Investigation, design, and integration of insert gradient coils in magnetic resonance imaging

Diffusion-weighted magnetic resonance imaging utilizes the magnetic gradients of the system to de-phase protons undergoing diffusion with respect to the overall mag­ netization. Areas of the image with reduced signal when compared to an un-weighted image represent where protons have undergone diffusion. The stronger the gradient applied during diffusion-weighting, the larger the signal loss due to diffusion, and the larger the b-value differentiating the diffusion coefficients. However, the maximum gradient strength during image acquisition is limited by both the original strength of the signal and peripheral nerve stimulation. Nerve stimulation is induced because the changing magnetic fields of the gradient pulse sequence induce electric fields that cause stimulation. The stimulation thresh­ old can be measured either in terms of the pulse sequence parameters of maximum gradient strength and slew rate, or in terms of the induced electric field and the duration of the electric field pulse. A finite-difference simulation was used to approximate the electric field induced inside a visible man model. The effect of varying the size, resolution, and position of the model inside the simulation was investigated with the wire pattern from a customized head/neck gradient coil. For accurate simulations, it was most important to ensure that the resolution of the model was sufficient to capture the air cavities of the sinus and trachea. m The peripheral nerve stimulation thresholds of a planar gradient coil were deter­ mined from human experiments. While the electrical stimulation threshold parame­ ters did not vary significantly from previous studies, the minimum gradient change and slew rate required to cause stimulation were significantly higher for the planar gradient than for reported thresholds of cylindrically designed gradient systems. Several non-cylindrical localized gradient designs were investigated for diffusion- weighted contrast as a fourth gradient, in addition to the three imaging axes. Both resistive and inductive merits were investigated. Of these, inductive values proved to be the limiting factor when designing coils sized to perform in a full body MRI system. Optimal merit and gradient strength were obtained from a butterfly design, and planar coils provided localized strength over a larger region. A butterfly coil was constructed with hollow copper wiring and powered to produce diffusion weighting during MRI. Diffusion contrast b—1300 s/mm2was obtained using the insert with significant time and signal to noise ratio improvement

System Characterization of a Human-Sized 3D Real-Time Magnetic Particle Imaging Scanner for Cerebral Applications

Since the initial patent in 2001, the Magnetic Particle Imaging (MPI) community has been striving to develop an MPI scanner suitable for human applications. Numerous contributions from different research fields, regarding tracer development, reconstruction methods, hardware engineering, and sequence design have been employed in pursuit of this objective. In this work, we introduce and thoroughly characterize an improved head-sized MPI scanner with an emphasis on human safety. The scanner is operated by open-source software that enables scanning, monitoring, analysis, and reconstruction, designed to be handled by end users. Our primary focus is to present all technical components of the scanner, with the ultimate objective to investigate brain perfusion imaging in phantom experiments. We have successfully achieved full 3D single- and multi-contrast imaging capabilities at a frame rate of 4 Hz with sufficient sensitivity and resolution for brain applications. To assess system characterization, we devised sensitivity, resolution, perfusion, and multi-contrast experiments, as well as field measurements and sequence analysis. The acquired images were captured using a clinically approved tracer and suitable magnetic field strengths, while adhering to the established human peripheral nerve stimulation thresholds. This advanced scanner holds potential as a tomographic imager for diagnosing conditions such as ischemic stroke or intracranial hemorrhage in environments lacking electromagnetic shielding. Furthermore, due to its low power consumption it may have the potential to facilitate long-term monitoring within intensive care units for various applications.Comment: 22 pages, 9 figure

