasibility of the setup an exemplary image simulation with an additional excitation field is performed and the resulting reconstruction is discussed.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009008, DOI: 10.18416/IJMPI.2020.200900

Further system characterization of the Single-Sided MPI Scanner with two- and three-dimensional measurements

With the single-sided MPI-Scanner unlimited object sizes can be measured and it is suitable for various medical applications. In previous measurements, the penetration depth for two-dimensional measurements has already been investigated, as well as the local resolution. The system has evolved from a one-dimensional to a three-dimensional system. Here, further measurements are carried out to characterize the system. To measure the penetration depth and spatial resolution as well as the influence of the orientation of the receiving coils, a four-layer phantom was measured three-dimensionally at different angles. Furthermore, a dilution series was measured to determine the linearity of the system reaction, which is a prerequisite for the quantifiability of the results. The results of both studies are shown and discussed in this work.   Int. J. Mag. Part. Imag. 7(2), 2021, Article ID: 2109001, DOI: 10.18416/IJMPI.2021.210900

Artifacts in field free line magnetic particle imaging in the presence of inhomogeneous and nonlinear magnetic fields

Introduction: Magnetic Particle Imaging (MPI) is an emerging medical imaging modality that detects super-paramagnetic particles exploiting their nonlinear magnetization response. Spatial encoding can be realized using a Field Free Line (FFL), which is generated, rotated and translated through the Field of View (FOV) using a combination of magnetic gradient fields and homogeneous excitation fields. When scaling up systems and/or enlarging the FOV in comparison to the scanner bore, ensuring homogeneity and linearity of the magnetic fields becomes challenging. The present contribution describes the first comprehensive, systematic study on the influence of magnetic field imperfections in FFL MPI. Methods: In a simulation study, 14 different FFL scanner setups have been examined. Starting from an ideal scanner using perfect magnetic fields, defined imperfections have been introduced in a range of configurations (nonlinear gradient fields, inhomogeneous excitation fields, or inhomogeneous receive fields, or a combination thereof). In the first part of the study, the voltage induced in the receive channels parallel and perpendicular to the FFL translation have been studied for discrete FFL angles. In the second part, an imaging process has been simulated comparing different image reconstruction approaches. Results: The induced voltage signals demonstrate illustratively the effect of the magnetic field imperfections. In images reconstructed using a Radon-based approach, the magnetic field imperfections lead to pronounced artifacts, especially if a deconvolution using the point spread function is performed. In images reconstructed using a system function based approach, variations in local image quality become visible. Conclusion: For Radon-based image reconstruction in FFL MPI in the presence of inhomogeneous and nonlinear magnetic fields, artifact correction methods will have to be developed. In this regard, a first approach has recently been presented by another group. Further research is required to elucidate the influence of magnetic field imperfections in MPI using a system function based approach

Novel Field Geometry Using Two Halbach Cylinders for FFL-MPI

Magnetic Particle Imaging (MPI) offers several methods for encoding magnetic materials in three-dimensions. The complexity of the imaging device in MPI using a field free line (FFL) is the biggest challenge in realizing such devices. This is especially the case for systems featuring large bores, high gradients or high temporal resolution. In this work, we suggest a novel Halbach cylinder based field generator that is able to generate an FFL with a gradient of 5 T/m. By rotating the Halbach array, FFL projections can be acquired. Furthermore, just one static drive field coil is needed for two-dimensional imaging. The presented concept could combine high gradient strength and high temporal resolution with minimum space requirements for FFL-MPI imaging in the future.Int. J. Mag. Part. Imag. 4(2), 2018, Article ID: 1811004, DOI: 10.18416/IJMPI.2018.181100