Pressure profiles of plasmas confined in the field of a dipole magnet

Understanding the maintenance and stability of plasma pressure confined by a strong magnetic field is a fundamental challenge in both laboratory and space plasma physics. Using magnetic and X-ray measurements on the Levitated Dipole Experiment (LDX), the equilibrium plasma pressure has been reconstructed, and variations of the plasma pressure for different plasma conditions have been examined. The relationship of these profiles to the magnetohydrodynamic (MHD) stability limit, and to the enhanced stability limit that results from a fraction of energetic trapped electrons, has been analyzed. In each case, the measured pressure profiles and the estimated fractional densities of energetic electrons were qualitatively consistent with expectations of plasma stability. LDX confines high temperature and high pressure plasma in the field of a superconducting dipole magnet. The strong dipole magnet can be either mechanically supported or magnetically levitated. When the dipole was mechanically supported, the plasma density profile was generally uniform while the plasma pressure was highly peaked. The uniform density was attributed to the thermal plasma being rapidly lost along the field to the mechanical supports. In contrast, the strongly peaked plasma pressure resulted from a fraction of energetic, mirror trapped electrons created by microwave heating at the electron cyclotron resonance (ECRH). These hot electrons are known to be gyrokinetically stabilized by the background plasma and can adopt pressure profiles steeper than the MHD limit. X-ray measurements indicated that this hot electron population could be described by an energy distribution in the range 50-100 keV. Combining information from the magnetic reconstruction of the pressure profile, multi-chord interferometer measurements of the electron density profile, and X-ray measurements of the hot electron energy distribution, the fraction of energetic electrons at the pressure peak was estimated to be about 35% of the total electron population. When the dipole was magnetically levitated the plasma density increased substantially because particle losses to the mechanical supports were eliminated so particles could only be lost via slower cross-field transport processes. The pressure profile was observed to be broader during levitated operation than it was during supported operation, and the pressure appeared to be contained in both a thermal population and an energetic electron population. X-ray spectra indicated that the X-rays came from a similar hot electron population during levitated and supported operation; however, the hot electron fraction was an order of magnitude smaller during levitated operation (<3% of the total electron population). Pressure gradients for both supported and levitated plasmas were compared to the MHD limit. Levitated plasmas had pressure profiles that were (i) steeper than, (ii) shallower than, or (iii) near the MHD limit dependent on plasma conditions. However, those profiles that exceeded the MHD limit were observed to have larger fractions of energetic electrons. When the dipole magnet was supported, high pressure plasmas always had profiles that exceeded the MHD interchange stability limit, but the high pressure in these plasmas appeared to arise entirely from a population of energetic trapped electrons

Diffusion dispersion imaging: Mapping oscillating gradient spin-echo frequency dependence in the human brain.

PURPOSE: Oscillating gradient spin-echo (OGSE) diffusion MRI provides information about the microstructure of biological tissues by means of the frequency dependence of the apparent diffusion coefficient (ADC). ADC dependence on OGSE frequency has been explored in numerous rodent studies, but applications in the human brain have been limited and have suffered from low contrast between different frequencies, long scan times, and a limited exploration of the nature of the ADC dependence on frequency. THEORY AND METHODS: Multiple frequency OGSE acquisitions were acquired in healthy subjects at 7T to explore the power-law frequency dependence of ADC, the diffusion dispersion. Furthermore, a method for optimizing the estimation of the ADC difference between different OGSE frequencies was developed, which enabled the design of a highly efficient protocol for mapping diffusion dispersion. RESULTS: For the first time, evidence of a linear dependence of ADC on the square root of frequency in healthy human white matter was obtained. Using the optimized protocol, high-quality, full-brain maps of apparent diffusion dispersion rate were also demonstrated at an isotropic resolution of 2 mm in a scan time of 6 min. CONCLUSIONS: This work sheds light on the nature of diffusion dispersion in the healthy human brain and introduces full-brain diffusion dispersion mapping at clinically relevant scan times. These advances may lead to new biomarkers of pathology or improved microstructural modeling

