International Journal on Magnetic Particle Imaging (IJMPI)
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Fourier Neural Operator for Coupled Brown-Neel Rotation Model
No full textModelling magnetization dynamics of magnetic nanoparticles (MNPs) is crucial to understand and predict their signal response in magnetic particle imaging (MPI). Coupled Brown-Ne?el rotation model expresses MNP magnetization as a system of ordinary differential equations (ODEs). However, numerical solution of these ODEs can be computationally intensive and time consuming using classical solvers. In this work, we propose a neural solver that utilizes a Fourier Neural Operator (FNO) to speed up the computation time for the coupled Brown-Ne?el rotation model. We show that the FNO model provides high signal fidelity with 5 orders of magnitude acceleration in computation time
Size- and shape-controlled iron oxide nanocrystals as MPI tracers based on a heating up synthesis technique
No full textA challenging aspect of the success of MPI is not only the development of sophisticated imaging scanners but also the synthesis of advanced tracers. Iron oxide nanocrystals (NCs) are promising candidates due to fascinating magnetic properties, including blocking temperature, coercivity, saturation magnetization, magnetic domain size, and exchange coupling effects. To achieve higher tracer sensitivity leading to better temporal and spatial resolution in a MPI scanner, NCs should have a steeper slope in the linear part of the magnetization curve and an approximate step function. The magnetic properties of magnetic NCs strongly depend on size, shape, and crystal structure. Therefore, we developed a sophisticated synthesis technique based on the thermal decomposition of iron oleate in order to control these parameters. The oleate was synthesized by use of a precursor system - Fe(II)CO3 and Fe2(III)(CO3)3. The method allows the synthesis of highly monodisperse NCs in various well-defined sizes and morphologies on a gram scale.
For a strong signal generation in MPI, a special oxidation treatment after synthesis was performed to maximize the magnetite phase\u27s content and increase the saturation magnetization. And moreover the uniformity and monodispersity of the sample make a strong signal generation in MPS. Based on the seeded emulsion polymerization, a phase transfer of the hydrophobic NCs into water, followed by subsequent encapsulation with a polystyrene shell, provides water solubility and biocompatibility. Thereby realizing the option for surface functionalization with e.g., antibodies or even the formation of defined clusters
Stent Tracking by Magnetic Particle Imaging
No full textMagnetic Particle Imaging (MPI) is a very promising methodology for periinterventional imaging. Interventional instruments have been considered invisible for MPI so far. Currently, MPI visibility is realized by special marking technologies. These modifications cause changes in the biomechanical characteristics of the instruments. In this work, a bare metal stent without nano-modification was tracked with high accuracy by MPI. To check potential MPI signal generation, nine commercial endovascular stents were tested in an MPI system. Two of the stents revealed sufficient MPI signal. As one of these two stents showed relevant heating, subsequent imaging experiments at different setups were performed with a single stent type (Boston Scientific/Wallstent-Uni Endoprothesis, diameter: 16 mm, length: 60 mm) only. The nitinol stent mounted on the delivery system, the fully expanded stent and solely the delivery system were placed at 49 defined spatial positions in a meandering pattern by a robot during MPI scans. Image reconstruction was performed and the mean absolute errors (MAE) between the signals’ centers of mass (COM) and the ground truth positions were calculated. The results of this work show that the tracking of the stent and its delivery system was feasible without nano-modifications. The MAE of the COM were 1.5 mm (pixel resolution: 2 mm) for the stent mounted on the delivery system, 3.7 mm for the expanded stent and 1.5 mm for the delivery system without the stent. In conclusion, accurate stent tracking without nano-modification is possible by MPI in the case of signal generating metal stents
Quantifying localized heating of synthetic antiferromagnet disk particles with an MPI-MFH platform
No full textRecently, a theranostic imaging platform which allows for switching between imaging and therapeutic modes has been realized by integrating a hyperthermia insert into a commercial magnetic particle imaging (MPI) scanner. The platform enables precisely localized heating to a specific target temperature deep in the tissue via magnetic fluid hyperthermia (MFH) as well as visualization of the distribution and quantification of the temperature of magnetic nanoparticle suspensions (MNPs). MPI-assisted MFH helps to overcome the typical constraint of MFH of delivering heat only to e.g., malignant regions without affecting the surrounding healthy tissues and it provides accurate control of the therapy. However, as a cancer therapy, MFH is still limited by the thermal outcome in the targeted region. The heating effectiveness of MFH strongly depends on the magnetization properties of the utilized MNPs. Synthetic antiferromagnet disk particles, feature the design of two ferromagnetic layers separated by a non-magnetic layer, can overcome this limitation by enhancing the hysteretic loss. In this work, the localized MFH properties of the synthetic antiferromagnet disk particles are explored using the theranostic imaging platform. The heat map is measured, and the thermal resolution is evaluated and compared to simulation results
Ellipsoidal Harmonic Expansions for Efficient Approximation of Magnetic Fields in Medical Imaging
No full textExact knowledge on the magnetic fields used in tomographic imaging systems is important for accurate sequence planning and model-based image reconstruction in existing devices. It is common to measure the magnetic fields on a small subset of points and use spherical harmonics to approximate the fields inside a spherical region. However, most scanner bores could be better filled using a cylindrical or ellipsoidal region. In this work, we present the application of ellipsoidal harmonics for efficient approximation of a typical magnetic field in magnetic particle imaging