Magnetic Resonance Systems Development for Point-of-Care MRI Platforms

Magnetic resonance imaging utilizes electromagnets to produce anatomical images in both clinical and research settings. In the race towards increasing performance head-optimized scanners have begun playing a significant role in providing high quality imaging of the head. However, they are implemented using smaller geometries and as such fail to allow entrance of the patient past their shoulders. This is overcome by designing asymmetric gradient coils which have their imaging region located towards one end of the gradient coil, as opposed to the geometric center, allowing brain imaging. There exists interest in compact configurations which allow imaging further into the cervical spine which is unfeasible using current asymmetric gradients. This work seeks to explore the design of asymmetric gradient coils with shoulder cut-outs to enable neck imaging by allowing the patient to enter further into the gradient coil while maintaining the small inner radius of a head-only platform. First, the relative trade-offs in designing an asymmetric shoulder cut-out gradient coil are explored and extended by rotating the transverse gradient axes to produce gradient coils which compensate for some of the electromagnetic burden due to the loss of conducting surfaces on the sides. Next, a complementary set of spherical harmonic active shims are designed and explored for implementation within this gradient coil configuration. From there the design of a cylindrical radiofrequency coil using gradient design techniques is investigated as preliminary work towards implementing these low-frequency design techniques which have had success designing gradient coils towards the design of radiofrequency coils. Finally, motivated by the complexity of the induced eddy currents in the surrounding conductive structures due to asymmetric gradient coils the final project explores the design of a multi-coil matrix array aimed at fitting within the compact gradient housing to dynamically compensate eddy currents during imaging. This work ultimately demonstrates the feasibility of implementing an asymmetric shoulder cut-out gradient coil with rotated transverse gradient axes to enable neck imaging in a compact MRI scanner while providing potential solutions to handle the increased eddy current complexity associated with a setup such as this

Optimization of a boundary element approach to electromagnet design with application to a host of current problems in Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) has proven to be a valuable methodological approach in both basic research and clinical practice. However, significant hardware advances are still needed in order to further improve and extend the applications of the technique. The present dissertation predominantly addresses gradient and shim coil design (sub-systems of the MR system). A design study to investigate gradient performance over a set of surface geometries ranging in curvature from planar to a full cylinder using the boundary element (BE) method is presented. The results of this study serve as a guide for future planar and pseudo-planar gradient systems for a range of applications. Additions to the BE method of coil design are developed, including the direct control of the magnetic field uniformity produced by the final electromagnet and the minimum separation between adjacent wires in the final design. A method to simulate induced eddy currents on thin conducting surfaces is presented. The method is used to predict the time-dependent decay of eddy currents induced on a cylindrical copper bore within a 7 T MR system and the induced heating on small conducting structures; both predictions are compared against experiment. Next, the method is extended to predict localized power deposition and the spatial distribution of force due to the Lorentz interaction of the eddy current distribution with the main magnetic field. New methods for the design of actively shielded electromagnets are presented and compared with existing techniques for the case of a whole-body transverse gradient coil. The methods are judged using a variety of shielding performance parameters. A novel approach to eliminate the interactions between the MR gradient system and external, non-MR specific, active devices is presented and its feasibility is discussed. A completely new approach to shimming is presented utilizing a network of current pathways that can be adaptively changed on a subject-by-subject basis and dynamically controlled. The potential benefits of the approach are demonstrated using computer simulations and a prototype coil is constructed and tested as a proof-of-principle

Magnetic Particle Imaging - Anwendungen von magnetischen Nanopartikeln in Analytik und Bildgebung