ENABLING INTERVENTIONAL MRI USING AN ULTRA-HIGH FIELD LOOPLESS ANTENNA

Interventional magnetic resonance imaging (iMRI) utilizes multi-functional capabilities of MRI, for targeting therapy and monitoring response. Up to recently, most of the iMRI procedures have been conducted at magnetic field strengths (B0) of 1.5T or lower. MRI at ultra-high B0 (UHF, B0≥3T) provides higher signal-to-noise ratio (SNR) that can be traded for better image resolution and/or shorter scan durations. This dissertation investigates the performance of the interventional loopless antenna at UHF and introduces new methods to enable UHF iMRI using interventional loopless antennae. First, a new MRI denoising method based on a spectral subtraction technique that can provide up to 45% of SNR improvement is introduced. While achievable SNR gains using post-processing methods are limited, the SNR of MRI is intrinsically improved by moving to higher B0. Here, the performance of a loopless antenna was tested at 7T. The results show that SNR increases quadratically with B0 (SNR~B02) up to 7T. The increased SNR of the loopless antenna at UHF enables MRI at sub-50µm in-plane resolutions. At UHF, excitation of tissue deep within the body becomes challenging due to the decreased penetration depth, and radiofrequency (RF) safety limitations when external coils are used. To overcome these problems, we used the loopless antenna for both transmitting the RF field, and receiving the MR signal from the tissue. Spatially selective B1-insensitive pulses were employed to improve the excitation homogeneity, providing a ~10cm FOV, which would be suitable for both device-tracking and localizing potential targets inside the body. Use of interventional devices in transmit and/or receive mode may elevate temperatures near the device above levels considered safe. To address this problem, we built a loopless antenna RF radiometer operating at 3T MRI frequency, 128MHz, to monitor the local temperature around the device. We investigated its performance inside bio-analogous phantoms and using electromagnetic and thermal numerical simulations. The radiometer was able to detect uniform temperature with an accuracy <0.3°C at 2 measurements/second, and estimate the peak 1g-averaged temperature rise within 0.4°C. The loopless antenna radiometer can be used to ensure safety of interventional procedures, without requiring any additional leads or sensors, or even MRI

Liquid cooled micro-scale gradient system for magnetic resonance

Schaltbare magnetische Feldgradientspulen sind ein geeignetes Werkzeug für die Modulation der Kernspinpräzession in der gepulsten Kernspinresonanzspektroskopie und Bildgebung. Die Magnetresonanztomographie von mikroskopischen Proben benötigt starke, schnell schaltbare Magnetfeldgradienten, um diffusionsbedingte Artefakte zu unterdrücken, Suszeptibilitätseffekte abzuschwächen und um die Messzeit zu verkürzen. Verschiedene Techniken können eingesetzt werden, um eine hohe Gradientenintensität zu erreichen, wie zum Beispiel die Erhöhung der Stromstärke oder die Steigerung der Windungsdichte der Feldspule. Ein weiterer, geeigneter technischer Ansatz besteht darin, die Gradientenspulen näher an der Probe zu platzieren. Als Konsequenz wird aber die durch die Joule-Erwärmung verursachte Wärmeentwicklung zu einem zentralen Problem. In dieser Arbeit wird ein neuartiges Design, ein Mikroherstellungsprozess und eine Kernspin-Evaluierung eines Feldgradientenchips präsentiert. Die Gradientenspulen wurden besonders hoch miniaturisiert und durch den Einsatz von verbesserten und neuartigen Strukturierungsverfahren entwickelt. Zuerst wird ein Fertigungsverfahren zur Herstellung einer kompakten Hochfrequenzspule vorgestellt. Durch den Einsatz einer maskenlosen Rückseitenlithographie konnte die Prozesskomplexität reduziert werden. Dieses Verfahren wurde durch Tintenstrahldruck mit Nanopartikeln realisiert, wobei die gedruckten Strukturen selbst als lithographische Maske für die Herstellung einer galvanischen Form dienen. Somit werden die Seitenwände der galvanischen Form durch die gedruckte Seed-Schicht optimal selbst ausgerichtet. Dies ermöglichte eine anisotrope Galvanisierung, um eine höhere elektrische Leitfähigkeit der gedruckten Leiterbahnen zu erzielen. Aus den Erkenntnissen der ausgearbeiteten Herstellungsprozesse wurde ein optimiertes Spulendesign für ein-axiale sowie drei-axiale linearen Gradientenchips entwickelt. Die einachsige lineare $z$-Gradientenspule wurde mit der Stream-Function-Methode berechnet, wobei die Optimierung darauf abgestimmt wurde, eine minimale Verlustleistung zu erzielen. Die Gradientenspulen wurden auf zwei Doppellagen implementiert, die mittels Cu-Galvanik in Kombination mit fotodefinierbaren Trockenfilm-Laminaten aufgebracht wurden. Bei dem hier vorgestellten Herstellungsverfahren diente die erste Metallisierungschicht gleichzeitig dazu, Widerstands-Temperaturdetektoren zu integrieren. Um niederohmige Spulen zu realisieren wurde der Galvanisierungsprozess soweit angepasst, um eine hohe Schichtdicke zu erzielen. Die Chipstruktur beinhaltet ein aktives Kühlsystem, um dem Aufheizen der Spulen entgegenzuwirken. Thermographische Aufnahmen in Kombination mit den eingebetteten Temperatursensoren ermöglichen es, die Erhitzung der Spule zu analysieren, um die Strombelastbarkeit zu ermitteln. Die Gradientenspule wurde mit einer Hochfrequenz-Mikrospule in einer Flip-Chip-Konfiguration zusammengebaut, und mit diesem Aufbau wurde ein eindimensionales Kernspinexperiment durchgeführt. Es wurde eine Gradienteneffizienz von 3.15 $T\,m^{−1}\,A^{−1}$ bei einer Profillänge von 1.2 $mm$ erreicht

