Wiebke Tenner

Magnetic particle imaging: Introduction to imaging and hardware realization

Magnetic Particle Imaging (MPI) is a recently invented tomographic imaging method that quantitatively measures the spatial distribution of a tracer based on magnetic nanoparticles. The new modality promises a high sensitivity and high spatial as well as temporal resolution. There is a high potential of MPI to improve interventional and image-guided surgical procedures because, today, established medical imaging modalities typically excel in only one or two of these important imaging properties. MPI makes use of the non-linear magnetization characteristics of the magnetic nanoparticles. For this purpose, two magnetic fields are created and superimposed, a static selection field and an oscillatory drive field. If superparamagnetic iron-oxide nanoparticles (SPIOs) are subjected to the oscillatory magnetic field, the particles will react with a non-linear magnetization response, which can be measured with an appropriate pick-up coil arrangement. Due to the non-linearity of the particle magnetization, the received signal consists of the fundamental excitation frequency as well as of harmonics. After separation of the fundamental signal, the nanoparticle concentration can be reconstructed quantitatively based on the harmonics. The spatial coding is realized with the static selection field that produces a field-free point, which is moved through the field of view by the drive fields. This article focuses on the frequency-based image reconstruction approach and the corresponding imaging devices while alternative concepts like x-space MPI and field-free line imaging are described as well. The status quo in hardware realization is summarized in an overview of MPI scanners.

Truncation artifacts in Magnetic Particle Imaging

Figure 1 clearly shows the effect of truncation artifacts when particles are not covered by the trajectory. With the removal of edge pixels these artifacts disappear. However, some information of the image is lost depending on the size of the necessary cut-off. With the multi-resolution approach the full information is used. Instead of removing pixels they are combined to large pixels. This method results in a better quality with small cut-offs compared to the non-compensated images.

A high power driving and selection field coil for an open MPI scanner

Three Litz wires have been used in parallel in order to decrease the layer to layer voltage down to 4 kV and the coil voltage down to 8 kV. A two millimeter thick epoxy plate has been glued between each layer, in order to prevent any electrical breakdown between the layers. A special epoxy has been used to minimize the inner temperature of the coil, which reached 180°C with a dissipated power of 5.2 kW. The use of parallel Litz wire leads to currents having different phase angle in every wire which has to be corrected.

Enlarging the Field of View in Magnetic Particle Imaging – A Comparison

Magnetic Particle Imaging evolves rapidly and human scanners are conceivable, already. However, the growing scanner size and therefore the increasing data within the field of view give rise to several unsolved problems. The reconstruction process, solving an inverse problem with the measured signal and the system function, is a storage consuming procedure for high resolution 3D imaging. Additionally, the size of the field of view strongly depends on the used gradient field and field amplitudes. Due to technical as well as medical limitations, such as specific absorption rates and peripheral nerve stimulation, the conventional procedures will not be sufficient to image large regions of interest. This paper compares and discusses approaches enlarging the field of view that might be used to reduce the reconstruction process and/or enlarge the field of view despite limited technical properties.

1D-image reconstruction for magnetic particle imaging using a hybrid system function

Magnetic Particle Imaging is a promising imaging technique using iron-oxide nanoparticle tracers. The spatial distribution of these particles can be determined by solving a system of linear equations. This reconstruction is based on a system function that either has to be measured or can be calculated with given information about scanner topology and particle characteristics. This paper introduces a new approach combining both possibilities. A Magnetic Particle Spectrometer is used to obtain a hybrid system function. Furthermore, we will show that the hybrid system function can be successfully used for 1D-image reconstruction and potentially is an alternative to the measurement-based system function.

Comparison of Open Scanner Designs for Interventional Magnetic Particle Imaging.

Magnetic Particle Imaging (MPI) allows for the quantitative determination of superparamagnetic iron oxide nanoparticles in vivo and offers exciting possibilities in functional, molecular and interventional imaging. In a simulation study, different open MPI scanner designs providing a varying extent of patient access (ranging from lateral to unrestricted) were assessed regarding their applicability to interventional MPI procedures. Criteria included power loss, size and shape of the field free point trajectory as well as spatial resolution. Each scanner design yields advantages and drawbacks, thus the decision for a certain scanner geometry depends on the individual circumstances and intended applications.

Visualization of Instruments in interventional Magnetic Particle Imaging (iMPI): A Simulation Study on SPIO Labelings

Due to its ability for quantitative 3D real time imaging with high sensitivity and spatial resolution but without ionizing radiation and iodine-based contrast agents, Magnetic Particle Imaging shows great promise for the application to the image guidance of cardiovascular interventions. For this purpose, the blood in the vessels and the instruments would have to be visualized, e.g. using a SPIO-based contrast agent and a SPIO labeling, respectively (SPIO: superparamagnetic iron oxide). In a simulation study of this situation, simple models of a guide wire and a catheter with a coated tip as well as a filled balloon catheter have been examined under a variety of conditions. The appearance of the instruments in the reconstructed images has been shown to be strongly dependent on the imaging parameters (gradient strength), the difference of the SPIO concentrations in adjacent structures as well as the geometric extensions of the instrument and its position inside the vessel (partial volume effect). It has been demonstrated that the visualization of instruments in a vessel may be possible with positive or negative contrast, depending on the individual circumstances.

Toward the Optimization of D-Shaped Coils for the Use in an Open Magnetic Particle Imaging Scanner

Magnetic particle imaging (MPI) is a novel medical imaging modality that allows for the quantitative detection of superparamagnetic iron oxide nanoparticles using static and oscillating magnetic fields. Essential aspects in the coil optimization for MPI include the magnetic field generated per unit current and the magnetic field homogeneity. For a set of D-shaped coils, which can be used in an open MPI scanner with lateral patient access to enable multidimensional imaging, these quantities were analyzed for a range of configurations. The results were compared with the situation in a four-wire-model and a four-wire-model with return paths (called eight-wire-model). It was found that for large coil radii, the effects of the D-coil set resemble those of the eight-wire-model; however, the local minimum in the magnetic field inhomogeneity is less pronounced.

Super-resolution approaches for resolution enhancement in magnetic particle imaging

Given a definition of spatial resolution that considers two objects as distinguished if the minimum value of the gap is less than 50% of the value at the object position [7], both approaches achieved an improvement of the spatial resolution in the 1D simulation study as visualized in Fig. 1 and 2. If the spatial resolution is considered as the minimum width of two distinguished lines, the first approach using spatially shifted images achieved a spatial resolution of 1.7 mm and the second approach using different sampling points achieved a resolution of 2.6 mm. This is an improvement in comparison to the used low resolution images with a spatial resolution of 2 mm (first approach) and 2.9 mm (second approach).

Concept of a rabbit-sized FFL-scanner

In the past years, different Magnetic Particle Imaging (MPI) scanners for small animals using a field free line (FFL) have been presented. In this work, a novel concept of a pre-clinical FFL MPI scanner which can accommodate rabbits is presented.