Magnetic Particle Imaging (MPI) is a new imaging modality that delivers tracer-based volume images with high spatial and temporal resolution. The properties of the nanoparticular tracer, that needs to be present in the imaging volume for MPI to render image contrast, have direct impact on the MPI performance. The magnetization dynamics of the superparamagnetic nanoparticles are a critical factor in MPI system design. However, once understood and numerically modelled the particle's magnetization dynamics are key to enabling functional imaging with MPI based on potential particle functionalization. This thesis describes the development of a magnetic particle imaging scanner and its accompanying particle characterization technique, magnetic particle spectroscopy (MPS). The devices have been designed, built and tested to deliver insights into particle dynamics and to function as a prototype platform for MPI research. That includes the scanner hardware as well as the software for modelling the particle's magnetization response and image reconstruction. The main focus is on the development and evolution of the so called 'Mobility MPI' (mMPI) which promises to provide an estimate of the particle mobility, including the hydrodynamic diameter of the particles and the viscosity of the surrounding medium, in additional to the standard concentration-weighted MPI image. By allowing a discrimination between Néel and Brownian contributions, mMPI in conjunction with a suitable tracer enables binding detection in the imaging volume. The harmonic spectrum connected with the dynamic magnetization response of the tracer is studied in MPS. The ability for conducting bio-assays with MPS is explored and the results are evaluated in context of appropriate numerical models. Furthermore, the effect of viscosity on the MPI system matrix is studied and different approaches for deducing mobility information from an MPI experiment are investigated.Magnetic Particle Imaging (MPI) ist eine neue Bildgebungsmodalität, die Volumenbilder mit hoher räumlicher und zeitlicher Auflösung liefert. Die Eigenschaften des nanopartikulären Markers, der im Bildgebungsvolumen anwesend einen Bildkontrast generiert, haben dabei direkten Einfluss auf die MPI-Performance. Die Magnetisierungsdynamik der superparamagnetischen Nanopartikel ist auch ein entscheidender Faktor im MPI Systemdesign. Ein eingehendes Verständnis und die numerische Modellierung der Partikel-Magnetisierungsdynamik kann dabei ein Schlüssel zur Realisierung von funktionaler Bildgebung im MPI sein, die auf einer möglichen Funktionalisierung der Partikel beruht. Diese Arbeit beschreibt die Entwicklung eines Magnetic Particle Imaging Scanners und der dazugehörigen Charakterisierungstechnik, der Magnetic Particle Spektroscopy (MPS). Die Geräte wurden dabei entwickelt, gebaut und getestet, um Einblicke in die Partikeldynamik zu geben und um als Prototyp-Plattform für die MPI-Forschung zu dienen. Das schließt sowohl die Scanner-Hardware als auch die Software zur Modellierung der dynamischen Partikelantwort und zur Bildrekonstruktion ein. Der Fokus liegt hierbei auf der Entwicklung des sogenannten 'Mobility MPI' (mMPI), welches eine Bestimmung der Partikelbeweglichkeit zusätzlich zur konventionellen konzentrations-gewichteten MPI-Bildgebung ermöglicht. Die Partikelbeweglichkeit umfasst dabei den hydrodynamischen Durchmesser der Partikel und die Viskosität des sie umgebenden Mediums. Durch die Unterscheidung von Néel'schen und Brown'schen Beiträgen zur Magnetisierung ermöglicht mMPI in Verbindung mit einem geeigneten Marker die Bindungsdetektion im Bildgebungsvolumen. Das Harmonischen-Spektrum und die dynamische Magnetisierungsantwort des MPI-Markers werden im MPS untersucht. Außerdem wird die Durchführung von Bio-Assays auf der Basis von MPS erkundet, und die Ergebnisse werden mit entsprechenden numerischen Modellen verglichen. Darüber hinaus wird der Einfluss der Viskosität auf die MPI System-Matrix analysiert und verschiedene Ansätze zur Ableitung der Mobilitätsinformation der Partikel aus den MPI Messdaten untersucht

Doctor of Philosophy

dissertationIn Chapter 1, an introduction to basic principles or MRI is given, including the physical principles, basic pulse sequences, and basic hardware. Following the introduction, five different published and yet unpublished papers for improving the utility of MRI are shown. Chapter 2 discusses a small rodent imaging system that was developed for a clinical 3 T MRI scanner. The system integrated specialized radiofrequency (RF) coils with an insertable gradient, enabling 100 'm isotropic resolution imaging of the guinea pig cochlea in vivo, doubling the body gradient strength, slew rate, and contrast-to-noise ratio, and resulting in twice the signal-to-noise (SNR) when compared to the smallest conforming birdcage. Chapter 3 discusses a system using BOLD MRI to measure T2* and invasive fiberoptic probes to measure renal oxygenation (pO2). The significance of this experiment is that it demonstrated previously unknown physiological effects on pO2, such as breath-holds that had an immediate (<1 sec) pO2 decrease (~6 mmHg), and bladder pressure that had pO2 increases (~6 mmHg). Chapter 4 determined the correlation between indicators of renal health and renal fat content. The R2 correlation between renal fat content and eGFR, serum cystatin C, urine protein, and BMI was less than 0.03, with a sample size of ~100 subjects, suggesting that renal fat content will not be a useful indicator of renal health. Chapter 5 is a hardware and pulse sequence technique for acquiring multinuclear 1H and 23Na data within the same pulse sequence. Our system demonstrated a very simple, inexpensive solution to SMI and acquired both nuclei on two 23Na channels using external modifications, and is the first demonstration of radially acquired SMI. Chapter 6 discusses a composite sodium and proton breast array that demonstrated a 2-5x improvement in sodium SNR and similar proton SNR when compared to a large coil with a linear sodium and linear proton channel. This coil is unique in that sodium receive loops are typically built with at least twice the diameter so that they do not have similar SNR increases. The final chapter summarizes the previous chapters

