In-Vitro MPI-Guided IVOCT Catheter Tracking in Real Time for Motion Artifact Compensation

Purpose: Using 4D magnetic particle imaging (MPI), intravascular optical coherence tomography (IVOCT) catheters are tracked in real time in order to compensate for image artifacts related to relative motion. Our approach demonstrates the feasibility for bimodal IVOCT and MPI in-vitro experiments. Material and Methods: During IVOCT imaging of a stenosis phantom the catheter is tracked using MPI. A 4D trajectory of the catheter tip is determined from the MPI data using center of mass sub-voxel strategies. A custom built IVOCT imaging adapter is used to perform different catheter motion profiles: no motion artifacts, motion artifacts due to catheter bending, and heart beat motion artifacts. Two IVOCT volume reconstruction methods are compared qualitatively and quantitatively using the DICE metric and the known stenosis length. Results: The MPI-tracked trajectory of the IVOCT catheter is validated in multiple repeated measurements calculating the absolute mean error and standard deviation. Both volume reconstruction methods are compared and analyzed whether they are capable of compensating the motion artifacts. The novel approach of MPI-guided catheter tracking corrects motion artifacts leading to a DICE coefficient with a minimum of 86% in comparison to 58% for a standard reconstruction approach. Conclusions: IVOCT catheter tracking with MPI in real time is an auspicious method for radiation free MPI-guided IVOCT interventions. The combination of MPI and IVOCT can help to reduce motion artifacts due to catheter bending and heart beat for optimized IVOCT volume reconstructions.Comment: 19 pages, 11 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

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

Iron core coil designs for MPI

In Magnetic Particle Imaging, much of the power consumed during an imaging sequence is used for the generation of the selection and focus fields. In today’s MPI scanners three different concepts are applied to generate the gradient fields: Air coils, permanent magnets and coils with soft iron. Air coils and permanent magnets have the great advantage of good calculability by the Biot-Savart Law. On the way to a clinical imaging modality, the needed power for sufficient gradient strength demand the use of soft iron. In order to make good use of the ferromagnetic amplification properties, much more complex simulations have to be done. A recently published head scanner uses a soft iron yoke for field generation. In this study, we investigated different coil geometries with soft iron with respect to this head scanner.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009042, DOI: 10.18416/IJMPI.2020.200904

Role of Phase Encoding in Pulsed Magnetic Particle Imaging

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

Modular MPI Component Testing Facility

The image quality of magnetic particle imaging depends on the interference-free measurement of the particlespectrum. Systematic errors due to harmonic distortions in the components used for implementation complicatethe development of efficient imaging devices. To test the suitability of parts for the development of MPIscanners, a test bench was developed in which the components to be tested can be integrated, to evaluate theirsuitability using the measured spectrum. Different forms of connectors integrated into the high-current pathwere analyzed for their influence on the signal quality using this test bench. An evaluation of this componentscan be made from the clearly visibible distortions in the spectrum recorded with the high-sensitive test system

Introducing a frequency-tunable magnetic particle spectrometer

Image quality in the new imaging modality magnetic particle imaging (MPI) heavily relies on the quality of the magnetic nanoparticles in use. Therefore, it is crucial to understand the behaviour of such particles. A common technique to analyze the behaviour of the particles is magnetic particle spectrometry (MPS). However, most spectrometers are limited to measurements at a single or multiple discrete excitation frequencies. This paper introduces a frequency-tunable spectrometer, able to perform measurements in the range of 100 Hz - 24kHz

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