Comparison of Reconstruction Methods for Elongated Multi-Patch Sequences in Magnetic Particle Imaging
No full textMagnetic particle imaging systems utilize one or more dynamic drive fields to excite the magnetization of the nanoparticles, along with a static selection field that spatially encodes the resulting magnetization signal. This selection field effectively suppresses signal generation outside a smaller volume surrounding the field-free region (FFR), while the drive fields swiftly move this region. However, the practical size of the possible field of view attainable with these single-patch sequences is limited. To upscale an MPI system, one can increase the effective field of view through two methods: moving the object or utilizing additional low-frequency focus fields. These extra fields can continuously or stepwise relocate the position of the FFR. Especially elongated sequences that involve slow and continuous shifts of focus fields along with simultaneous movements of drive fields in the FFR are of particular interest due to their flexibility and operability at full duty cycle. For reconstruction one either considers the entire sequence as a single patch or as a collection of multiple patches, each of which does impact image artifacts, reconstruction time, and calibration complexity. Dependent of the interpretation of the raw data particular challenges include the processing of the measured data, which is no longer periodic, and the length of the data sets that is now determined by the duration of the focus field sequence rather than by the comparably short drive field excitation cycles. In this work, various methods for reconstructing elongated multi-patch sequences are examined and contrasted based on their impact on image artifacts, runtime, and calibration complexity
Biocompatibility of sulfobetaine and PEG 25 kDa functionalized synomag®-D
No full textThe method of magnetic particle imaging relies crucially on the accessibility of adequate tracer materials. Those have to fulfill different technical requirements like high signal strength but also high biocompatibility, in particular hemocompatibility, and high blood circulation time since magnetic nanoparticles are applied intravascular in most applications. In order to further improve these properties, high-performing synomag®-D was optimized within the present study by coupling of sulfobetaine or PEG 25 kDa-OMe. Relative cell viability of these functionalized magnetic nanoparticles was found to be 8 to 18% higher compared to commercially available Resotran® while observed activation of the coagulation system was slightly faster. Thus, sulfobetaine and PEG 25 kDa-OMe functionalized synomag®-D present promising candidates for further investigations with respect to their behavior in vivo
Multi-contrast MPI channel leakage reduction using a two-step measurement and reconstruction method
No full textMagnetic particle imaging (MPI) is an imaging modality that measures the response of magnetic nanoparticle tracersto alternating magnetic fields. There has recently been exploration into multi-contrast MPI, in which the signalfrom different tracer materials or environments is separately reconstructed, resulting in multi-channel images thatenable temperature or viscosity quantification. In this work, we investigate the channel leakage in multi-contrastMPI reconstruction and we introduce a two-step measurement and reconstruction method to quantify and reducechannel leakage between multi-contrast MPI channels
Image reconstruction in a data space for MPI
No full textSystem matrix based image reconstruction in MPI requires the acquisition of calibration measurements, which is a time consuming process during which the imaging device is not available for other measurements. In a clinical setting, the time would be reduced significantly in which the imaging device is available for patient care. In order to enable fast reconstruction algorithms, the system matrix must be kept in main memory, which places special demands on the hardware of the reconstruction computer due to the size of the system matrix. Previously, it has been shown that system matrices may be acquired in dedicated devices which enables off-site calibration measurements. Based on this, we propose a data space for image reconstruction in MPI. In the data space, a network of imaging facilities, calibration and reconstruction experts will be created to facilitate system matrix based image reconstruction. Imaging facilities send measurement data across the data space to the calibration experts, who send back reconstructed images. By integrating reconstruction experts in the data space, new reconstruction techniques can be provided quickly. Following this approach, the scanning devices do not need to take calibration measurements and are always available for patient care. Further, the imaging facilities do not require dedicated hardware for image reconstruction
Simulation study of a magnetic particle imaging device capable of hyperthermia
No full textMagnetic fluid hyperthermia (MFH) is a promising non-invasive therapy. Magnetic particle imaging (MPI) is a novel quantitative imaging technique determining the local concentration of magnetic nanoparticles with high sensitivity, and high temporal-spatial resolution. Successful implemention of MPI-guided localized MFH have been reported, which shows great potential for precision diagnosis and treatment of tumors. However, real-time 3D MPI-guided localized MFH hasn’t been reported, which is promising for understaning the the treatment process and molecular mechanism of MFH. Here, we present a proof-of-concept design of a 3D Cartesian scanning MPI device with a radiofrequency coil capable of localized MFH. A proof-of-concept simulation study is conducted to evaluate the feasibility of designed coils, designed 3D MPI mode and designed localized mode. The simultion results show the designed MPI device has the potential to realize the real-time 3D MPI-guided localized MFH