Jörn Borgert

Three-dimensional real-time in vivo magnetic particle imaging

Magnetic particle imaging (MPI) is a new tomographic imaging method potentially capable of rapid 3D dynamic imaging of magnetic tracer materials. Until now, only dynamic 2D phantom experiments with high tracer concentrations have been demonstrated. In this letter, first in vivo 3D real-time MPI scans are presented revealing details of a beating mouse heart using a clinically approved concentration of a commercially available MRI contrast agent. A temporal resolution of 21.5 ms is achieved at a 3D field of view of 20.4 x 12 x 16.8 mm(3) with a spatial resolution sufficient to resolve all heart chambers. With these abilities, MPI has taken a huge step toward medical application.

Magnetization response spectroscopy of superparamagnetic nanoparticles for magnetic particle imaging

Magnetic particle imaging (MPI) is a tomographic imaging modality sensitive to the spatial distribution of magnetic particles. The spectrometer, described in this paper, is capable of measuring the remagnetization spectrum of superparamagnetic nanoparticles. With this spectrometer the suitability of particles, for MPI, can be characterized. Furthermore, the spectrometer can be used to estimate the particle size distribution, which allows for more accurate simulations in MPI.

Motion-compensated and gated cone beam filtered back-projection for 3-D rotational X-ray angiography

This paper presents a method to reconstruct moving objects from cone beam X-ray projections acquired during a single rotational run using a given motion vector field. The method is applicable to voxel driven cone-beam filtered back-projection reconstruction approaches. Here, a formulation based on the algorithm of Feldkamp, Davis, and Kress (FDK) is presented. The motion correction is applied during the back-projection step by shifting the voxel to be reconstructed according to the motion vector field. The method is applied to three-dimensional (3-D) rotational X-ray angiography. Projections from a beating coronary heart phantom are simulated. Motion-compensated reconstructions with varying accuracy of the applied motion field are carried out for a late diastolic heart phase and compared to the reconstruction obtained with the standard FDK-method from projections of the corresponding motion-free model in the same heart phase. Furthermore, gated reconstructions are calculated by weighting the projections according to their cardiac phase without using a motion vector field. Different gating window widths are applied, and the reconstructions are compared. Using the correct motion field with the motion-compensated reconstruction, the image quality of the standard reconstruction from the corresponding motion-free coronary model can almost be recovered. The reconstructed image quality stays acceptable if the accuracy of the motion field sampling points is better than 1 mm. The gated reconstructions with a window width of 15%-20% of the cardiac cycle lead to superior results compared to nearest neighbor gating, especially for histogram based visualization and analysis. The motion-compensated reconstructions provide sharp images of the coronaries far surpassing the image quality of gated reconstructions

Experimental results on fast 2D-encoded magnetic particle imaging

This paper presents the first experimental results on magnetic particle imaging with full 2D encoding. The encoding speed achieved was 3.88 ms for a field of view of 1x1 cm2. Small phantoms composed of several dots each filled with 200 nl undiluted Resovist (500 mmol(Fe) l(-1)) were scanned. A resolution of better than 1 mm was achieved for a frame rate of 25 frames s(-1).

Model-Based Reconstruction for Magnetic Particle Imaging

Magnetic particle imaging (MPI) is a new imaging modality capable of imaging distributions of superparamagnetic nanoparticles with high sensitivity, high spatial resolution and, in particular, high imaging speed. The image reconstruction process requires a system function, describing the mapping between particle distribution and acquired signal. To date, the system function is acquired in a tedious calibration procedure by sequentially measuring the signal of a delta sample at the positions of a grid that covers the field of view. In this work, for the first time, the system function is calculated using a model of the signal chain. The modeled system function allows for reconstruction of the particle distribution in a 1-D MPI experiment. The approach thus enables fast generation of system functions on arbitrarily dense grids. Furthermore, reduction in memory requirements may be feasible by generating parts of the system function on the fly during reconstruction instead of keeping the complete matrix in memory.

Magnetic Particle Imaging With Tailored Iron Oxide Nanoparticle Tracers

Magnetic particle imaging (MPI) shows promise for medical imaging, particularly in angiography of patients with chronic kidney disease. As the first biomedical imaging technique that truly depends on nanoscale materials properties, MPI requires highly optimized magnetic nanoparticle tracers to generate quality images. Until now, researchers have relied on tracers optimized for MRI T2*-weighted imaging that are sub-optimal for MPI. Here, we describe new tracers tailored to MPIs unique physics, synthesized using an organic-phase process and functionalized to ensure biocompatibility and adequate in vivo circulation time. Tailored tracers showed up to 3 × greater signal-to-noise ratio and better spatial resolution than existing commercial tracers in MPI images of phantoms.

