Search

Gastrointestinal bleeding, as a potentially life-threatening condition, is typically diagnosed by radiation-based imaging modalities like computed tomography or more invasively catheter-based angiography. Endoscopy enables examination of the upper gastrointestinal tract and the colon but not of the entire small bowel. Magnetic Particle Imaging (MPI) enables non-invasive, volumetric imaging without ionizing radiation. The aim of this study was to evaluate the feasibility of detecting gastrointestinal bleeding by single- and multi-contrast MPI using human-sized organs. A 3D-printed small bowel phantom and porcine small bowel specimens were prepared with a defect within the bowel wall as the source of a bleeding. For multi-contrast MPI, the bowel lumen was filled with an intestinal tracer representing an orally administered tracer. MPI was performed to evaluate the fluid exchange between the vascular compartment of the bowel wall and the lumen while a blood pool tracer was applied. Leakage of the blood pool tracer was observed to the bowel lumen. Multi-contrast MPI enabled co-registration of both tracers at the same location within the bowel lumen indicating gastrointestinal bleeding. Single- and multi-contrast MPI are feasible to visualize gastrointestinal bleeding. Therefore, MPI might emerge as a useful tool for radiation-free detection of bleeding within the entire gastrointestinal tract.

Gastrointestinal (GI) bleeding is a potentially life-threatening condition that is typically diagnosed using radiation-based imaging modalities such as computed tomography (CT) or catheter-based angiography. Magnetic Particle Imaging (MPI) could provide non-invasive, real-time volumetric imaging without ionizing radiation in future human-sized scanners that covers the entire GI tract. We have developed a human-sized (3D printed) phantom that represents both the bowel lumen and the vascular compartment of the bowel wall. One version has a perforation between the two compartments and a control phantom does not. For single contrast MPI, we evaluate the fluid exchange between the two lumen by observing an administered blood pool tracer. For multi-contrast MPI, the intestinal lumen was filled with an intestinal tracer, which represents an orally administered tracer, to allow co-registration of both tracers at the same location. Both single-and multi-contrast MPI are feasible to visualize GI bleeding and MPI may prove to be a useful tool for radiation-free detection of bleeding throughout the GI tract.

The 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 theoretical work, a matching condition that governs inductors 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.

Image reconstruction in Magnetic Particle Imaging (MPI) is an ill-posed linear inverse problem. A standard method for solving such a problem is the regularized least squares approach, which uses, a regularization function to reduce the impact of measurement noise in the reconstructed image by leveraging prior knowledge. Various optimization algorithms, including the Kazcmarz method or the Alternating Direction Method of Multipliers (ADMM), and regularization functions, such as l2 or Fused Lasso priors have been employed. Therefore, the creation and implementation of cutting-edge image reconstruction techniques necessitate a robust and adaptable optimization framework. In this work, we present the open-source Julia package RegularizedLeastSquares.jl, which provides a large selection of common optimization algorithms and allows flexible inclusion of regularization functions. These features enable the package to achieve state-of-the-art image reconstruction in MPI.

Model-based reconstruction is still one of the key challenges in magnetic particle imaging (MPI) when using multidimensional 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 MPI scanner parameters on the system matrix pattern and review methods to calibrate each parameter.

In magnetic particle imaging, a field-free region is maneuvered throughout the field of view using a time-varying magnetic field known as the drive-field. Human-sized systems operate the drive-field in the kHz range and generate it by utilizing strong currents that can rise to the kA range within a coil called the drive field generator. Matching and tuning between a power amplifier, a band-pass filter, and the drive-field generator is required. Here, for reasons of safety in future human scanners, a symmetrical topology and a transformer called an inductive coupling network are used. Our primary objectives are to achieve floating potentials to ensure patient safety while attaining high linearity and high gain for the resonant transformer. We present a novel systematic approach to the design of a loss-optimized resonant toroid with a D-shaped cross section, employing segmentation to adjust the inductance-to-resistance ratio while maintaining a constant quality factor. Simultaneously, we derive a specific matching condition for a symmetric transmit-receive circuit for magnetic particle imaging. The chosen setup filters the fundamental frequency and allows simultaneous signal transmission and reception. In addition, the decoupling of multiple drive field channels is discussed, and the primary side of the transformer is evaluated for maximum coupling and minimum stray field. Two prototypes were constructed, measured, decoupled, and compared to the derived theory and method-of-moment based simulations.

Magnetic Particle Imaging hardware has reached human scale and thus patient safety questions and clinical application scenarios are in the focus of current research. In this work, we present a safe real-time 3D MPI system for cerebral applications. High voltages are avoided to ensure patient safety by a low voltage-high current transmit coil design. The developed 2D drive-field generator generates a field-free-point trajectory in the sagittal xz-plane that is shifted with low frequency by a dynamic selection-field sequence along the y-axis. The scanner generates 3D images with 4 frames/second and allows for direct visualization of the clinically preferred transversal yz-plane, which is crucial for future brain examinations. Advanced reconstruction techniques reach a system sensitivity of 4 µgFe with respect to the iron mass in a sensitivity study., Deutsche Forschungsgemeinschaft (DFG)

In Magnetic Particle Imaging, the spatial distribution of a tracer is measured and depicted with a concentration dependent signal intensity for any location that contains particles, whereas surrounding tissue does not provide any signal. After tracer injection, the signal over time (positive contrast) can be utilized as a transient response to calculate dynamic diagnostic parameters like perfusion parameter maps. In this work, a bolus of physiological saline solution without any particles (negative contrast) is proposed, where the remaining steady state concentration contributes to the image contrast. This opens up the possibility to stretch the total monitoring time of a patient by utilizing a positive-negative contrast sequence, while keeping the total iron dose constant in the subject. Resulting time responses show that normalized signals from positive and negative boli are concurrent in the phantom experiments, indicating identical diagnostic parameters for in-vivo use.

