Magnetic-field measurement and simulation of a field-free line magnetic-particle scanner

In 2005, B. Gleich and J. Weizenecker initially presented the tracer based medical imaging modality Magnetic Particle Imaging (MPI). It uses the nonlinear magnetization behavior of super paramagnetic iron oxide nanoparticles (SPIONs). MPI has the potential to perform real-time imaging in the sub millimeter-range without the use of harmful radiation. To acquire a particle signal from the tracer, an alternating homogenous magnetic field (drive field) is applied. Due to the nonlinearity of the particle magnetization, the magnetic field is distorted and higher harmonics are generated that indicate a particle concentration within the field of view (FOV). For the spatial distribution, another magnetic field that exhibits a high gradient (selection field) is applied simultaneously. Basically, there are two different types of selection fields containing either a field- free point (FFP) or a field-free line (FFL). Because of magnetic saturation, only SPIONs within the close vicinity of the FFP or FFL contribute to the particle signal. As the FFP is moved by the drive field through the FOV a spatial distribution of the SPIONs can be obtained. In the other encoding concept, the FFL rotates and is additionally translated by the drive field to obtain one dimensional projections for various angles. In this work, the currently world’s largest FFL MPI Scanner is investigated. Single components of the generated magnetic field are measured precisely to accomplish an accurate simulation of a translating and rotating FFL.

The effects of metal artifact reduction on the retrieval of attenuation values

BACKGROUND

The quality of CT slices can be drastically reduced in the presence of high-density objects such as metal implants within the patients' body due to the occurrence of streaking artifacts. Consequently, a delineation of anatomical structures might not be possible, which strongly influences clinical examination.

PURPOSE

The aim of the study is to clinically evaluate the retrieval of attenuation values and structures by the recently proposed Augmented Likelihood Image Reconstruction (ALIR) and linear interpolation in the presence of metal artifacts.

MATERIAL AND METHODS

A commercially available phantom was equipped with two steel inserts. At a position between the metal rods, which shows severe streaking artifacts, different human tissue-equivalent inserts are alternately mounted. Using a single-source computer tomograph, raw data with and without metal rods are acquired for each insert. Images are reconstructed using the ALIR algorithm and a filtered back projection with and without linear interpolation. Mean and standard deviation are compared for a region of interest in the ALIR reconstructions, linear interpolation results, uncorrected images with metal rods, and the images without metal rods, which are used as a reference. Furthermore, the reconstructed shape of the inserts is analyzed by comparing different profiles of the image.

RESULTS

The measured mean and standard deviation values show that for all tissue classes, the metal artifacts could be reduced using the ALIR algorithm and the linear interpolation. Furthermore, the HU values for the different classes could be retrieved with errors below the standard deviation in the reference image. An evaluation of the shape of the inserts shows that the reconstructed object fits the shape of the insert accurately after metal artifact correction. Moreover, the evaluation shows a drop in the standard deviation for the ALIR reconstructed images compared to the reference images while reducing artifacts and keeping the shape of the inserts, which indicates a noise reduction ability of the ALIR algorithm.

CONCLUSION

HU values, which are distorted by metal artifacts, can be retrieved accurately with the ALIR algorithm and the linear interpolation approach. After metal artifact correction, structures, which are not perceptible in the original images due to streaking artifacts, are reconstructed correctly within the image using the ALIR algorithm. Furthermore, the ALIR produced images with a reduced noise level compared to reference images and artifact images. Linear interpolation results in a distortion of the investigated shapes and features remaining streaking artifacts.

Non-Equispaced System Matrix Acquisition for Magnetic Particle Imaging Based on Lissajous Node Points

Magnetic Particle Imaging (MPI) is an emerging technology in the field of (pre)clinical imaging. The acquisition of a particle signal is realized along specific sampling trajectories covering a defined field of view (FOV). In a system matrix (SM) based reconstruction procedure, the commonly used acquisition path in MPI is a Lissajous trajectory. Such a trajectory features an inhomogeneous coverage of the FOV, i.e. the center region is sampled less dense than the regions towards the edges of the FOV. Conventionally, the respective SM acquisition and the subsequent reconstruction do not reflect this inhomogeneous coverage. Instead, they are performed on an equispaced grid. The objective of this work is to introduce a sampling grid that inherently features the aforementioned inhomogeneity by using node points of Lissajous trajectories. Paired with a tailored polynomial interpolation of the reconstructed MPI signal, the entire image can be recovered. It is the first time that such a trajectory related non-equispaced grid is used for image reconstruction on simulated and measured MPI data and it is shown that the number of sampling positions can be reduced, while the spatial resolution remains constant.