Zweifrequenz Magnetic-Particle-Imaging-Scanner - Hardware für mobilityMPI

Magnetic Particle Imaging (MPI) ist eine neue tomographische Bildgebungsmethode, welche magnetische Nanopartikel als Marker nutzt und potentielle Anwendungsmöglichkeiten in der Medizin und medizinischen Forschung bietet. Durch seine Auflösung im mm-Bereich und seine hohe zeitliche Auflösung ist es für viele Anwendungen geeignet. Da MPI ohne ionisierende Strahlung arbeitet, bietet eine Substitution von radiologischen Untersuchungen Vorteile für Patient und medizinisches Personal. Eine Herausforderung ist die Entwicklung von Anwendungsmöglichkeiten, die über die Fähigkeiten der etablierten Methoden hinausgehen. Ein möglicher Vorteil von MPI ist die Nutzung von Nanopartikeln, welche z.B. durch Funktionalisierung der Oberfläche kontrolliert mit ihrer Umgebung interagieren können. Durch das Ausnutzen der Partikeldynamik können diese Interaktionen magnetisch mit MPI gemessen und eine räumliche Darstellung des Partikelzustands gewonnen werden. Die vorliegende Arbeit beschreibt die Entwicklung und Anwendung eines MPI-Scanners, welcher speziell für die Demonstration und Erforschung der funktionalen Bildgebungsmöglichkeiten mit MPI unter Ausnutzung der Mobilitätsinformation der Partikel entworfen wurde. Dieses Konzept, welches als mobility-MPI (mMPI) bezeichnet wird, kann durch verschiedene Verfahren unter Ausnutzung der Brownschen Relaxation realisiert werden. Ein Verfahren ist die Verwendung zweier Anregungsfrequenzen, was die Separation der Partikeldynamik von der räumlichen Verteilung ermöglicht. Da aktuelle MPI Scanner Sende- und Empfangsschaltungen besitzen, welche an die Anregungsfrequenz angepasst sind, erfordert das Mehrfrequenz-mMPI Hardwareerweiterungen gegenüber konventionellen Scannern. Die vorliegende Arbeit beschreibt detailliert das Design und die Konstruktion eines Magnetic Particle Spectrometers und eines mMPI-Systems und ihrer Komponenten. Mehrere Entwurfsverfahren, wie z.B. eine Methode zur präzisen Vorhersage der parasitären Effekte bei Spulen, wurden im Zuge des Design-Prozesses entwickelt. Erwähnenswert ist ebenfalls das Design der Sende- und Empfangsfilter, welche hohe Dämpfungswerte im Sperrbereich bei guter Linearität liefern. Die Entwicklung einer volldifferentiellen Empfangsspule ist ein weiterer interessanter Aspekt. Der neue Scanner wurde ausgehend von konventionellen ein- und zweidimensionalen MPI-Bildern genutzt, um die Eignung von MPI für die räumlich aufgelöste funktionale Bildgebung mittels Partikeldynamik zu zeigen.Magnetic Particle Imaging (MPI), being a new tomographic imaging modality based on magnetic nanoparticle tracers, offers potential applications in medicine and medical research. Possessing spatial resolution in the millimeter range and high temporal resolution, it is well suited for many applications. Since MPI doesn't rely on ionizing radiation, substitution of radiological methods by MPI benefits the safety of patients and medical staff. One of the challenges MPI faces is to provide capabilities that surpass what is currently possible with tracer based imaging systems. A possible advantage of MPI is the presence of nanoparticle tracers that can be modified to interact in well defined ways with their environment through surface functionalization. By exploiting particle dynamics, these interactions can be measured magnetically and can provide a spatially resolved map of particles states, enabling quantitative, functional imaging. This work describes the development and application of a MPI scanner designed specifically to demonstrate and further research the functional imaging capability of MPI through the measurement of particle mobility. This concept, called moblity MPI (mMPI), can be realized based on the Brownian relaxation mechanism. One possible technique involves the use of multiple excitation frequencies, that can be used to separate information on the spatial distribution from particle dynamics. Since the particle relaxation is inherently dependent on the excitation frequency, acquiring the same image at several drive frequencies provides the additional data required for functionally and spatially resolved MPI. Since current MPI scanners rely on transmit and receive chains tuned to the excitation frequency, the scanner hardware needs to be adapted. This thesis describes in detail the design and construction of a MPS and a mMPI system and their components. Several design techniques, such as a method for the accurate prediction of coil parasitics, have been developed for this task. Other noteworthy aspects are the design of transmit and receive filters, offering high stop band attenuation and good linearity. The development of a fully differential receive coil completes the effort to provide a system suitable for the demonstration of mMPI. After acquiring conventional one- and two-dimensional MPI images, the new scanner was used to demonstrate the capability of MPI to provide spatially resolved information on particle mobility