Optimization of Design Procedures for Delta Relaxation Enhanced Magnetic Resonance

Delta relaxation enhanced magnetic resonance (dreMR) is a magnetic resonance imaging (MRI) method that produces contrast based on longitudinal relaxation dispersion. Through modulation of the magnetic field using an actively-shielded, field-cycling insert coil, this technique increases probe specificity and suppresses remaining signal. However, significant improvements are needed. This thesis addresses two advancements in dreMR with a focus on optimizing design procedures. A general procedure was developed to design split power solenoid magnets. The procedure was then applied to the design of a switched-field exposure system. A coil was constructed and the method was validated. This procedure can be used for to optimize dreMR coil primary windings. Next, a simulation tool was developed to model tissue magnetization as a function of time and magnetic field. Polarization sequences were discovered that maximize dispersion-based contrast. These optimized design procedures may add to future developments in dreMR technology

The feasibility of Quadrupole Dip Imaging with PMRI: focus on multiple sclerosis

Due to the character of the original source materials and the nature of batch digitization, quality control issues may be present in this document. Please report any quality issues you encounter to [email protected], referencing the URI of the item.Includes bibliographical references (leaves 80-97).Magnetic Resonance (MR) techniques provide valuable information for the diagnosis, monitoring, treatment, and study of many diseases. However, limitations on the sensitivity and specificity warrant the development of new imaging techniques. Quadrupole Dip Imaging (QDI) is a novel MR technique based on the magnitude of the quadrupole dip in the T₁ dispersion profile of substances containing rotationally immobilized proteins. The implementation of QDI requires field-cycled (FC) relaxometry. Prepolarized NM (PNW could potentially provide a low-cost way to conduct FC experiments and thus implement QDI. I have conducted a literature review and analysis to predict the value of using QDI to study Multiple Sclerosis (MS), to determine the feasibility of implementing QDI with PMRI, and to identify obstacles to successful penetration of the technology to the clinical environment. QDI could potentially be used to non-invasively create protein density maps in vivo, which could provide clinically valuable information on the histopathological substrate of MS that is not available through present imaging techniques. It appears that this information will be most valuable for studies of the development and nature of the diseases instead of for diagnosis and disease monitoring. Factors that will affect the development and dissemination of QDI with PNM include the development of PMRI T₁-measuring pulse sequences that are robust to inhomogeneity and field ramping, the inherently small signal and dynamic range of QDI, and MR hardware acquisition trends towards high-field devices. QDI with PMRI will probably maintain or exceed conventional MRI safety, patient tolerance, and cost. I have also conducted experiments that demonstrate that PNM can, in fact, be used to create dispersion profiles. Using the home-made PNM scanner at the Magnetic Resonance Systems Laboratory at Texas A&M I have verified the linearity of SNR with increasing prepolarizing field strength and demonstrated qualitatively the feasibility of T₁ measurement at different field strengths for CuSO₄ (aq) and Bovine Serum Albumin/gluteraldehyde phantoms

Scalable strategies for tumour targeting of magnetic carriers and seeds

With the evolving landscape of medical oncology, focus has shifted away from nonspecific cytotoxic treatment strategies toward therapeutic paradigms more characteristic of targeted therapies. These therapies rely on delivery vehicles such as nano-carriers or micro robotic devices to boosts the concentration of therapeutics in a specific targeted site inside the body. The use of externally applied magnetic field is suggested to be a predominant approach for remote localisation of magnetically responsive carriers and devices to the target region that could not be otherwise reached. However, the fast decline of the magnetic fields and gradients with increasing distances from the source is posing a major challenge for its clinical application. The aim of this thesis was to investigate potential magnetic delivery strategies which can circumvent some of the typical limitations of this technique. Two different approaches were explored to this end. The first approach was to characterise the ability of a conventional permanent magnet on targeting individual nano-carriers and develop novel magnetic designs which improve the targeting efficiency. The second approach was evaluating the feasibility of a magnetic resonance imaging system to move a millimetre-sized magnetic particle within the body. Phantom and in vivo magnetic targeting experiments illustrated the significant increase in effective targeting depth when our novel magnetic design was used for targeting nano-carriers compared with conventional magnets. In the later part of the thesis, the proof of concept and characterisation experiments showed that a 3 mm magnetic particle can be moved in ex vivo brain tissue using a magnetic resonance imaging system using clinically relevant gradient strengths. The magnetic systems introduced in this thesis provide the potential to target nano-carriers and millimetre-sized thermoseeds to tumours located at deep regions of human body through vasculature and soft tissue respectively

High-fidelity, near-field microwave gates in a cryogenic surface trap

We present a novel dynamical decoupling strategy for near field microwave gradient driven, Mølmer-Sørensen style, two-ion quantum logic gates, which suppresses errors from both fluctuations in the qubit frequency and imperfection in the decoupling drive itself. Using a microwave-integrated surface-trap which is operated cryogenically at 25 K and a magnetically insensitive 43-Ca+ qubit at 288 G, we demonstrate a 331 us two-ion quantum logic gates, with 4.9(11)e-3 logic error probability. This is below the 1% error threshold required for quantum error correction and represents a ~10x gate time reduction when compared to previously demonstrated near field gradient driven microwave gates below the 1% error probability threshold. Additionally, two faster gates were demonstrated without the use of dynamical decoupling. Respectively, these two gates had gate operation durations of 216.8 us & 153.8 us and measured gate error probabilities of 8.5(20)e-3 & 9.8(21)e-3. Further, we develop a method for rapid calculation of ion transport operations. We successfully demonstrate ion transport as well as crystal splitting and merging operations within two different ion traps using the waveforms calculated by this ion transport toolbox