Novel cardiovascular magnetic resonance phenotyping of the myocardium

INTRODUCTION Left ventricular (LV) microstructure is unique, composed of a winding helical pattern of myocytes and rotating aggregations of myocytes called sheetlets. Hypertrophic cardiomyopathy (HCM) is a cardiovascular disease characterised by left ventricular hypertrophy (LVH), however the link between LVH and underlying microstructural aberration is poorly understood. In vivo cardiovascular diffusion tensor imaging (cDTI) is a novel cardiovascular MRI (CMR) technique, capable of characterising LV microstructural dynamics non-invasively. In vivo cDTI may therefore improve our understanding microstructural-functional relationships in health and disease. METHODS AND RESULTS The monopolar diffusion weighted stimulated echo acquisition mode (DW-STEAM) sequence was evaluated for in vivo cDTI acquisitions at 3Tesla, in healthy volunteers (HV), patients with hypertensive LVH, and HCM patients. Results were contextualised in relation to extensively explored technical limitations. cDTI parameters demonstrated good intra-centre reproducibility in HCM, and good inter-centre reproducibility in HV. In all subjects, cDTI was able to depict the winding helical pattern of myocyte orientation known from histology, and the transmural rate of change in myocyte orientation was dependent on LV size and thickness. In HV, comparison of cDTI parameters between systole and diastole revealed an increase in transmural gradient, combined with a significant re-orientation of sheetlet angle. In contrast, in HCM, myocyte gradient increased between phases, however sheetlet angulation retained a systolic-like orientation in both phases. Combined analysis with hypertensive patients revealed a proportional decrease in sheetlet mobility with increasing LVH. CONCLUSION In vivo DW-STEAM cDTI can characterise LV microstructural dynamics non-invasively. The transmural rate of change in myocyte angulation is dependent on LV size and wall thickness, however inter phase changes in myocyte orientation are unaffected by LVH. In contrast, sheetlet dynamics demonstrate increasing dysfunction, in proportion to the degree of LVH. Resolving technical limitations is key to advancing this technique, and improving the understanding of the role of microstructural abnormalities in cardiovascular disease expression.Open Acces

Segmented simultaneous multi-slice diffusion-weighted imaging with navigated 3D rigid motion correction

Purpose To improve the robustness of diffusion-weighted imaging (DWI) data acquired with segmented simultaneous multi-slice (SMS) echo-planar imaging (EPI) against in-plane and through-plane rigid motion.Theory and Methods The proposed algorithm incorporates a 3D rigid motion correction and wavelet denoising into the image reconstruction of segmented SMS-EPI diffusion data. Low-resolution navigators are used to estimate shot-specific diffusion phase corruptions and 3D rigid motion parameters through SMS-to-volume registration. The shot-wise rigid motion and phase parameters are integrated into a SENSE-based full-volume reconstruction for each diffusion direction. The algorithm is compared to a navigated SMS reconstruction without gross motion correction in simulations and in vivo studies with four-fold interleaved 3-SMS diffusion tensor acquisitions.Results Simulations demonstrate high fidelity was achieved in the SMS-to-volume registration, with submillimeter registration errors and improved image reconstruction quality. In vivo experiments validate successful artifact reduction in 3D motion-compromised in vivo scans with a temporal motion resolution of approximately 0.3 s.Conclusion This work demonstrates the feasibility of retrospective 3D rigid motion correction from shot navigators for segmented SMS DWI.Radiolog

Development of novel magnetic resonance methods for advanced parametric mapping of the right ventricle

The detection of diffuse fibrosis is of particular interest in congenital heart disease patients, including repaired Tetralogy of Fallot (rTOF), as clinical outcome is linked to the accurate identification of diffuse fibrosis. In the Left Ventricular (LV) myocardium native T1 mapping and Diffusion Tensor Cardiac Magnetic Resonance (DT-CMR) are promising approaches for detection of diffuse fibrosis. In the Right Ventricle (RV) current techniques are limited due to the thinner, mobile and complex shaped compact myocardium. This thesis describes technical development of RV tissue characterisation methods. An interleaved variable density spiral DT-CMR method was implemented on a clinical 3T scanner allowing both ex and in vivo imaging. A range of artefact corrections were implemented and tested (gradient timing delays, off-resonance and T2* corrections). The off- resonance and T2* corrections were evaluated using computational simulation demonstrating that for in vivo acquisitions, off-resonance correction is essential. For the first-time high-resolution Stimulated Echo Acquisition Mode (STEAM) DT-CMR data was acquired in both healthy and rTOF ex-vivo hearts using an interleaved spiral trajectory and was shown to outperform single-shot EPI methods. In vivo the first DT-CMR data was shown from the RV using both an EPI and an interleaved spiral sequence. Both sequences provided were reproducible in healthy volunteers. Results suggest that the RV conformation of cardiomyocytes differs from the known structure in the LV. A novel STEAM-SAturation-recovery Single-sHot Acquisition (SASHA) sequence allowed the acquisition of native T1 data in the RV. The excellent blood and fat suppression provided by STEAM is leveraged to eliminate partial fat and blood signal more effectively than Modified Look-Locker Imaging (MOLLI) sequences. STEAM-SASHA T1 was validated in a phantom showing more accurate results in the native myocardial T1 range than MOLLI. STEAM-SASHA demonstrated good reproducibility in healthy volunteers and initial promising results in a single rTOF patient.Open Acces