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Visualization of Instruments in interventional Magnetic Particle Imaging (iMPI): A Simulation Study on SPIO Labelings
Abstract
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.
... According to the Langevin theory [22], the particle size detectable by the MPI system is limited by the constant gradient magnetic field. Using a permanent magnet will keep the MPI system working and the system safety will be reduced, while the closed test environment formed by the solenoid structure used in MPI is an unfavorable factor for patients with claustrophobia [23][24][25][26][27][28]. In this paper, the permanent magnet was replaced by electromagnetic coil structure to form a gradient coil capable of generating an FFP. ...
... Then, atherosclerosis tracer and tracer-labeled RBCs are imaged for atherosclerotic plaque assessment and long-term monitoring several hours or days after injection. Besides diagnostic purposes, MPI-guided intervention is feasible, as well, and instruments for visualizing SPIO-labeled intervention in MPI are under investigation [100][101][102]. In contrast to digital subtraction angiography (DSA), which yields 2D projections, MPI is free of ionizing radiation and could produce 3D continuous imaging of vessel trees by using long-lasting tracer labeled RBCs for more precise intervention guidance. ...
Magnetic particle imaging (MPI) is a promising medical imaging technique producing quantitative images of the distribution of tracer materials (superparamagnetic nanoparticles) without interference from the anatomical background of the imaging objects (either phantoms or lab animals). Theoretically, the MPI platform can image with relatively high temporal and spatial resolution and sensitivity. In practice, the quality of the MPI images hinges on both the applied magnetic field and the properties of the tracer nanoparticles. Langevin theory can model the performance of superparamagnetic nanoparticles and predict the crucial influence of nanoparticle core size on the MPI signal. In addition, the core size distribution, anisotropy of the magnetic core and surface modification of the superparamagnetic nanoparticles also determine the spatial resolution and sensitivity of the MPI images. As a result, through rational design of superparamagnetic nanoparticles, the performance of MPI could be effectively optimized. In this review, the performance of superparamagnetic nanoparticles in MPI is investigated. Rational synthesis and modification of superparamagnetic nanoparticles are discussed and summarized. The potential medical application areas for MPI, including cardiovascular system, oncology, stem cell tracking and immune related imaging are also analyzed and forecasted.
... In principle, there are two different ways of visualizing conventional instruments: labeling with an SPIO-based enamel or loading a lumen of the instrument with SPIOs and, in terms of a negative or passive contrast, visualization of an unlabeled instrument in tracer-containing volumes (eg, in contrasted vessels) (6). ...
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.
First presented in 2005, magnetic particle imaging (MPI) is a new imaging modality able to acquire and reconstruct the distribution of a magnetic tracer included in a non-ferromagnetic volume. In 2009, Weizenecker et al. presented a video showing a tracer distribution flowing through the beating heart of a mouse. It was acquired in a 20.4x12.0x16.8 mm^3 volume at 46 frames per seconds. The spatial encoding was done using a field free point (FFP) moved along a Lissajous trajectory. Since then, many efforts have been done to improve the technology. To further increase the sensitivity of the acquisition process, it has been proposed to replace the FFP by a field free line (FFL). In 2014, Weber et al. presented the first continuously rotating FFL scanner, acquiring and displaying 50 circular images per seconds within a Field of View (FOV) with a diameter of 25 mm.
However, to image a tracer distribution in the human body, up-scaled version of those scanners have to be realised. In this thesis, the design of two FFL scanners having a bore diameter of 173 mm and 500 mm are investigated, to highlight and investigate some of the challenges awaiting the up-scaling process.
The first part of this work defines precisely the acquisition sequence and the imaging properties of the MPI systems.
It is shown that the coils designed in this work have magnetic-field topologies which cannot be considered as ideal.
This, in turn, has an impact on the reconstruction of the tracer distribution.
Using a model based approach which assumes perfectly straight lines, as the filtered back projection, leads to artefacts in the reconstructed images.
