International Journal on Magnetic Particle Imaging (IJMPI)
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Disk Shaped Magnetic Thin-Film Nanoparticles Tailored for Optimal MPI Signal Generation
No full textMagnetic Particle Imaging (MPI) generates signals through the nonlinear magnetization response of magnetic nanoparticle tracers to an external magnetic field. The common tracers used so far are superparamagnetic iron oxide (SPIO) nanoparticles. These particles exhibit a modest saturation magnetization and follow a Langevin-type magnetization curve, which restricts their dM/dH response. Consequently, this leads to a limited generation of higher harmonic signals in magnetic field drive oscillations, thereby affecting the sensitivity and spacial resolution achievable in MPI setups.
In our research, we have adopted a top-down approach for nanoparticle fabrication. This process begins with the sputter-deposition of multilayers on a Germanium sacrificial layer, which is evaporated onto a silicon wafer. From these layers, circular nanoscale islands are then patterned. The analysis of the magnetic properties of these islands revealed an M(H) loop characterized by a narrow switching field distribution and coercive fields below 1 mT. To create a nanoparticle suspension, these circular islands were detached from the wafer through the dissolution of the sacrificial layer. The recorded magnetic particle spectra of these disk-shaped magnetic nanoparticles showed a significant enhancement in the amplitudes of the higher harmonics when compared to perimag® particles. This advancement allows for the detection of signals up to the nth harmonic, leading to a marked improvement in MPI performance
Single-Shot Magnetic Field Measurements for MPI
No full textMagnetic field measurements play a central role in Magnetic Particle Imaging (MPI). However, their determination is often a time-consuming process that involves sequentially scanning a given volume at predefined locations with a single magnetic field sensor. Therefore, an extensive amount of measurements is required to accurately characterize magnetic field generators, particularly those utilizing non-linear field generation processes. In this work, we introduce a single-shot magnetic field measurement system capable of capturing magnetic fields within a sphere with a 9 cm diameter at a measurement rate of 10 Hz. In comparison, a similar measurement conducted with a single 3D Hall probe moved by a robot previously took 4 min per volume measurement. We present magnetic field measurements of static MPI fields and characterize the accuracy of the single-shot measurement system. In addition, we discuss limitations on the field shapes that can be accurately measured
Influence of switched-mode power supplies in an arbitrary waveform magnetic particle spectroscope
No full textAt Fraunhofer IMTE an arbitrary waveform Magnetic Particle Spectroscope (MPS) is developed to gain further insight into particle behavior. To enable these arbitrary fields, no transmit filters, resonant circuits or receive filters are used, as both the send and the receive signal are broadband. To power the auxiliary components of the system in an initial setup Switched-Mode Power Supplies (SMPS) were used. SMPS are commonly used in modern electronic equipment as they are energy efficient and cost-effective. The downside of these converters is their output ripple and EMI due to the switching nature. The Low Noise Amplifier (LNA) used in the system was supplied with an SMPS. The developed four-stage LNA consists of a J-FET stage followed by two low noise, low distortion operational amplifiers, with a final differential amplifier stage. While the latter stages offer power supply rejection, the first stage of the LNA offers near to none. As the signal disturbance caused by the SMPS within the first stage is amplified in further stages a linear power supply with a very low output noise and high supply voltage rejection is a promising measure to reduce overall noise in the system. To evaluate the influence of different power supplies, the spectra of the receive signals of background measurements of the RedPitaya IO boards are compared. A comparison was made between a SMPS, a commercially available linear power supply and a specific developed low noise linear power supply
Preventing Interference Between Saturation Coil and Receive Coil in MPI
No full textIn magnetic particle imaging (MPI), the signals received from magnetic nanoparticles (MNPs) are directly proportional to concentration. Hence, accumulation of MNPs in off-target organs may overpower the signal from actual regions-of-interest that contain MNPs at a smaller concentration. We previously proposed placing a saturation coil over the off-target organ to locally suppress its signal. However, the saturation coil caused a large interference signal on the receive coil, necessitating the acquisition of a separate baseline to determine the interference signal. In this work, we propose methods to prevent the interference between the saturation coil and the receive coil to enable localized signal suppression without the need for an additional baseline acquisition
Simulations of magnetic nanoparticles with internal magnetization dynamics for magnetic hyperthermia
No full textFerrofluids, suspensions of magnetic nanoparticles, are versatile materials used for magnetic hyperthermia, among other applications. We developed a simulation framework to study the complex behavior of a multiparticle system in a viscous medium and to understand the structure formation and its impact on relaxation processes and heat dissipation. The simulations combine Langevin dynamics and the internal magnetization dynamics by solving the coupled Landau-Lifshitz-Gilbert equation. We simulate a system of multiple single-domain particles with uniaxial anisotropy in an aqueous fluid with thermal fluctuations and dipole-dipole interactions. The simulations reveal the self-aggregated structures of the particles depending on temperature, external field, and material parameters. Self-agglomeration must be avoided to ensure the stability of the ferrofluid. With the right parameters, the particles in our simulations can behave superparamagnetically due to the inclusion of a stochastic term in the effective field. In general it is difficult to find parameters for stable ferrofluids that also have high hysteresis losses, but this simulation framework can facilitate the search for optimal parameters. It can also help to more accurately predict the behavior of ferrofluids in the high-frequency regime, as well as provide deeper insight into the behavior of the individual magnetic nanoparticles