Weighted iterative reconstruction for magnetic particle imaging

Magnetic particle imaging (MPI) is a new imaging technique capable of imaging the distribution of superparamagnetic particles at high spatial and temporal resolution. For the reconstruction of the particle distribution, a system of linear equations has to be solved. The mathematical solution to this linear system can be obtained using a least-squares approach. In this paper, it is shown that the quality of the least-squares solution can be improved by incorporating a weighting matrix using the reciprocal of the matrix-row energy as weights. A further benefit of this weighting is that iterative algorithms, such as the conjugate gradient method, converge rapidly yielding the same image quality as obtained by singular value decomposition in only a few iterations. Thus, the weighting strategy in combination with the conjugate gradient method improves the image quality and substantially shortens the reconstruction time. The performance of weighting strategy and reconstruction algorithms is assessed with experimental data of a 2D MPI scanner.

Human erythrocytes as nanoparticle carriers for magnetic particle imaging

The potential of red blood cells (RBCs) loaded with iron oxide nanoparticles as a tracer material for magnetic particle imaging (MPI) has been investigated. MPI is an emerging, quantitative medical imaging modality which holds promise in terms of sensitivity in combination with spatial and temporal resolution. Steady-state and dynamic magnetization measurements, supported by semi-empirical modeling, were employed to analyze the MPI signal generation using RBCs as novel biomimetic constructs. Since the superparamagnetic iron oxide (SPIO) bulk material that is used in this study contains nanoparticles with different sizes, it is suggested that during the RBC loading procedure, a preferential entrapment of nanoparticles with hydrodynamic diameter ≤60 nm occurs by size-selection through the erythrocyte membrane pores. This affects the MPI signal of an erythrocyte-based tracer, compared to bulk. The reduced signal is counterbalanced by a higher in vivo stability of the SPIO-loaded RBCs constructs for MPI applications.

Analysis of a 3-D System Function Measured for Magnetic Particle Imaging

Magnetic particle imaging (MPI) is a new tomographic imaging approach that can quantitatively map magnetic nanoparticle distributions in vivo. It is capable of volumetric real-time imaging at particle concentrations low enough to enable clinical applications. For image reconstruction in 3-D MPI, a system function (SF) is used, which describes the relation between the acquired MPI signal and the spatial origin of the signal. The SF depends on the instrumental configuration, the applied field sequence, and the magnetic particle characteristics. Its properties reflect the quality of the spatial encoding process. This work presents a detailed analysis of a measured SF to give experimental evidence that 3-D MPI encodes information using a set of 3-D spatial patterns or basis functions that is stored in the SF. This resembles filling 3-D k-space in magnetic resonance imaging, but is faster since all information is gathered simultaneously over a broad acquisition bandwidth. A frequency domain analysis shows that the finest structures that can be encoded with the presented SF are as small as 0.6 mm. SF simulations are performed to demonstrate that larger particle cores extend the set of basis functions towards higher resolution and that the experimentally observed spatial patterns require the existence of particles with core sizes of about 30 nm in the calibration sample. A simple formula is presented that qualitatively describes the basis functions to be expected at a certain frequency.

Magnetic Particle Imaging: Visualization of Instruments for Cardiovascular Intervention

PURPOSE To evaluate the feasibility of different approaches of instrument visualization for cardiovascular interventions guided by using magnetic particle imaging (MPI). MATERIALS AND METHODS Two balloon (percutaneous transluminal angioplasty) catheters were used. The balloon was filled either with diluted superparamagnetic iron oxide (SPIO) ferucarbotran (25 mmol of iron per liter) or with sodium chloride. Both catheters were inserted into a vessel phantom that was filled oppositional to the balloon content with sodium chloride or diluted SPIO (25 mmol of iron per liter). In addition, the administration of a 1.4-mL bolus of pure SPIO (500 mmol of iron per liter) followed by 5 mL of sodium chloride through a SPIO-labeled balloon catheter into the sodium chloride-filled vessel phantom was recorded. Images were recorded by using a preclinical MPI demonstrator. All images were acquired by using a field of view of 3.6 × 3.6 × 2.0 cm. RESULTS By using MPI, both balloon catheters could be visualized with high temporal (21.54 msec per image) and sufficient spatial (≤ 3 mm) resolution without any motion artifacts. The movement through the field of view, the inflation and deflation of the balloon, and the application of the SPIO bolus were visualized at a rate of 46 three-dimensional data sets per second. CONCLUSION Visualization of SPIO-labeled instruments for cardiovascular intervention at high temporal resolution as well as monitoring the application of a SPIO-based tracer by using labeled instruments is feasible. Further work is necessary to evaluate different labeling approaches for diagnostic catheters and guidewires and to demonstrate their navigation in the vascular system after administration of contrast material. SUPPLEMENTAL MATERIAL http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12120424/-/DC1.