Design of a Transceive Coil Array for Parallel Imaging at 9.4T

The main goal of this thesis is to design and develop a transmit/receive (transceive) coil array for small animal imaging at 9.4T. The goal is achieved by following basic RF design principles with a methodical construction approach and demonstrating viable applications. As operational frequencies increase linearly with higher static fields, the wavelength approaches the size of the sample being imaged. The resulting standing wave mode deteriorates image homogeneity. Fortunately, with multi-channel coil arrays, the produced Bi field can be tailored to produce a homogeneous excitation in the region of interest, thus overcoming the so called dielectric resonance effect. We examined a solution to achieve a higher level of Bx homogeneity and we compared the improvement of RF wavelength effects reduction against the results obtained with a similar-sized conventional birdcage coil. An additional benefit of this design lies in the fact that the use of multiple receiving coil elements is necessary for the implementation of fast imaging acquisition techniques such as parallel imaging. This is possible because the distinct element sensitivities are used to reconstruct conventional images from undersampled (or accelerated) data. The greatest advantage of parallel imaging is thus the reduction of total acquisition time. In functional MRI (fMRI), single-shot EPI is one of the standard imaging technique. Unfortunately, EPI suffers from significant limitations, precisely because all of the data is acquired following a single RF excitation. As a result EPI images can manifest artifacts and blurring due to susceptibility mismatch, off-resonance effects and reduced signal at the edges of k-space. Fortunately, parallel imaging can be used to decrease such unwanted effects by reducing the total k-space data acquired. Presented in this thesis is the logical progression of the construction of a transceive coil from surface coil fundamentals to high field applications such as field focusing and parallel imaging techniques

Radio Frequency Coils for Ultra-high Field Magnetic Resonance Imaging

Magnetic resonance imaging (MRI) has become a powerful tool not only to analyze the anatomical structures of the human body non invasively but also to investigate brain activity with functional MRI. The promise of increase in signal to noise ratio and spectral resolution proportional to the main magnetic field strength motivated a few research laboratories to pursue even higher field strengths. The 9.4T whole body human scanner and the 16.4T animal scanner installed at the Max Planck Institute for Biological Cybernetics, Tuebingen were, for many years, the worlds strongest magnets in their respective categories. In addition to the strong magnets, radio frequency (RF) coils are also equally important in realising the benefits offered by the high field MRI scanners. The aim of this thesis work is to develop optimized RF coils and RF hardware for ultra-high high field MRI

Entwicklung von Fluor-19 und Protonen-Magnetresonanztomographie und ihre Anwendung bei Neuroentzündung