2D Images Recorded With a Single-Sided Magnetic Particle Imaging Scanner

Magnetic Particle Imaging is a new medical imaging modality, which detects superparamagnetic iron oxide nanoparticles. The particles are excited by magnetic fields. Most scanners have a tube-like measurement field and therefore, both the field of view and the object size are limited. A single-sided scanner has the advantage that the object is not limited in size, only the penetration depth is limited. A single-sided scanner prototype for 1D imaging has been presented in 2009. Simulations have been published for a 2D single-sided scanner and first 1D measurements have been carried out. In this paper, the first 2D single-sided scanner prototype is presented and the first calibration-based reconstruction results of measured 2D phantoms are shown. The field free point is moved on a Lissajous trajectory inside a 30 × 30 mm2 area. Images of phantoms with a maximal distance of 10 mm perpendicular to the scanner surface have been reconstructed. Different cylindrically shaped holes of phantoms have been filled with 6.28 μl undiluted Resovist. After the measurement and image reconstruction of the phantoms, particle volumes could be distinguished with a distance of 2 mm and 6 mm in vertical and horizontal direction, respectively.

Steering of Magnetic Devices With a Magnetic Particle Imaging System

Small magnetic devices have been steered in arbitrary direction and with variable force using a preclinical demonstrator system for magnetic particle imaging (MPI). Fast localization due to the high imaging rate of over 40 volumes/s and strong forces due to the high field gradient of more than 1 T/m render an MPI system, a good platform for image-guided steering of magnetic devices. In this paper, these capabilities are demonstrated in phantom experiments, where a closed feedback loop has been realized to exert translational forces in horizontal and vertical direction on a magnetic device moving in a viscous medium. The MPI system allows for the controlled application of those forces by combining variable homogeneous fields with strong field gradients.

Artifact free reconstruction with the system matrix approach by overscanning the field-free-point trajectory in magnetic particle imaging

Magnetic particle imaging is a tracer-based imaging method that utilizes the non-linear magnetization response of iron-oxide for determining their spatial distribution. The method is based on a sampling scheme where a sensitive spot is moved along a trajectory that captured a predefined field-of-view (FOV). However, particles outside the FOV also contribute to the measurement signal due to their rotation and the non-sharpness of the sensitive spot. In the present work we investigate artifacts that are induced by particles not covered by the FOV and show that the artifacts can be mitigated by using a system matrix that covers not only the region of interest but also a certain area around the FOV. The findings are especially relevant when using a multi-patch acquisition scheme where the boundaries of neighboring patches have to be handled.

Contribution of brownian rotation and particle assembly polarisation to the particle response in magnetic particle spectrometry

The spectrometry of super-paramagnetic iron-oxide nanoparticles is a central tool for characterising particles that are used in Magnetic Particle Imaging. In Magnetic Particle Imaging, nanoparticles are excited by a magnetic field and the particle response is measured. Until now, the influence of the trajectory sequence on the dynamic particle relaxation has not been scoped. With a multi-dimensional Magnetic Particle Spectrometer, analysing the behaviour of different trajectories on the particles becomes possible. In this paper, the contribution of Brownian rotation and assembly polarisation on the particle signal is being analysed.

Sequences for real-time magnetic particle imaging

Magnetic Particle Imaging (MPI) is a new imaging modality with the potential to be a new medical tool for angiographic diagnostics. It is capable of visualizing the spatial distribution of super-paramagnetic nanoparticles in high temporal and spatial resolution. Furthermore, the new spatial encoding scheme of a field free line (FFL) promises a ten-fold higher sensitivity. So far, all know imaging devices featuring this new technique feature slow data acquisition and thus, are far away from real-time imaging capability. An actual real-time approach requires a complex field generator and an application of currents with very precise amplitude and phase. Here, we present the first implementation and calibration of a dynamic FFL field sequence enabling the acquisition of 50 MPI images per second in a mouse sized scanner.

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.

