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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.

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.

Magnetic Particle Imaging (MPI) is a tracer based medical imaging modality with great potential due to its high sensi-tivity, high spatial and temporal resolution, and its ability to quantify the tracer concentration. Image reconstruction in MPI is an ill-posed problem, which can be addressed by regularization methods that lead to a reconstruction bias, which is apparent in a systematic mismatch between true and reconstructed tracer distribution. This is expressed in a background signal, a mismatch of the spatial support of the tracer distribution and a mismatch of its values. In this work, MPI reconstruction bias and its impact are investigated and a recently proposed debiasing method with significant bias reduction capabilities is adopted.

A recent work introduced a balloon catheter from a polymer mixed with magnetic nanoparticles that was visible in magnetic particle imaging (MPI) in initial trials. In this work, dynamic multi-patch and multi-contrast experiments were performed to validate the applicability of the balloon catheter for complex application scenarios in MPI. In both cases, the balloon was successfully imaged, although there are limitations in resolution and dynamic imaging range.

Magnetic particle imaging is a very useful tool in the detection of stroke. To study the ability of stroke in a mouse model the data acquisition is challenging as a mouse brain contains only a very small ratio of blood compared to large animals or humans. The effective concentration within the whole organ is therefore very small, especially compared to the heart or the liver. Typical MPI receiver coils however cover a sensitive region of around 30 mm to 50 mm and have a bore size of above 40 mm. This leads on the one hand to non-optimal signal coupling due to the distance to the particles and on the other hand strong signals from the heart can cause artifacts in the low signal regions. In this work we present a coil optimized for mouse brain imaging, which due to its small size, also dampens signal from regions outside of the coil.

Although Magnetic Particle Imaging (MPI) is not yet in clinical use, it is highly promising for several medical ap-plications, and especially for applications in diagnostic vascular in vivo imaging and imaging-guided vascular interventions. Furthermore, in the last years, different superparamagnetic iron oxide (SPIO) based contrast agents have been developed and approved for niche clinical applications in Magnetic Resonance Imaging (MRI) as alterna-tive to Gadolinium-based contrast agents (GBCAs) due to the risk for patients suffering from kidney dysfunction or nephrogenic systemic fibrosis (NSF). Recently, the potential of RBCs loaded with different SPIO nanoparticles as blood-pool tracer agents with longer blood retention time for MRI and MPI has been investigated. Here, we report the first in vitro results with the highly efficient dextran-based MPI tracer particles perimag® and synomag®-D to study their eligibility to be encapsulated into human RBCs and the potential of these new SPIO-RBC constructs as tracer material for MPI.

One approach of image reconstruction in MPI is the system matrix based reconstruction. With this approach, in addition to the particle behavior, the sequence and the scanner properties are also calibrated and stored in a system matrix, so that a linear system of equations for the image reconstruction must be solved. However, the measurement of the system matrix is very time-consuming, depending on the desired spatial resolution. Independently of this, there are some remarkable symmetries within the system matrix that could be exploited to significantly reduce the calibration time. In the context of this work the theoretical description of a system matrix about Chebyshev polynomials is used to completely build a 3D system matrix by mirroring an octant and to successfully reconstruct an image.