In recent works, arbitrary waveform or pulsed excitation in Magnetic Particle Imaging (MPI) was proposed to offer better resolution and sensitivity. Generating these excitation fields poses a new challenge in MPI hardware design. This work proposes a method which models the excitation chain as a linear system and predicts the required input voltage for the desired output field. The initial prediction is then iteratively improved to compensate for inaccuracies of the model. The method is demonstrated to achieve accurate field waveforms in both linear and slew rate limited regions of the amplifier.

The sampling trajectory is an important parameter of a magnetic particle imaging (MPI) system and should be selected in order to guarantee the best image quality constrained by hardware limitations. A simulation study is performed with the side conditions of a permanent magnet-based field-free line (FFL) scanner system evaluating multiple trajectory types (radial, spiral, uniform spiral, flower) and trajectory densities in terms of spatial resolution. The findings provide information on suitable FFL trajectories and indicate initial trends for advantageous sampling patterns in the reference system. While the study presented here uses simulated data, the used software framework has the potential to generate sequenced signals based on real measurement data. Within this framework, we present a novel approach to FFL trajectory analysis., Europäischer Fonds für Regionale Entwicklung (EFRE)

Many different concepts for selection-field generators have been introduced for Magnetic Particle Imaging so far. In this work, the field generation characteristics of an optimized iron core selection-field generator consisting of two coil arrays with a total of 18 coils are presented. Due to the high number of degrees of freedom, a wide variety of field configurations are possible. The setup allows the generation of arbitrarily shaped fields, including the standard Magnetic Particle Imaging fields such as field-free points and field-free lines. In this work, measurements of static magnetic fields are presented and the current calculation method for generating a specific field configuration is discussed., Deutsche Forschungsgemeinschaft (DFG)

Improving resolution and sensitivity will widen possible medical applications of magnetic particle imaging. Pulsed excitation promises such benefits, at the cost of more complex hardware solutions and restrictions on drive field amplitude and frequency. State-of-the-art systems utilize a sinusoidal excitation to drive superparamagnetic nanoparticles into the non-linear part of their magnetization curve, which creates a spectrum with a clear separation of direct feed-through and higher harmonics caused by the particles response. One challenge for rectangular excitation is the discrimination of particle and excitation signals, both broad-band. Another is the drive-field sequence itself, as particles that are not placed at the same spatial position, may react simultaneously and are not separable by their signal phase or shape. To overcome this potential loss of information in spatial encoding for high amplitudes, a superposition of shifting fields and drive-field rotations is proposed in this work. Upon close view, a system matrix approach is capable to maintain resolution, independent of the sequence, if the response to pulsed sequences still encodes information within the phase. Data from an Arbitrary Waveform Magnetic Particle Spectrometer with offsets in two spatial dimensions is measured and calibrated to guarantee device independence. Multiple sequence types and waveforms are compared, based on frequency space image reconstruction from emulated signals, that are derived from measured particle responses. A resolution of 1.0 mT (0.8 mm for a gradient of (-1.25,\,-1.25,\,2.5) T/m) in x- and y-direction was achieved and a superior sensitivity for pulsed sequences was detected on the basis of reference phantoms.

The achievable sensitivity inMagnetic 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.

For imaging and magnetic manipulation experiments in Magnetic Particle Imaging several field generating coils are required to produce sufficiently high and flexible magnetic fields. To minimize the power consumption, coils with iron cores are the best choice for low and medium frequency ranges. Such coils have comparatively high reactance and are often inductively coupled. The trivial approach to ensure target currents is to provide each coil with a current controlled source resulting in high system complexity and high costs. This paper presents a circuit design to distribute bipolar target currents from a single unipolar source with high accuracy, reducing unwanted coil coupling by a feedback controller. Thus, the number of current sources can be significantly reduced. With a regenerative concept, reactive power is stored and can be reused, allowing efficient and fast current switching., Bundesministerium für Bildung und Forschung (BMBF)

Non-sinusoidal excitation waveforms have the ability to improve the signal-to-noise ratio and image resolution under certain conditions. Yet, the ability to use phase information for spatial encoding is expected to diminish as sharp pulses lead to concurrent signal response due to steep slopes and therefore less phase information. This motivates investigations into alternate sampling approaches that mitigate a loss in spatial encoding. However, measurements and image reconstruction results indicate that 10 times faster slew rates compared to sine excitation lead to enough phase information to resolve basic features using system matrix reconstruction.