The experimental autoimmune encephalomyelitis (EAE) is used to study multiple sclerosis (MS) pathology and develop novel technologies to quantify inflammation over time. Magnetic resonance imaging (MRI) with gadolinium-based contrast agents (GBCAs) is the state-of-the-art method to assess inflammation in MS patients and its animal model. Fluorine (19F)-MRI is one novel technology to quantify inflammatory immune cells in vivo using 19F-nanoparticles. T1 mapping of contrast-enhancing images is another method that could be implemented to quantify inflammatory lesions. Transient macroscopic changes in the EAE brain confound quantification and necessitate registration methods to spatially align images in longitudinal studies. For 19F-MRI, an additional challenge is the low signal-to-noise ratio (SNR) due to low number of 19F-labeled immune cells in vivo. Transceive surface radiofrequency (RF) probes and SNR-efficient imaging techniques such as RARE (Rapid Acquisition with Relaxation Enhancement) are combined to increase sensitivity in 19F-MRI. However, the strong spatially-varying RF field (B1 inhomogeneity) of transceive surface RF probes further hampers quantification. Retrospective B1 correction methods typically use signal intensity equations, unavailable for complex acquisition methods like RARE. The main goal of this work is to investigate novel B1 correction and registration methods to enable the study of inflammatory diseases using 1H- and 19F-MRI following GBCA and 19F-nanoparticle administration, respectively. For correcting B1 inhomogeneities in 1H- and 19F-MR transceive surface RF probes, a model-based method was developed using empirical measurements and simulations, and then validated and compared with a sensitivity method and a hybrid of both. For 19F-MRI, a workflow to measure anatomical images in vivo and a method to compute 19F-concentration uncertainty after correction using Monte Carlo simulations were developed. To overcome the challenges of EAE brain macroscopic changes, a pipeline for registering images throughout longitudinal studies was developed. The proposed B1 correction methods demonstrated dramatic improvements in signal quantification and T1 contrast on images of test phantoms and mouse brains, allowing quantitative measurement with transceive surface RF probes. For low-SNR scenarios, the model-based method yielded reliable 19F-quantifications when compared to volume resonators. Uncertainty after correction depended linearly on the SNR (≤10% with SNR≥10.1, ≤25% when SNR≥4.25). The implemented registration approach provided successful image alignment despite substantial morphological changes in the EAE brain over time. Consequently, T1 mapping was shown to objectively quantify gadolinium lesion burden as a measure of inflammatory activity in EAE. The 1H- and 19F-MRI methods proposed here are highly relevant for quantitative MR of neuroinflammatory diseases, enabling future (pre)clinical investigations.Die experimentelle Autoimmun-Enzephalomyelitis (EAE) wird zur Untersuchung Multipler Sklerose (MS) und zur Entwicklung neuer Technologien zur Entzündungsquantifizierung eingesetzt. Magnetresonanztomographie (MRT) mit Gadolinium-haltigen Kontrastmitteln (GBCAs) ist die modernste Methode zur Beurteilung von Entzündungen bei MS-Patienten und im Tiermodell. Fluor (19F)-MRT unter Verwendung von 19F-Nanopartikeln ist eine neue Technologie zur Quantifizierung entzündlicher Immunzellen in vivo. T1-Kartierung ist eine MRT-Methode, die zur Quantifizierung entzündlicher Läsionen eingesetzt werden könnte. Temporäremorphologische Veränderungen im EAE-Gehirn erschweren die Quantifizierung und erfordern Registrierungsmethoden, um MRT-Bilder in Längsschnittstudien räumlichabzugleichen. Das niedrige Signal-Rausch-Verhältnis (SNR) ist aufgrund der geringen Anzahl 19F-markierter Immunzellen in vivo eine zusätzliche Herausforderung der 19F-MRT. Um deren Empfindlichkeit zu erhöhen, werden Sende-/Empfangsoberflächen-Hochfrequenzspulen (TX/RX-HF-Spule) und SNR-effiziente MRT-Techniken wie RARE (Rapid Acquisition with Relaxation Enhancement) kombiniert. Jedoch verhindert die starke räumliche Variation des HF-Feldes (B1-Inhomogenität) dieser Spulen die Signalquantifizierung. Retrospektive B1-Korrekturmethoden verwenden in der Regel Signalintensitätsgleichungen, die für komplexe MRT-Techniken wie RARE nicht existieren. Das Hauptziel dieser Arbeit ist die Untersuchung neuartiger B1-Korrektur- und Bildregistrierungsmethoden, um in vivo 1H- und 19F-MRT Studien von Entzündungsprozessen zu ermöglichen. Zur Korrektur von B1-Inhomogenitäten wurde eine modellbasierte Methode entwickelt. Diese verwendet empirische Messungen und Simulationen, wurde in Phantomexperimenten validiert und mit Referenzmethoden verglichen. Für 19F-MRT wurden ein Protokoll zur Messung anatomischer Bilder in vivo und eine Methode zur Berechnung der 19F-Konzentrationsunsicherheit nach Korrektur mittels Monte-Carlo-Simulationen entwickelt. Um morphologische Veränderungen im EAE-Gehirn in longitudinalen Studien zu kompensieren, wurde zur Bildregistrierung eine Software-Bibliothek entwickelt. Die B1-Korrekturmethoden zeigten in Testobjekten und Mäusehirnen drastische Verbesserungen der Signal- und T1 Quantifizierung und ermöglichten so quantitative Messungen mit TX/RX-HF-Spulen. Die modellbasierte Methode lieferte für geringe SNRs zuverlässige 19F-Quantifizierungen, deren Genauigkeit mit dem SNR korrelierte. Die implementierte Registrierungsmethode ermöglichte einen erfolgreichen Abgleich von Bildserientrotz erheblicher morphologischer Veränderungen im EAE-Hirn. Folglich wurde gezeigt, dass MRT basierte T1-Kartierung die Gadolinium-Läsionslast als Maß entzündlicher Aktivität bei EAE objektiv quantifizieren kann. Die hier unterscuhten Methoden sind für quantitative 1H- und 19F-MRT neuroinflammatorischer Erkrankungen sehr relevant und ermöglichen künftige (prä)klinische Untersuchungen

