International Journal on Magnetic Particle Imaging (IJMPI)
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Trade-off between power consumption and receive signal strength for inductively coupled transmit-receive circuits in MPI
No full textThe signal chain of a Magnetic Particle Imaging system can be designed to include a dedicated receive-only coil or to combine transmit and receive coils. More common are circuits with separate transmit and receive chains, using dedicated receive coil(s) that cancel the excitation feedthrough. However, combined transmit-receive systems may prove to have several benefits, such as reducing the system complexity, providing a lower resistive noise contribution due to larger copper cross-section, facilitating a transition from 1D to multidimensional signal generation and acquisition, and implementing an embedded band-stop filter. In this work, a matching condition that governsinductors for resonant combined transmit-receive systems is investigated. To tap the signal, a compromise between the obtained signal strength and power consumption is considered, caused by the chosen circuit topology, that balances both signal loss and power consumption at a -3 dB benchmark
A study on different Halbach arrays of magnets in relation to magnetic field gradients in a handheld MPI system
No full textHandheld MPI devices have promising clinical application prospects due to their portability. However, there is a lack of detailed analysis of Halbach magnet arrays, which are critical for generating field-free regions in handheld MPI system imaging. Therefore, investigating the relationship between the Halbach array configuration and the resulting magnetic field is necessary. In this study, we modify the magnet rotation angle, magnetic field rotation angle, magnet thickness, and magnet outer radius of cylindrical magnets to vary the Halbach array configuration, and further explore and confirm their relationship with the magnetic field gradients. In the future, these results can be utilized to adjust the configuration of the Halbach magnet array to better meet clinical requirements and further promote the application of handheld MPI
Gradiometric human head coil-coupled magnetoresistive sensor for sensitivity improvement in magnetic particle imaging
No full textRealization of magnetic particle imaging (MPI) for human brain scanner concerns safety limits while demanding high sensitivity to probe low magnetic signal from allowable tracer dosage. In order to reduce magnetostimulation effects, applying few kHz excitation field is optional with a high inductance receive coil having noisy output voltage around self-resonance frequencies. Here, we employed magnetoresistive (MR) sensor to improve signal quality of using gradiometric human head coil via flux transformer. This inductive current-sensitive detection of MR sensor provides low noise level to detect Langevin signal from a mini coil equivalent to magnetic moment of few microg Fe Resovist®. While our split gradiometer setup leads to considerable signal attenuation and feed-through suppression, we later preliminary confirmed the 3rd harmonic response of few mg Fe Resovist® sample. This value was still higher than the expected iron-mass sensitivity limit of the magnetometric system. Although ac current filtering (e.g., high-order resonant band-pass filter) is further necessary to compensate for harmonic noises and secure pure sinusoidal excitation, MR sensor-embedded receive chain appears applicable toward clinical MPI system
Synthetic Antiferromagnet Disk Particles for Hyperthermia Applications
No full textMagnetic hyperthermia is a technique for the destruction of cancerous cells by heat arising from magnetization losses from magnetic nanoparticles (MNPs) in an oscillatory magnetic field. For the typically employed chemically synthesized superparamagnetic nanoparticles the heat generation remains limited by the low magnetization of the oxidic materials, the wide particle size distribution, and the narrow shape of their magnetic hysteresis loop. To overcome these limits, synthetic antiferromagnet magnetic disk particles (SAF MDP) consisting of two ferromagnetic (F) layers separated by a non-magnetic layer were designed. The geometry and magnetic system parameters were optimized via micromagnetic modeling to obtain an antiferromagnetically coupled (zero moment) ground state and an abrupt switching into a ferromagnetically aligned state in an applied field HAF->F to maximize the hysteretic loss. The magnetic multilayer was sputter-deposited onto a 50nm-thick Ge sacrificial layer on a silicon wafer. Self—assembled polysterene spheres served as an etch mask for the successive nanopatterning of disk-shaped islands. These were then detached from the supporting wafer by dissolution of the Ge layer and subsequently encapsulated in thin silica shells using tetraethyl orthosilicate (TEOS). The magnetic properties of the SAF MDP analyzed by vibrating sample and Kerr magnetometry, high-resolution in-field magnetic force microscopy closely matched the design goals and micromagnetic simulation results. A turn-on/turn-off magnetism of the SAF MDP and a hysteretic loss close to the theoretical limit given by the magnetic material with the highest saturation magnetization and a rectangular SAF hysteresis loop could be demonstrated. Experiments mapping the hysteretic loss of SAF MDP suspensions for different operation conditions demonstrated a superior performance compared to classical superparamagnetic nanoparticles
Multicontrasting MPS by dual-tone nonlinearity probing
No full textNovel MPI-based modalities such as multi-contrast imaging or remote viscosity recording require independent measurement of at least two or more magnetic particle types simultaneously. Particle response models based on the Fokker-Plank equation allow independent reconstruction of core and hydrodynamic diameters. However, due to complexity and stochastic character, they remain a black box for explaining the origin of measured nonlinear distortions. A model is required to suggest which frequency lines to measure, which fields to apply for reconstruction of particle core size distribution, and to explicitly show limitations of setup and measurement scheme (e.g., the range of core diameters available for reconstruction). Assuming that the amplitude of the sample magnetic moment and its distortions are determined by the number of excited magnetic moments, and response phase is governed by amplitude-dependent relaxation mechanisms, we show how and under what conditions two-tone systems become the key to independent measurement of the response from particles of different diameters
Recent Progress in Model-Based Reconstruction using Approximate Particle Models and low Calibration Effort