Experimental evaluation of different weighting schemes in magnetic particle imaging reconstruction

Magnetic Particle Imaging (MPI) is a new imaging technique with an outstanding sensitivity, a high temporal and spatial resolution. MPI is based on the excitation and detection of magnetic tracer material by using magnetic fields. The spatial resolution strongly depends on the reconstruction parameters and on the selection and weighting of the system function frequency components. Currently, no fundamental strategy to weight the system function for the reconstruction is given. In this contribution, the influence on the spatial resolution of different selection and weighting methods is analyzed. Thereby, a new strategy is proposed to select and weight the components with respect to their mixing order. As a result, it is confirmed that a weighted system function provides better results of image reconstruction than a non-weighted one. In addition to this, it is shown that the usage of the mixing order in combination with established weightings improves the resolution.

Magnetic particle imaging: current developments and future directions

Magnetic particle imaging (MPI) is a novel imaging method that was first proposed by Gleich and Weizenecker in 2005. Applying static and dynamic magnetic fields, MPI exploits the unique characteristics of superparamagnetic iron oxide nanoparticles (SPIONs). The SPIONs’ response allows a three-dimensional visualization of their distribution in space with a superb contrast, a very high temporal and good spatial resolution. Essentially, it is the SPIONs’ superparamagnetic characteristics, the fact that they are magnetically saturable, and the harmonic composition of the SPIONs’ response that make MPI possible at all. As SPIONs are the essential element of MPI, the development of customized nanoparticles is pursued with the greatest effort by many groups. Their objective is the creation of a SPION or a conglomerate of particles that will feature a much higher MPI performance than nanoparticles currently available commercially. A particle’s MPI performance and suitability is characterized by parameters such as the strength of its MPI signal, its biocompatibility, or its pharmacokinetics. Some of the most important adjuster bolts to tune them are the particles’ iron core and hydrodynamic diameter, their anisotropy, the composition of the particles’ suspension, and their coating. As a three-dimensional, real-time imaging modality that is free of ionizing radiation, MPI appears ideally suited for applications such as vascular imaging and interventions as well as cellular and targeted imaging. A number of different theories and technical approaches on the way to the actual implementation of the basic concept of MPI have been seen in the last few years. Research groups around the world are working on different scanner geometries, from closed bore systems to single-sided scanners, and use reconstruction methods that are either based on actual calibration measurements or on theoretical models. This review aims at giving an overview of current developments and future directions in MPI about a decade after its first appearance.

Electronic field free line rotation and relaxation deconvolution in magnetic particle imaging

It has been shown that magnetic particle imaging (MPI), an imaging method suggested in 2005, is capable of measuring the spatial distribution of magnetic nanoparticles. Since the particles can be administered as biocompatible suspensions, this method promises to perform well as a tracer-based medical imaging technique. It is capable of generating real-time images, which will be useful in interventional procedures, without utilizing any harmful radiation. To obtain a signal from the administered superparamagnetic iron oxide (SPIO) particles, a sinusoidal changing external homogeneous magnetic field is applied. To achieve spatial encoding, a gradient field is superimposed. Conventional MPI works with a spatial encoding field that features a field free point (FFP). To increase sensitivity, an improved spatial encoding field, featuring a field free line (FFL) can be used. Previous FFL scanners, featuring a 1-D excitation, could demonstrate the feasibility of the FFL-based MPI imaging process. In this work, an FFL-based MPI scanner is presented that features a 2-D excitation field and, for the first time, an electronic rotation of the spatial encoding field. Furthermore, the role of relaxation effects in MPI is starting to move to the center of interest. Nevertheless, no reconstruction schemes presented thus far include a dynamical particle model for image reconstruction. A first application of a model that accounts for relaxation effects in the reconstruction of MPI images is presented here in the form of a simplified, but well performing strategy for signal deconvolution. The results demonstrate the high impact of relaxation deconvolution on the MPI imaging process.

Axially elongated field-free point data acquisition in magnetic particle imaging

The magnetic particle imaging (MPI) technology is a new imaging technique featuring an excellent possibility to detect iron oxide based nanoparticle accumulations in vivo. The excitation of the particles and in turn the signal generation in MPI are achieved by using oscillating magnetic fields. In order to realize a spatial encoding, a field-free point (FFP) is steered through the field of view (FOV). Such a positioning of the FFP can thereby be achieved by mechanical or electromagnetical movement. Conventionally, the data acquisition path is either a planar 2-D or a 3-D FFP trajectory. Assuming human applications, the size of the FOV sampled by such trajectories is strongly limited by heating of the body and by nerve stimulations. In this work, a new approach acquiring MPI data based on the axial elongation of a 2-D FFP trajectory is proposed. It is shown that such an elongation can be used as a data acquisition path to significantly increase the acquisition speed, with negligible loss of spatial resolution.

