System Characterization of a Human-Sized 3D Real-Time Magnetic Particle Imaging Scanner for Cerebral Applications

Since the initial patent in 2001, the Magnetic Particle Imaging (MPI) community has been striving to develop an MPI scanner suitable for human applications. Numerous contributions from different research fields, regarding tracer development, reconstruction methods, hardware engineering, and sequence design have been employed in pursuit of this objective. In this work, we introduce and thoroughly characterize an improved head-sized MPI scanner with an emphasis on human safety. The scanner is operated by open-source software that enables scanning, monitoring, analysis, and reconstruction, designed to be handled by end users. Our primary focus is to present all technical components of the scanner, with the ultimate objective to investigate brain perfusion imaging in phantom experiments. We have successfully achieved full 3D single- and multi-contrast imaging capabilities at a frame rate of 4 Hz with sufficient sensitivity and resolution for brain applications. To assess system characterization, we devised sensitivity, resolution, perfusion, and multi-contrast experiments, as well as field measurements and sequence analysis. The acquired images were captured using a clinically approved tracer and suitable magnetic field strengths, while adhering to the established human peripheral nerve stimulation thresholds. This advanced scanner holds potential as a tomographic imager for diagnosing conditions such as ischemic stroke or intracranial hemorrhage in environments lacking electromagnetic shielding. Furthermore, due to its low power consumption it may have the potential to facilitate long-term monitoring within intensive care units for various applications.Comment: 22 pages, 9 figure

Organ Specific Head Coil for High Resolution Mouse Brain Perfusion Imaging using Magnetic Particle Imaging

Magnetic Particle Imaging (MPI) is a novel and versatile imaging modality developing towards human application. When up-scaling to human size, the sensitivity of the systems naturally drops as the coil sensitivity depends on the bore diameter. Thus, new methods to push the sensitivity limit further have to be investigated to cope for this loss. In this paper a dedicated surface coil improving the sensitvity in cerebral imaging applications was developed. Similar to MRI the developed surface coil improves the sensitivity due to the closer vicinity to the region of interest. With the developed surface coil presented in this work, it is possible to image tracer samples containing only 896 pg iron and detect even small vessels and anatomical structures within a wild type mouse model. As current sensitivity measures are dependent on the tracer system a new method for determining a sensitivity measure without this dependence on the tracer is presented and verified to enable comparison between MPI receiver systems.Comment: 9 pages 7 figures original articl

Efficient 3D Drive-Field Characterization for Magnetic Particle Imaging Systems

Magnetic particle imaging uses time-dependent drive fields for signal generation, which are designed to be homogeneous in order to achieve a similar image quality all over the volume of interest. In practice, the fields are not exactly homogeneous and in turn a precise knowledge of the spatial field profile is necessary when using a model-based reconstruction approach. In this work, we propose an efficient method for the measurement and compact  representation of the drive fields using a small 3D calibration coil and solid harmonics

Heat it up: Thermal stabilization by active heating to reduce impedance drifts in capacitive matched networks

The achievable sensitivity in Magnetic Particle Imaging is not only limited by noise, but also depends on the stability of the system. Thermal dependencies of the current carrying components lead to drive-field distortions in amplitude and phase causing drifting background signals. In this work, an active capacitor heating system is developed that allows for thermal stabilization and trimming a resonance circuit to the desired frequency

Quasi-simultaneous magnetic particle imaging and navigation of nanomag/synomag-D particles in bifurcation flow experiments

Magnetic Particle Imaging (MPI) is used to visualize the distribution of superparamagnetic nanoparticles within 3D volumes with high sensitivity in real time. Recently, MPI is utilized to navigate micron-sized particles and micron-sized swimmers, since the magnetic field topology of the MPI scanner is well suited to apply magnetic forces. In this work, we analyze the magnetic mobility and imaging performance of nanomag/synomag-D for Magnetic Particle Imaging/Navigation (MPIN). With MPIN the focus fields are constantly switching between imaging and magnetic force mode, thus enabling quasi-simultaneous navigation and imaging of particles. In flow bifurcation experiment with a 100 % stenosis on one branch, we determine the limiting flow velocity of 1.36 mL/s, which allows all particles to flow only through one branch towards the stenosis. During this experiment, we image the accumulation of the particles within the stenosis. In combination with therapeutic substances, this approach has high potential for targeted drug delivery.Deutsche Forschungsgemeinschaft (DFG)Bundesministerium für Bildung und Forschung (BMBF

MPI Signal Performance of Resotran

An important ingredient for clinical translation of Magnetic Particle Imaging (MPI) is the availability of superparamagnetic iron-oxid nanoparticles (SPIOs) with given medical approval for human interventions. Many SPIOs used as tracer-material in magnetic resonance imaging (MRI), do not have the magnetic properties needed for a potent signal generation in MPI. In particular they are often too small and thus the thermal energy dominates the magnetic energy leading to a linear magnetization behavior, which is not suitable for signal generation and spatial encoding in MPI. Some particle types are too large and block the Neél relaxation process due to strong magnetic anisotropies, reducing their ability to follow the field at excitation frequencies between 10 kHz to 150 kHz. At the same time, the medical approval of dedicated MPI tracers with optimal signal performance is very costly and tedious and will only turn profitable for companies with a clear clinical business case. Fortunately, there are some tracers that are suitable for both MRI and MPI and thus evaluating newly introduced MRI tracers is essential for potential human MPI studies. We show that the new MRI contrast agent Resotran (b.e.imaging GmbH, Baden-Baden, Germany, medically approved in 10/2022 under reg. no. 7002837.00.00 in Germany) is suitable for MPI. Initial Magnetic Particle Spectroscopy measurements indicate that Resotran shows a similar performance as the formerly approved tracer Resovist (Bayer Schering Pharma, Berlin, Germany) and the pre-clinical MPI tracer perimag (micromod Partikeltechnologie, Rostock, Germany). In combination with human-sized MPI systems, this paves the way towards first human MPI experiments

MPI Transfer-Function Estimation with Receive-Coil Coupling

Time- and memory-consuming calibration measurements are a major drawback in system-matrix-based reconstructions in magnetic particle imaging (MPI). Especially, exchanging the receive coils requires new system matrices and therefore new calibration measurements. To reduce the number of such measurements, the MPI transfer function can be used to transfer the system matrix of one receive coil set to another. The transfer function can be obtained by a direct measurement or by estimation using a measured system matrix of each setup. In this abstract, we extend the latter to incorporate coupling of the receive coils while using only a few voxels of the system matrices. In this way, we can transfer system matrices between two different receive setups, both of which may contain non-orthogonal coils

Gradient power reducing using pulsed selection-field sequences

Large selection-field power is required to generate a sufficient gradient strength in Magnetic Particle Imaging (MPI). Without cooling, the subsequent heat generation can limit the maximum experiment time. For commercially available MPI scanners a lot of effort was put into active cooling requiring space and infrastructure to dissipate heat. In this abstract, a promising power handling for the selection-field generation is presented. Using a pulsed instead of a continuous selection-field the gradient strength can be increased and no active cooling is required