No full textIn magnetic particle imaging (MPI) one of the key challenges is still the model-based reconstruction problem particularly when using multi-dimensional Lissajous-type excitation. Already during the last IWMPI 2023 promising results towards a solution to this key problem have been presented from different groups. A combination of realistic physical modeling of the nanoparticles\u27 magnetization behavior and a careful calibration of the remaining scanner components result in improved model-based reconstructions when compared to the fully calibrated system matrix approach. In addition, the question has been asked to which extent the equilibrium model can be extended to include particle anisotropy, which significantly increases the accuracy of the model while still keeping the computational effort low. The previous findings from different groups at the last IWMPI provided the starting point for a larger joint collaboration where we extended the previous results by an extensive series of experiments which showcase that the model-based reconstruction can be applied in aligned and unaligned immobilized cases but even more important also to fluid cases. In this IWMPI contribution we present an update on the ongoing works in the consortium addressing the key problem of model-based reconstruction in MPI for multi-dimensional Lissajous-type excitation. We showcase the accuracy and the computational effort for model evaluation on different system matrices and various particle phantoms being measured recently on the Bruker preclinical MPI system
Exploiting the Fourier Neural Operator for Parameter Identification in MPI
No full textModel-based magnetic particle imaging (MPI) is a challenging task both due to the complicated underlying physical model and the high numerical effort required for the solution of the corresponding equations of motion. A second challenge for practical applications is the identification of model parameters that are consistent with the given experimental setting and produce accurate predictions of the MPI signals. In this work, we show how the parameter identification problem can be addressed using a learned physics simulator based on the Fourier neural operator. As an application, we show how model-based system matrices can be estimated from a small set of calibration measurements, which can also be interpreted as a model-based approach to system matrix recovery. We compared our approach to established compressed sensing and interpolation schemes and found that it outperformed both
Design and Optimization of a Selection Field Generator for a Human-Sized Magnetic Particle Imaging Head Scanner
No full textMagnetic particle imaging (MPI) is currently on the verge of moving from a pre-clinical to a clinical imaging method. In 2019, the first version of a head-scanner and its selection field generator was shown by Graeser et al.. An improvement of the two-coil selection field generator was introduced by Foerger et al., containing 18 iron core coils that can be driven separately. The resulting 9 coils per side are arranged in a 3 x 3 square shape. In this work, this concept is adapted to a human sized scanner examining a set of parameters, including the coil and core size, their material, the current distribution and a division into multiple coils. The setup is simulated in COMSOL 6.1 and optimized with the aim of achieving the target gradient of 0.5 T/m, while minimizing the average power consumption. However, while analyzing the parameters the coil arrangement showed a very promising result in terms of a significant reduction in dissipated power. A MATLAB optimization algorithm, that solves the inverse problem for an optimal current combination for a given target field, reflects the estimated power for each FFP position in the field of view. From this solution a power map can be generated. With this the original sqare- and a new circular arrangement are compared and a reduction of almost half the power consumption can be seen. However, this circular design needs further optimization regarding constructive challenges, but it shows how power-saving selection field generators could look like in the future
Simulation of a Modular Coil Unit for a Preclinical MPI Scanner
No full textThe adoption of preclinical Magnetic Particle Imaging (MPI) as a standard technique in research heavily relies on the availability of scanners with a low entry barrier. One of the barriers is the infrastructure demand of such a system in terms of space, electric supply power and cooling. Scanners that try to tackle this require power-efficient transmit coil assemblies. Here, the design process of such a set of coils is presented and the optimization steps are discussed. With a total power loss of 1.2 kW for the drive and focus fields and a field of view (FoV) of 33.8 mm at 5 T/m the whole system can potentially be powered from a single power outlet
Multi-harmonic Gridded 3D Deconvolution (MH3D) for Image Reconstruction in MPI: MH3D
No full textWe have developed a new effective methodology called multi-harmonic gridded 3D deconvolution (MH3D) for image reconstruction in magnetic particle imaging (MPI). This new methodology in turn has led to important new theoretical insights into this complex problem. The technique works by first mapping the MPI harmonics into the spatial domain by gridding the time domain data to their respective field free regions (FFRs). We refer to these spatially gridded harmonic data as harmonic portraits since they represent only a portrait-like representation of the true image. The final true image is then formed by inverting these harmonic portraits, which is effectively accomplished via a 3D deconvolution with harmonic point spread functions (PSFs). The PSFs are the harmonic portrait signatures resulting from a scanned point source. They are formed through a simulated MPI system model of the scanner and magnetic tracers, while additional results indicate that they can be modeled analytically. Additional MPI scanner parameters can be modeled into MH3D, such as multiple and non-uniform receive coils.
Analytical and numerical results indicate that our method is lossless, in the sense that it retains all the information coming from an MPI signal. The primary computational advantage of our approach is the ability to easily perform artifact analysis and correction techniques made available to us in the intermediate harmonic portrait domain, which has greatly aided in the hardware development of a clinical scale system. Our new approach also provides new key theoretical insights into the MPI reconstruction problem. Namely, we have shown that it is possible to faithfully reconstruct images using only the second harmonic, albeit at the cost of lower spatial resolution. Alternatively, high resolution imaging is possible with only the second harmonic provided sufficiently dense FFR scanning patterns. This also suggested that a narrow band receive chain could be implemented in the hardware. However, higher harmonics become increasingly important at higher drive field amplitudes