On the formulation of the image reconstruction problem in magnetic particle imaging

In magnetic particle imaging (MPI), the spatial distribution of magnetic nanoparticles is determined by applying various static and dynamic magnetic fields. Due to the complex physical behavior of the nanoparticles, it is challenging to determine the MPI system matrix in practice. Since the first publication on MPI in 2005, different methods that rely on measurements or simulations for the determination of the MPI system matrix have been proposed. Some methods restrict the simulation to an idealized model to speed up data reconstruction by exploiting the structure of an idealized MPI system matrix. Recently, a method that processes the measurement data in x-space rather than frequency space has been proposed. In this work, we compare the different approaches for image reconstruction in MPI and show that the x-space and the frequency space reconstruction techniques are equivalent.

Safety measurements for heating of instruments for cardiovascular interventions in magnetic particle imaging (MPI) - first experiences

Magnetic particle imaging (MPI) has emerged as a new imaging method with the potential of delivering images of high spatial and temporal resolutions and free of ionizing radiation. Recent studies demonstrated the feasibility of differentiation between signal-generating and non-signal-generating devices in Magnetic Particle Spectroscopy (MPS) and visualization of commercially available catheters and guide-wires in MPI itself. Thus, MPI seems to be a promising imaging tool for cardiovascular interventions. Several commercially available catheters and guide-wires were tested in this study regarding heating. Heating behavior was correlated to the spectra generated by the devices and measured by the MPI. The results indicate that each instrument should be tested separately due to the wide spectrum of measured temperature changes of signal-generating instruments, which is up to 85°C in contrast to non-signal-generating devices. Development of higher temperatures seems to be a limitation for the use of these devices in cardiovascular interventions.

Magnetic particle imaging: kinetics of the intravascular signal in vivo

BACKGROUND

Magnetic particle imaging (MPI) uses magnetic fields to visualize superparamagnetic iron oxide nanoparticles (SPIO). Today, Resovist(®) is still the reference SPIO for MPI. The objective of this study was to evaluate the in vivo blood half-life of two different types of Resovist (one from Bayer Pharma AG, and one from I'rom Pharmaceutical Co Ltd) in MPI.

METHODS

A Resovist concentration of 50 μmol/kg was injected into the ear artery of ten New Zealand White rabbits. Five animals received Resovist distributed by I'rom Pharmaceutical Co Ltd and five received Resovist by Bayer Pharma AG. Blood samples were drawn before and directly after injection of Resovist, at 5, 10, and 15 minutes, and then every 15 minutes until 120 minutes after the injection. The MPI signal of the blood samples was evaluated using magnetic particle spectroscopy.

RESULTS

The average decline of the blood MPI signal from the two distributions differed significantly (P=0.0056). Resovist distributed by Bayer Pharma AG showed a slower decline of the MPI signal (39.7% after 5 minutes, 20.5% after 10 minutes, and 12.1% after 15 minutes) compared with Resovist produced by I'rom Pharmaceutical Co Ltd (20.4% after 5 minutes, 7.8% after 10 minutes, no signal above noise level after 15 minutes).

CONCLUSION

In MPI, the blood half-life of an SPIO tracer cannot be equalized to the blood half-life of its MPI signal. Resovist shows a very rapid decline of blood MPI signal and is thus not suitable as a long circulating tracer. For cardiovascular applications in MPI, it may be used as a bolus tracer.

Improved field free line magnetic particle imaging using saddle coils

Magnetic particle imaging (MPI) is a novel tracer-based imaging method detecting the distribution of superparamagnetic iron oxide (SPIO) nanoparticles in vivo in three dimensions and in real time. Conventionally, MPI uses the signal emitted by SPIO tracer material located at a field free point (FFP). To increase the sensitivity of MPI, however, an alternative encoding scheme collecting the particle signal along a field free line (FFL) was proposed. To provide the magnetic fields needed for line imaging in MPI, a very efficient scanner setup regarding electrical power consumption is needed. At the same time, the scanner needs to provide a high magnetic field homogeneity along the FFL as well as parallel to its alignment to prevent the appearance of artifacts, using efficient radon-based reconstruction methods arising for a line encoding scheme. This work presents a dynamic FFL scanner setup for MPI that outperforms all previously presented setups in electrical power consumption as well as magnetic field quality.