Focused torque delivery and low frequency inductive sensing of micromagnets in rotating magnetic fields
No full textMagnetically controlled biomedical microrobots show promise for facilitating site-specific drug delivery and performing other therapeutic functions in the body. Dual use of magnetic particle imaging apparatuses for the control and detection of microrobots is a possibility that is increasingly being explored, avoiding the use of ionizing radiation to provide real-time feedback on their position and trajectory. Nevertheless, the methods that have been demonstrated typically need to switch between distinct actuation and sensing modes, limiting the duty cycle of each and neglecting an opportunity to directly utilize simultaneous inductive feedback during actuation. Here, we show that, under a low-frequency rotating magnetic field (1 to 100 Hz), it is possible to inductively detect magnetic torques applied to a model microrobot. This is made possible by a prototype inductive sensing apparatus that finely adjusts both phase and amplitude to achieve cancellation between a sense and compensation coil, suppressing background signal from the rotating field by 90 dB. Further, by monitoring inductive signals, we show that combining rotating magnetic fields with selection fields enables the selective delivery of torque to multiple microrobots within a working volume. We discuss how the sensitivity of inductive detection using the techniques we are developing compares favorably to alternative methods for measuring the fringing fields produced by the microrobots. These concepts for low-frequency inductive detection of microrobots serve to lay a foundation for future closed-loop control schemes based on simultaneous actuation and sensing
Optimization of wall thickness for water-cooled hollow conductor drive coils in human-sized MPI
No full textThe reduction and removal of heat dissipated in the drive coil emerges as a major concern when Magnetic Particle Imaging is scaled to human size. The low ac-resistance of Litz wire can reduce the power that generates the heat, but cooling Litz wire bundles can pose a challenge. Hollow conductors with axially flowing coolant present an attractive alternative, but because of skin and proximity effects, the wall thickness is a critical design parameter affecting both the heat created and its rate of removal. Here, we introduce and experimentally validate an analytical method for calculating drive coil heating and optimizing the wall thickness. We demonstrate that the optimal wall thickness provides a 3.3x improvement in cooling efficiency for a fixed pressure with only marginal changes to the coil resistance. A 3.3x improvement in cooling efficiency would allow for ~1.8x more current for the same coil temperature rise, and thus higher drive fields
Experimental Parameter Calibration of the Scanner Model for Model-Based MPI
No full textModel-based reconstruction is still one of the key challenges in magnetic particle imaging (MPI) when using multi-dimensional Lissajous-type excitation. Besides an appropriate particle model, the model of the MPI system for signal generation and reception has a major impact on the modeled system matrix. We outline the influence of the latter MPI scanner parameters on the system matrix pattern and review methods to calibrate each parameter
Improving the Resolution of Single-harmonic MPI Using Perpendicular Signal Transformation
No full textIn single-harmonic magnetic particle imaging (MPI), image reconstruction is typically achieved through deconvolution using the point spread function (PSF) corresponding to the 3rd harmonic signal received parallel to the excitation field. In this study, we present an improved single-harmonic MPI reconstruction method. The method is based on transforming the 3rd harmonic signal received perpendicular to the excitation field. Experimental results demonstrated that our method, compared to the reconstruction using the raw original parallel 3rd harmonic signals and raw perpendicular 3rd harmonic signals, not only effectively improves resolution but also better recovers the shape contour of the phantom. Specifically, the proposed method achieves at least a 2-fold increase in resolution compared to the deconvolution reconstruction by the raw parallel 3rd harmonic signals
Imaging performance of thin-film disk particles tailored for optimal MPI signal generation
No full textThe imaging performance of a Magnetic Particle Imaging (MPI) scanner is fundamentally constrained by the magnetic characteristics of the employed tracer. The achievable spatial resolution depends on the magnetic particle used, particularly on its magnetic moment\u27s reaction to the applied oscillatory field, which, for commonly used superparamagnetic particles, is limited by their Langevin-type magnetization behavior. An optimal nanoparticle would demonstrate a high magnetic moment and a step-like magnetization response to the magnetic field oscillations. This sharp response leads to a narrow point spread function and enhances the amplitude of the higher harmonics. Thus, higher harmonics, which encode the fine spatial resolution, become detectable above measurement noise.
In this study, we utilized magnetic disk particles fabricated via a top-down approach. With this method of fabrication, the optimization of the particles’ magnetic properties is not constrained by chemical synthesis limitations. We then evaluated the imaging performance of these particles by employing multi-dimensional (one-, two-, and three-dimensional) excitations and determining the system matrices using a multi-dimensional magnetic particle spectrometer. We then benchmarked them against Perimag, the established gold-standard MPI tracer. The system matrices of the disk particles revealed significantly stronger signals, especially noticeable in the high-order harmonics. Furthermore, the structure of these one- and two-dimensional matrices closely resembled Chebyshev polynomials, without the typical vignette effects visible by wet synthesis particle systems. Finally, an estimation for the achievable resolution improvement is given by using the sensitivity and noise level of a preclinical MPI scanner within a simulated software phantom