Hanne Wojtczyk

Magnetic Particle Imaging: Visualization of Instruments for Cardiovascular Intervention

PURPOSE To evaluate the feasibility of different approaches of instrument visualization for cardiovascular interventions guided by using magnetic particle imaging (MPI). MATERIALS AND METHODS Two balloon (percutaneous transluminal angioplasty) catheters were used. The balloon was filled either with diluted superparamagnetic iron oxide (SPIO) ferucarbotran (25 mmol of iron per liter) or with sodium chloride. Both catheters were inserted into a vessel phantom that was filled oppositional to the balloon content with sodium chloride or diluted SPIO (25 mmol of iron per liter). In addition, the administration of a 1.4-mL bolus of pure SPIO (500 mmol of iron per liter) followed by 5 mL of sodium chloride through a SPIO-labeled balloon catheter into the sodium chloride-filled vessel phantom was recorded. Images were recorded by using a preclinical MPI demonstrator. All images were acquired by using a field of view of 3.6 × 3.6 × 2.0 cm. RESULTS By using MPI, both balloon catheters could be visualized with high temporal (21.54 msec per image) and sufficient spatial (≤ 3 mm) resolution without any motion artifacts. The movement through the field of view, the inflation and deflation of the balloon, and the application of the SPIO bolus were visualized at a rate of 46 three-dimensional data sets per second. CONCLUSION Visualization of SPIO-labeled instruments for cardiovascular intervention at high temporal resolution as well as monitoring the application of a SPIO-based tracer by using labeled instruments is feasible. Further work is necessary to evaluate different labeling approaches for diagnostic catheters and guidewires and to demonstrate their navigation in the vascular system after administration of contrast material. SUPPLEMENTAL MATERIAL http://radiology.rsna.org/lookup/suppl/doi:10.1148/radiol.12120424/-/DC1.

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.

Toward cardiovascular interventions guided by magnetic particle imaging: First instrument characterization

Magnetic particle imaging has emerged as a new technique for the visualization and quantification of superparamagnetic iron oxide nanoparticles. It seems to be a very promising application for cardiovascular interventional radiology. A prerequisite for interventions is the artifact‐free visualization of the required instruments and implants. Various commercially available catheters, guide wires, and a catheter experimentally coated with superparamagnetic iron oxide nanoparticles were tested regarding their signal characteristics using magnetic particle spectroscopy to evaluate their performance in magnetic particle imaging. The results indicate that signal‐generating and non‐signal‐generating instruments can be distinguished. Furthermore, coating or loading non‐signal‐generating instruments with superparamagnetic iron oxide nanoparticles seems to be a promising approach, but optimized nanoparticles need yet to be developed. Magn Reson Med, 2013.

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.

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.

Fundamentals and Potential of Magnetic Particle Imaging

Cardiovascular interventions are standard treatment for numerous cardiovascular conditions and require high fidelity imaging tools to accurately visualize both vessels and interventional devices. Currently, digital subtraction angiography (DSA) is the standard method for peripheral arterial angiography. Magnetic particle imaging (MPI) is a new imaging modality, free of ionizing radiation, that utilizes static and oscillating magnetic fields to provide high temporal resolution, sub-millimeter spatial resolution images and high sensitivity. Superparamagnetic iron oxide nanoparticles (SPIOs) are used as tracers in MPI and signals are based on non-linear magnetization characteristics of those SPIOs. Regarding the magnetic moment of used tracers in MPI imaging is much faster in MPI, compared to imaging in CT and MRI. This makes MPI also very attractive for cardiovascular imaging and cardiovascular interventions. First in vivo visualization of a beating mouse heart demonstrated the feasibility of the visualization of the cardiovascular system by MPI. Different scanner designs and acquisition methods have already emerged addressing the requirements of cardiovascular interventions. Early studies have demonstrated MPI as an interesting and promising cardiovascular imaging modality. Technical improvement in hardware MPI imaging systems are currently being addressed in ongoing research which will facilitate former image acquisition with higher resolution in larger animals and/or human.

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.