Sodium Magnetic Resonance Imaging at 9.4 Tesla

The motivation to perform magnetic resonance imaging (MRI) at ultra-high field strength (UHF) (B0 ≥ 7 Tesla) is primarily driven by the increased sensitivity compared to low field MRI. This is especially true for nuclei which exhibit intrinsically a low signal-to-noise ratio (SNR) either due to their physical properties or their small in vivo concentrations. The aim of this thesis was to establish the measurement techniques required for sodium magnetic resonance imaging at 9.4 Tesla and to overcome some of the limitations faced at lower field strengths. For this purpose, the hardware as well as the software used for the acquisition of the MR signal were designed and adapted to each other with great care in order to harness the full potential offered by UHF MRI. In the first part of this thesis, a novel coil setup consisting of a single-tuned sodium birdcage coil and a proton patch antenna was used to acquire high-resolution quantitative sodium images of several healthy volunteers. This setup provided a satisfactory sensitivity at the sodium frequency and offered at the same time the possibility to acquire the proton signal for anatomical localization and B0 shimming. Correction methods for inhomogeneities of the B0 and radio-frequency (RF) transmit field (B1) were implemented and partial volume effects were mitigated by the reduced voxel size, which enabled a more accurate quantification of the sodium concentration in the human brain. However, the spatial resolution was insufficient to completely avoid quantifications errors at tissue boundaries, although the achieved sensitivity was considerably higher compared to previous studies. The second part of the thesis focused on further increasing the sensitivity of the coil setup at the sodium frequency without sacrificing the proton imaging capability. The final coil design was made up of an assembly of three coils arranged in layers. The innermost layer consisted of a multi-channel receiver array to boost the sensitivity for sodium imaging. The middle layer comprised the sodium transmit array and the outer layer was formed by a dipole array to enable proton imaging. It could be shown that the proposed coil setup possessed all the required features needed for efficient multi-nuclear MRI at UHF and enabled the acquisition of sodium images having a quality not previously achieved. In the last part of the thesis, the high sensitivity provided by the multi-channel coil array and the strong static magnetic field was used to perform sodium triple quantum filtered (TQF) imaging, which is known to be an SNR-critical application. The latter allows differentiating between intra- and extracellular sodium, which might be valuable information for disease diagnosis and monitoring. Apart from the low SNR, the high power deposition rates associated with this type of imaging technique are challenging, especially at UHF. To overcome this problem, at least partially, a modulation of the flip angles of the TQ preparation module was proposed and shown to improve the sensitivity by about 20%.Das Bestreben Magnetresonanztomographie (MRT) bei ultra-hohen Feldstärken (UHF) (B0 ≥ 7 Tesla) durchzuführen kann in erster Linie mit der deutlich erhöhten MR-Empfindlichkeit im Gegensatz zu niedrigeren Feldstärken erklärt werden. Dies gilt insbesondere für die Bildgebung mit Kernen, die an sich schon ein niedriges Signal-Rausch-Verhältnis (SNR) ausweisen; dies entweder aufgrund ihrer physikalischen Eigenschaften oder ihrer geringen In-vivo-Konzentrationen. Das Ziel dieser Arbeit war es die erforderlichen Messverfahren für die Natriumbildgebung bei 9,4 Tesla zu erarbeiten und einige Einschränkungen, die bei niedrigeren Feldstärken auftreten, zu überwinden. Zu diesem Zweck wurde maßgeschneiderte Hard- und Software für die Erfassung des MR-Signals entwickelt und aufeinander abgestimmt um das volle Potential, das UHF-MRT bietet, zu nutzen. Im ersten Teil der Arbeit wurde ein neuartiger Spulenaufbau, bestehend aus einer mono-resonanten Natrium-Birdcage-Spule und einer Protonen-Patch-Antenne, verwendet um hochauflösende quantitative Natriumbilder von mehreren gesunden Probanden aufzunehmen. Dieser Aufbau stellte eine zufriedenstellende Empfindlichkeit bei der Natriumfrequenz sicher und bot gleichzeitig die Möglichkeit das Protonensignal für anatomische Lokalisation und B0-Shimming zu nutzen. Korrekturverfahren wurden implementiert und angewendet um Inhomogenitäten des B0 und Radiofrequenz- (RF) Feldes (B1) entgegenzuwirken. Durch die Reduzierung der Voxelgröße konnten Partialvolumeneffekte gemindert und eine genauere Quantifizierung der Natriumkonzentration im menschlichen Gehirn erreicht werden. Jedoch war die erreichte räumliche Auflösung unzureichend um Quantifizierungsfehler an Gewebegrenzen gänzlich zu vermeiden, obwohl die erzielte Empfindlichkeit deutlich höher war als bei vorhergehenden Studien. Der zweite Teil der Arbeit konzentrierte sich auf eine weitere Erhöhung der Empfindlichkeit des Spulenaufbaus für die Natriumbildgebung ohne dabei die Möglichkeit der Protonenbildgebung zu verlieren. Der endgültige Messaufbau bestand aus drei in Schichten angeordneten Spulen. Die innerste Schicht bildete eine Mehrkanalempfangsanordnung, welche eine möglichst hohe Empfindlichkeit für das Natriumsignal gewährleisten sollte. Die Natriumsendespule stellte die mittlere Schicht dar. Eine Dipolantennenanordnung bildete die äußerste Schicht und wurde für die Protonenbildgebung benutzt. Es konnte gezeigt werden, dass der vorgeschlagene Spulenaufbau alle erforderlichen Funktionen besitzt, die für eine effiziente Mehrkern-MRT-Messung bei ultra-hohem Feld benötig werden, und es erlaubt Natriumbilder mit einer vorher unerreichten Qualität aufzunehmen. Im letzten Teil der Arbeit wurde die hohe MR-Empfindlichkeit, resultierend aus der Verwendung einer Mehrkanalspule und eines starken statischen Magnetfeldes, genutzt um Tripelquanten (TQ)-Kohärenzen zu messen, welche nur ein sehr geringes SNR aufweisen. Tripelquanten-gefilterte (TQF) Bilder ermöglichen die Unterscheidung zwischen intra- und extrazellulären Natrium und können möglicherweise wertvolle Informationen für die Diagnose und Überwachung von Krankheiten liefern. Abgesehen von dem niedrigen SNR, bereiten die hohen RF-Sendeleistungen, die für diese Bildgebungstechnik benötigt werden, Probleme insbesondere bei UHF. Um dieses Problem zumindest teilweise zu mindern wurde eine Modulation der Flipwinkel, welche die TQ-Kohärenzen erzeugen, vorgeschlagen und gezeigt, dass sich so die Sensitivität der TQ Sequenz um etwa 20% steigern lässt