Using a system-function reconstruction approach with properly resolved spectra suppresses those artefacts, but the acquisition rate is divided by two.
To cover a larger FOV, a new FFL sequence using focus fields is introduced.
It requires the use of a system function reconstruction, as no model-based reconstruction are available for a generic sequence.
The first reconstruction shows artefacts, which have to be investigated in further studies.
The second part of this work is focused on safety limitations associated with MPI scanners. The main foreseen safety risk associated with an MPI scanner is the peripheral nerves stimulation (PNS). As no precise data exist on the PNS thresholds for the frequencies and magnetic field-strengths usable in MPI, an approximation is proposed to evaluate the PNS risk of MPI sequences. Applying this relation to a human-sized FFL scanner imaging an FOV of 400x405 mm^2 10 times per second, it is found that the patient will probably experience PNS. Further experimental validations have to be carried out to determine more precisely the PNS thresholds for such sequences. Afterwards, the sequences may be adapted to avoid PNS in the patient. A first example of such an adjustment is also presented.
The third part of this work is focused on the construction of the electromagnets used in an FFL scanner with a bore diameter of 173 mm. A boundary element method formulation is found to be adequate to optimise all the necessary coils. A scanner design with a gradient on the line of 0.8 T.m^(-1) and two 15 mT drive fields has been implemented. The fields strengths are mainly limited by the cooling infrastructure available and the technical complexity associated with all the parts of the scanner. The actual design requires around 40 kW of electrical power. The rotation frequency of the line is actually limited by the filters, power factor correction elements and the shielding of the different elements.
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.
Distinct magnetic nanoparticle designs can have unique spectral responses to an AC
magnetic field in a technique called the magnetic spectroscopy of Brownian motion (MSB).
The spectra of the particles have been measured using desktop spectrometers and in vivo
measurements. If multiple particle types are present in a region of interest, the unique
spectral signatures allow for the simultaneous quantification of the various particles. We
demonstrate such a potential experimentally with up to three particle types. This ability to
concurrently detect multiple particles will enable new biomedical applications.
Porcine models have become increasingly popular in cardiovascular research, because physiological and anatomical features of the cardiovascular system from pigs including the coronary vascular system and the coronary collateral vessels are comparable to humans. The standard farm pig rapidly increases in body weight and size, potentially confounding serial measurements of cardiac function and morphology. In contrast Göttingen® minipigs have a characteristic growth curve that avoids the dramatic increase in weight in adulthood seen in farm pigs. The Göttingen® minipig is especially suitable for long-term studies because of its inherent small size and ease of handling, even at full maturity, which is reached at 2 y of age compared with 3 y for domestic pigs. However, there is still a need on detailed information about the macro- and microvascular characteristics of the vascular system from the Göttingen® minipigs. The study was aimed to describe the macro- and microvascular characteristics of adult Göttingen® minipigs (n=18) by use of CT-imaging and histology over a time period of 4 months starting from 16 months of age up to 20 months. The animals showed no clinical symptoms of disease and were kept in-house at a light/dark rhythm of 12:12 under defined climatic conditions. The experiments were licensed by the regional authorities for health and social affairs (LaGeSo), Berlin. The study included the measurement of the length and of the luminal diameter of arteries and veins from the neck, thorax, abdomen, and limbs which are frequently used in experiments with pigs. In addition microscopical and histological parameters (luminal vessel diameter, thickness of the tunica externa, tunica media, and tunica interna) were studied on hematoxylin-eosin-stained sections of the blood vessel.
Magnetic particle imaging (MPI) is a new tomographic imaging technique capable of imaging magnetic tracer material at high temporal and spatial resolution. Image reconstruction requires solving a system of linear equations, which is characterized by a "system function" that establishes the relation between spatial tracer position and frequency response. This paper for the first time reports on the structure and properties of the MPI system function.
An analytical derivation of the 1D MPI system function exhibits its explicit dependence on encoding field parameters and tracer properties. Simulations are used to derive properties of the 2D and 3D system function.