Magnetic and Plasmonic Contrast Agents in Optical Coherence Tomography

Optical coherence tomography (OCT) has gained widespread application for many biomedical applications, yet the traditional array of contrast agents used in incoherent imaging modalities do not provide contrast in OCT. Owing to the high biocompatibility of iron oxides and noble metals, magnetic and plasmonic nanoparticles, respectively, have been developed as OCT contrast agents to enable a range of biological and pre-clinical studies. Here we provide a review of these developments within the past decade, including an overview of the physical contrast mechanisms and classes of OCT system hardware addons needed for magnetic and plasmonic nanoparticle contrast. A comparison of the wide variety of nanoparticle systems is also presented, where the figures of merit depend strongly upon the choice of biological application

Study of Magnetization Switching in Coupled Magnetic Nanostructured Systems

A study of magnetization dynamics experiments in nanostructured materials using the rf susceptibility tunnel diode oscillator (TDO) method is presented along with a extensive theoretical analysis. An original, computer controlled experimental setup that measures the change in susceptibility with the variation in external magnetic field and sample temperature was constructed. The TDO-based experiment design and construction is explained in detail, showing all the elements of originality. This experimental technique has proven reliable for characterizing samples with uncoupled magnetic structure and various magnetic anisotropies like: CrO2 , FeCo/IrMn and Co/SiO2 thin films. The TDO was subsequently used to explore the magnetization switching in coupled magnetic systems, like synthetic antiferromagnet (SAF) structures. Magnetoresistive random access memory (MRAM) is an important example of devices where the use of SAF structure is essential. To support the understanding of the SAF magnetic behavior, its configuration and application are reviewed and more details are provided in an appendix. Current problems in increasing the scalability and decreasing the error rate of MRAM devices are closely connected to the switching properties of the SAF structures. Several theoretical studies that were devoted to the understanding of the concepts of SAF critical curve are reviewed. As one can notice, there was no experimental determination of SAF critical curve, due to the difficulties in characterizing a magnetic coupled structure. Depending of the coupling strength between the two ferromagnetic layers, on the SAF critical curve one distinguishes several new features, inexistent in the case of uncoupled systems. Knowing the configuration of the SAF critical curve is of great importance in order to control its switching characteristics. For the first time a method of experimentally recording the critical curve for SAF is proposed in this work. In order to overcome technological limitations, a new way of recording the critical curve by using an additional magnetic bias field was explored

Morphology-properties studies in laser synthesized nanostructured materials

Synthesis of well-defined nanostructures by pulsed laser melting is an interesting subject from both a funda- mental and technological point of view. In this thesis, the synthesis and functional properties of potentially useful materials were studied, such as tin dioxide nanostructured arrays, which have potential applications in hydrogen gas sensing, and ferromagnetic Co nanowire and nanomagnets, which are fundamentally im- portant towards understanding magnetism in the nanoscale. First, the formation of 1D periodic tin dioxide nanoarrays was investigated with the goal of forming nanowires for hydrogen sensing. Experimental obser- vations combined with theoretical modeling successfully explained the mechanisms of structure formation. One of the primary findings was that evaporation of tin dioxide was the most significant contributor to the pattern formation. Next, the spontaneous liquid film spinodal dewetting process under pulsed laser melting was modeled using the viscous dissipation approach. We found that the fluid condition for spinodal dewet- ting is where the film-substrate tangential stress is zero. Following this, the remainder of the thesis focused on synthesis and characterization of magnetic nanostructures. We first successfully installed a home-built Surface Magneto-optical Kerr Effect (SMOKE) system. Using SMOKE we measured the Kerr rotation from potential plasmonic-ferromagnetic magneto-optical materials made from Co-Ag thin films and nanoparti- cles as a function of composition. We found that films made by co-deposition of Co and Ag showed higher Kerr rotation in contrast to bilayer film structures with same effective amount of Co and Ag. Next, we inves- tigated the shape and size dependence of magnetic properties of nanostructures, specifically hemispherical nanoparticles, nanowires and nanorods, prepared by the pulsed laser process. The magnetic anisotropy was studied by using the SMOKE system complemented with magnetic force microscopy (MFM) analy- sis. Results from magnetic hysteresis measurements of the nanostructures in different geometries showed coercivity and remanence that could be attributed to magnetic shape anisotropy. MFM analysis showed that domain orientation was found to depend on the aspect ratio of the nanostructure. These investigations generally helped advance the science of nanostructure synthesis using nanosecond pulsed laser techniques as well demonstrate that SMOKE is a promising method to investigate nanostructure magnetism

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