In situ theranostic platform uniting highly localized magnetic fluid hyperthermia, magnetic particle imaging, and thermometry in 3D

In all of medical profession a broad field of research is dedicated to seek less invasive and low-risk forms of therapy with the ultimate goal of non-invasive therapy, particularly in neoplasmic diseases. Theranostic platforms, combining diagnostic and therapeutic approaches within one system, have thus garnered interest to augment invasive surgical, chemical, and ionizing interventions. Magnetic particle imaging (MPI) offers, with its versatile tracer material (superparamagnetic iron oxide nanoparticles, SPIOs), a quite recent alternative to established radiation based diagnostic modalities. In addition, MPI lends a bimodal theranostic frame allowing to combine tomographic imaging with therapy techniques using the very same SPIOs. In this work, we show for the first time the interleaved combination of MPI-based imaging, therapy (highly localized magnetic fluid hyperthermia) and therapy safety control (MPI-based thermometry) within one theranostic platform in all three spatial dimensions

Spatial selectivity enhancement in magnetic fluid hyperthermia by magnetic flux confinement

Aiming to increase spatial selectivity to enhance the precision in Magnetic Fluid Hyperthermia (MFH) therapy and the spatial resolution in imaging, we propose a strategy to increase the selection field gradient in Magnetic Particle Imaging (MPI). In this study, a solution for an existing MPI system topology was simulated, using an additional soft magnetic material as iron core retrofit at the center of the selection field coil. Due to the core's high magnetic permeability relative to air, the magnetic flux is confined, increasing the selection field gradient. Within this simulation study, the optimal core position is evaluated, whilst its effects on the magnet system are validated. According to our results, this strategy can achieve a 27 % reduction in theranostic field of therapy. We found that this technique increases the magnetic field gradient up to a factor of 1.4 (from 2.5 to 3.4 T/m) in z-direction, without significant loading of the drive field resonance circuit caused by eddy currents in the MPI compatible iron core shielding.   Int. J. Mag. Part. Imag. 7(1), 2021, Article ID: 2103002, DOI: 10.18416/IJMPI.2021.210300

Eigen-reconstructions: a closer look into the System Matrix

Magnetic particle imaging (MPI) offers an exceptional set of advantages including high sampling efficiency and sub-millimeter spatial resolution. The former is maximized using ad-hoc sampling trajectories; the latter relies on the compensation of the point spread function of the superparamagnetic iron oxide particles (SPIOs) necessary for the MPI signal. The System Matrix (SM) approach for reconstruction uses a pre-calibration measurement and achieves both purposes simultaneously, relating the concentration of SPIOs and the particle response to the signal. Consequently, the SM’s quality will largely influence the reconstruction. Considering the multitude of factors involved in the reconstruction, it is difficult to identify sources of image artifacts.  In this work, we demonstrate the potential to use reconstructions of individual measurements within the SM (eigen-reconstruction) as a test for both SM and reconstruction quality. We also present an algorithm to enhance image quality using eigen-reconstructions.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009072, DOI: 10.18416/IJMPI.2020.200907

Enhancing spatial resolution in magnetic particle imaging using eigen-reconstructions: opportunities and limitations

Enhancements in spatial resolution can open new avenues for novel applications, but acquiring data at higher resolutions generally comes with penalties in measurement times, signal-to-noise ratios and safety concerns. Therefore, maximizing the spatial resolution of the available data during image reconstruction is paramount. Magnetic Particle Imaging (MPI) has already reached sub-millimeter spatial resolutions. With standard tracers, this has been achieved using a reconstruction method that compensates for the point spread function of the system and the superparamagnetic iron oxide particles (SPIOs). This method is known as the system matrix approach and uses a calibration measurement, relating the concentration of SPIOs and the true particle response to the measured signal. Using a calibration measurement for reconstruction requires a comprehensive assessment of the quality of the system matrix in addition to the measured image data. Analyzing the system matrix by reconstructing selected measurements contained in itself and visualizing them in image space (henceforward called eigen-reconstructions) can provide clear information regarding image quality and artifacts. This is equivalent to using ideal measurement data. Thus, it is possible to identify sources of image artifacts arising solely from the reconstruction, which can be then compensated. In a preliminary report, we presented the principle of eigen-reconstructions to identify and reduce reconstruction-induced artifacts. In this work, we focus on the application of the method to enhance spatial resolution in MPI reconstructions. The principles of an iterative algorithm based on eigen-reconstructions are also further detailed. It is shown that the algorithm compensates the blur arising during image reconstruction effectively. For validation, we present tests of our method using 2D and 3D datasets including an homogeneous and a resolution phantom to demonstrate potential opportunities and limitations.   Int. J. Mag. Part. Imag. 7(2), 2021, Article ID: 2112003, DOI: 10.18416/IJMPI.2021.211200

Spatial selectivity enhancement in RF-hyperthermia by magnetic flux confinement

Aiming to increase spatial selectivity which provides the precision in hyperthermia therapy and high resolution in imaging, we propose a strategy to increase field gradient for Magnetic Particle Imaging (MPI) modality. In this study, a solution for an existing MPI system topology was simulated, which uses additional soft magnetic material as iron core retrofit at the center of selection field coil. Due to core property of high magnetic permeability relative to air, magnetic flux gets confined to increase selection field gradient field slope. Within this simulation study, the optimal core position is evaluated whilst its effects on the magnet system is validated. We found that this technique increases the magnetic field gradient up to a factor of 1.4 from 2.5 T/m to 3.4 T/m in z-direction, without significant loading of the drive field resonance circuit due to power losses caused by eddy currents in the MPI compatible iron core shielding.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009015, DOI: 10.18416/IJMPI.2020.200901

Validation of spatial selectivity enhancement for magnetic fluid hyperthermia by introducing ferromagnetic cores

Spatial selectivity plays a crucial role in magnetic fluid hyperthermia because it can define the precision of thermal dose localization and spatial resolution. We propose an application of additional ferromagnetic cores, with high magnetic permeability, to confine the magnetic flux of the selection field coil. An increased gradient leads to increasing spatial selectivity in theranostic therapy of MPI-assisted magnetic fluid hyperthermia. This work validates our recent simulation study [1] by actual experiments of iron core prototypes. This study shows that our core prototypes can increase the gradient by a factor of 1.3 which suggests a 21% improvement in thermal localization in hyperthermia therapy.  &nbsp

MPI-based spatio-temporal estimation of a temperature profile induced by an IR laser

The separation of signals based on particle type, their environment or temperature has added a useful layer to the foundation of spatial MP imaging. The multi-color approach of signal reconstruction offers a far-reaching option for clinical interventions such as the controllable and precise application of hyperthermia. In this study, a multi-color reconstruction approach was applied to highlight the potential of MPI for temperature monitoring. For this purpose, we heated a solution of SPIO nanoparticles with a high-power laser and reconstructed the corresponding dynamic temperature maps by MPI data acquisition. A good temporal and spatial correlation between the MPI-based temperature maps and fiber optic thermometer measurements was observed