It is found that for ideal tracer particles in a harmonic excitation field and constant selection field gradient, the 1D system function can be represented by Chebyshev polynomials of the second kind. Exact 1D image reconstruction can thus be performed using the Chebyshev transform. More realistic particle magnetization curves can be treated as a convolution of the derivative of the magnetization curve with the Chebyshev functions. For 2D and 3D imaging, it is found that Lissajous excitation trajectories lead to system functions that are closely related to tensor products of Chebyshev functions.
Since to date, the MPI system function has to be measured in time-consuming calibration scans, the additional information derived here can be used to reduce the amount of information to be acquired experimentally and can hence speed up system function acquisition. Furthermore, redundancies found in the system function can be removed to arrive at sparser representations that reduce memory load and allow faster image reconstruction.
Due to the possibility of high temporal and spatial resolution, high sensitivity and three-dimensional imaging, Magnetic Particle Imaging is a promising new imaging approach for interventional cardiovascular procedures. In this contribution we present first MPI-images of a specifically labeled, though commercially available device for cardiovascular intervention. Furthermore, different approaches to label those instruments are discussed.
The binding of nanoparticles to in vivo targets impacts their use for medical imaging, therapy, and the study of diseases and disease biomarkers. Though an array of techniques can detect binding in vitro, the search for a robust in vivo method continues. The spectral response of magnetic nanoparticles can be influenced by a variety of changes in their physical environment including viscosity and binding. Here, the authors show that nanoparticles in these different environmental states produce spectral responses, which are sufficiently unique to allow for simultaneous quantification of the proportion of nanoparticles within each state.
The authors measured the response to restricted Brownian motion using an array of magnetic nanoparticle designs. With a chosen optimal particle type, the authors prepared particle samples in three distinct environmental states. Various combinations of particles within these three states were measured concurrently and the authors attempted to solve for the quantity of particles within each physical state.
The authors found the spectral response of the nanoparticles to be sufficiently unique to allow for accurate quantification of up to three bound states with errors on the order of 1.5%. Furthermore, the authors discuss numerous paths for translating these measurements to in vivo applications.
Multiple nanoparticle environmental states can be concurrently quantified using the spectral response of the particles. Such an ability, if translated to the in vivo realm, could provide valuable information about the fate of nanoparticles in vivo or improve the efficacy of nanoparticle based treatments.
Magnetic particle imaging (MPI) is a new tomographic imaging method potentially capable of rapid 3D dynamic imaging of magnetic tracer materials. Until now, only dynamic 2D phantom experiments with high tracer concentrations have been demonstrated. In this letter, first in vivo 3D real-time MPI scans are presented revealing details of a beating mouse heart using a clinically approved concentration of a commercially available MRI contrast agent. A temporal resolution of 21.5 ms is achieved at a 3D field of view of 20.4 x 12 x 16.8 mm(3) with a spatial resolution sufficient to resolve all heart chambers. With these abilities, MPI has taken a huge step toward medical application.
The use of contrast agents and tracers in medical imaging has a long history. They provide important information for diagnosis and therapy, but for some desired applications, a higher resolution is required than can be obtained using the currently available medical imaging techniques. Consider, for example, the use of magnetic tracers in magnetic resonance imaging: detection thresholds for in vitro and in vivo imaging are such that the background signal from the host tissue is a crucial limiting factor. A sensitive method for detecting the magnetic particles directly is to measure their magnetic fields using relaxometry; but this approach has the drawback that the inverse problem (associated with transforming the data into a spatial image) is ill posed and therefore yields low spatial resolution. Here we present a method for obtaining a high-resolution image of such tracers that takes advantage of the nonlinear magnetization curve of small magnetic particles. Initial 'phantom' experiments are reported that demonstrate the feasibility of the imaging method. The resolution that we achieve is already well below 1 mm. We evaluate the prospects for further improvement, and show that the method has the potential to be developed into an imaging method characterized by both high spatial resolution as well as high sensitivity.