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

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

Influence of the Receive Channel Number on the Spatial Resolution in Magnetic Particle Imaging

Magnetic Particle Imaging (MPI) is a fast and highly sensitive tomographic imaging modality. When applying 3D Lissajous imaging sequences, the region of interest is rapidly sampled by moving a field-free point along a predefined trajectory. Since the field excitation is done using three orthogonal excitation coils, usually also the magnetization response is measured with three independent and orthogonal receive coils. In this work the influence of selecting a subset of receive channels during reconstruction on the resulting image quality is analyzed. It is shown that using a single receive channel a slight loss of spatial resolution in the order of 12–22% in the direction perpendicular to the receiving direction can be observed while in direction of the receive coil the resolution is preserved and partially even improved. Since the construction of decoupled 3D receive coil units is a major engineering effort, the findings can be used to simplify the construction of 3D Lissajous type scanners

Determination of 3D system matrices using a mirroring approach based on mixing theory

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

Towards accurate modeling of the multidimensional magnetic particle imaging physics

The image reconstruction problem of the tomographic imaging technique magnetic particle imaging (MPI) requires the solution of a linear inverse problem. One prerequisite for this task is that the imaging operator that describes the mapping between the tomographic image and the measured signal is accurately known. For 2D and 3D excitation patterns, it is common to measure the system matrix in a calibration procedure, that is both, very time consuming and adds noise to the operator. The need for measuring the system matrix is due to the lack of an accurate model that is capable of describing the nanoparticles’ magnetization behavior in the MPI setup. Within this work we exploit a physical model that is based on Néel rotation for large particle ensembles and we find model parameters that describe measured 2D MPI data with much higher precision than state of the art MPI models. With phantom experiments we show that the simulated system matrix can be used for image reconstruction and reduces artifacts due to model-mismatch considerably.TK acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation)—project number 281474342/GRK2224/1 'Pi3: Parameter Identification—Analysis, Algorithms, Applications' and support by the project 'MPI2' funded by the Federal Ministry of Education and Research (BMBF, project no. 05M16LBA). TK acknowledges the financial support by the German Research Foundation (DFG, grant number KN 1108/2-1) and the Federal Ministry of Education and Research (BMBF, grant number 05M16GKA and 13XP5060B). The publication is also funded by the German Research Foundation (DFG, project no. 392323616) and the Hamburg University of Technology (TUHH) in the funding programme Open Access Publishing

Towards accurate modeling of the multidimensional MPI physics

The MPI image reconstruction problem requires, particularly for 2D and 3D excitation patterns, a measured system matrix due to the lack of an accurate model that is capable of describing the nanoparticles’ magnetization behavior in the MPI setup. Here we exploit a model based on Néel rotation for large particle ensembles and we find model parameters that describe measured 2D MPI data with much higher precision than state of the art MPI models, which is also illustrated in phantom experiments. This is a short summary of the recent work [4] to which we refer to for all further details.   Int. J. Mag. Part. Imag. 6(2), Suppl. 1, 2020, Article ID: 2009004, DOI: 10.18416/IJMPI.2020.200900

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)