Towards Picogram Detection of Superparamagnetic Iron-Oxide Particles Using a Gradiometric Receive Coil

Superparamagnetic iron-oxide nanoparticles can be used in a variety of medical applications like vascular or targeted imaging. Magnetic particle imaging (MPI) is a promising tomographic imaging technique that allows visualizing the 3D nanoparticle distribution concentration in a non-invasive manner. The two main strengths of MPI are high temporal resolution and high sensitivity. While the first has been proven in the assessment of dynamic processes like cardiac imaging, it is unknown how far the detection limit of MPI can be lowered. Within this work, we will present a highly sensitive gradiometric receive-coil unit combined with a noise-matching network tailored for the measurement of mice. The setup is capable of detecting 5 ng of iron in vitro at 2.14 sec acquisition time. In terms of iron concentration we are able to detect 156 {\mu}g/L marking the lowest value that has been reported for an MPI scanner so far. In vivo MPI mouse images of a 512 ng bolus at 21.5 ms acquisition time allow for capturing the flow of an intravenously injected tracer through the heart of a mouse. Since it has been rather difficult to compare detection limits across MPI publications we propose guidelines improving the comparability of future MPI studies.Comment: 15 Pages, 7 Figures, V2: Changed the initials of Author Kannan M Krishnan, added two citations, corrected typo

OpenMPIData: an initiative for freely accessible magnetic particle imaging data

Magnetic particle imaging is a tomographic imaging technique capable of measuring the local concentration of magnetic nanoparticles that can be used as tracers in biomedical applications. Since MPI is still at a very early stage of development, there are only a few MPI systems worldwide that are primarily operated by technical research groups that develop the systems themselves. It is therefore difficult for researchers without direct access to an MPI system to obtain experimental MPI data. The purpose of the OpenMPIData initiative is to make experimental MPI data freely accessible via a web platform. Measurements are performed with multiple phantoms and different image sequences from 1D to 3D. The datasets are stored in the magnetic particle image data format (MDF), an open document standard for storing MPI data. The open data is mainly intended for mathematicians and algorithm developers working on new reconstruction algorithms. Each dataset is designed to pose a specific challenge to image reconstruction. In addition to the measurement data, computer aided design (CAD) drawings of the phantoms are also provided so that the exact dimensions of the particle concentrations are known. Thus, the phantoms can be reproduced by other research groups using additive manufacturing. These reproduced phantoms can be used to compare different MPI systems.Supported by the German Research Foundation (DFG, grant number KN 1108/2-1) and the Federal Ministry of Education and Research (BMBF, grant numbers 05M16GKA and 13XP5060B)

In-vitro MPI-guided IVOCT catheter tracking in real time for motion artifact compensation

This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. 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

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

3D printed anatomical model of a rat for medical imaging

For medical research, approximately 115 million animals are needed every year. Rodents are used to test possible applications and procedures for the diagnosis of anatomical and physiological diseases. However, working with living animals increases the complexity of an experiment. Accurate experimental planning is essential in order to fulfill the 3R rules (replace, reduce and refine). Especially in tracer-based imaging modalities, such as magnetic particle imaging (MPI), where only nanoparticles give a positive contrast, the anatomical structure of the rodent is not visible without co-registration with another imaging modality. This leads to problems in the experimental planning, as parameters, such as field of view, rodent position and tracer concentration, have to be determined without visual feedback. In this work, a 3D CAD rat model is presented, which can be used to improve the experiment planning and thus reduce the number of animals required. It was determined using an anatomy atlas and 3D printed with stereolithography. The resulting model contains the most important organs and vessels as hollow cavities. By filling these with appropriate tracer materials, the phantom can be used in different imaging modalities such as MPI, magnetic resonance imaging (MRI) or computed tomography (CT). In a first MPI measurement, the phantom was filled with superparamagnetic nanoparticles. Finally, a successful visualization of all organs and vessels of the phantom was possible. This enables the planning of the experiment and the optimization of experimental parameters for a region of interest, where certain organs in a living animal are localized.Research funding: The authors thankfully acknowledge the financial support by the DFG (grant number KN 1108/2-1) and the BMBF (grant number 05M16GKA)

Hybrid system calibration for multidimensional magnetic particle imaging

Magnetic particle imaging visualizes the spatial distribution of superparamagnetic nanoparticles. Because of its key features of excellent sensitivity, high temporal and spatial resolution and biocompatibility of the tracer material it can be used in multiple medical imaging applications. The common reconstruction technique for Lissajous-type trajectories uses a system matrix that has to be previously acquired in a time-consuming calibration scan, leading to long downtimes of the scanning device. In this work, the system matrix is determined by a hybrid approach. Using the hybrid system matrix for reconstruction, the calibration downtime of the scanning device can be neglected. Furthermore, the signal to noise ratio of the hybrid system matrix is much higher, since the size of the required nanoparticle sample can be chosen independently of the desired voxel size. As the signal to noise ratio influences the reconstruction process, the resulting images have better resolution and are less affected by artefacts. Additionally, a new approach is introduced to address the background signal in image reconstruction. The common technique of subtraction of the background signal is replaced by extending the system matrix with an entry that represents the background. It is shown that this approach reduces artefacts in the reconstructed images.DFG, grant number BU 1436/10-1, DFG grant numbers KN 1108/2-1, AD 125/5-

Model-based voltage predictions for arbitrary waveform excitation in Magnetic Particle Imaging

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

Comparison of Reconstruction Methods for Measured FFL Data

System matrix and x-space resonstruction are two of the main approaches for image reconstruction of field free line magnetic particle imaging. A comparative studiy of both options is performed on data from a phantommeasurement in a permanent magnet-based scanner system. The system matrix reconstruction is performed in a hybrid fashion with data obtained in a spectrometer. This data is also used to obtain the relaxation time and particle diameter of the particles used. A deconvolution can then be used to enhance the image quality of the x-space approach which is compared to the system matrix reconstruction. A slight blurring can be depicted in the x-space reconstruction but overall a good agreement between both approaches is reached with the Structural Similarity Index yielding a value of 0.79