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

Real-time Feedback of B0 Shimming at Ultra High Field MRI

Magnetic resonance imaging(MRI) is moving towards higher and higher field strengths. After 1.5T MRI scanners became commonplace, 3T scanners were introduced and once 3T scanners became commonplace, ultra high field (UHF) scanners were introduced. UHF scanners typically refer to scanners with a field strength of 7T or higher. The number of sites that utilise UHF scanners is slowly growing and the first 7T MRI scanners were recently CE certified for clinical use. Although UHF scanners have the benefit of higher signal-to-noise ratio (SNR), they come with their own challenges. One of the many challenges is the problem of inhomogeneity of the main static magnetic field(B0 field). This thesis addresses multiple aspects associated with the problem of B0 inhomogeneity. The process of homogenising the field is called "shimming". The focus of this thesis is on active shimming where extra shim coils drive DC currents to generate extra magnetic fields superimposed on the main magnetic field to correct for inhomogeneities. In particular, we looked at the following issues: algorithms for calculating optimal shim currents; global static shimming using very high order/degree spherical harmonic-based (VHOS) coils; dynamic slice-wise shimming using VHOS coils compared to a localised multi-coil array shim system; B0 field monitoring using an NMR field camera; characterisation of the shim system using a field camera; and designing a controller based on the shim system model for real-time feedback

Geometrically Decoupled Phased Array Coils for Mouse Imaging

Phased array surface coils offer high SNR over a large field of view. Phased array volume coils have high SNR at the surface and centre of the volume. Most array coil designs typically employ a combination of geometrical and additional techniques, such as isolating preamplifiers for element-to-element decoupling. The development of array coils for small animal MRI is of increasing interest. However isolation preamplifiers are expensive and not ubiquitous at the field strengths typically employed for small animal work (4.7T, 9.4T, etc). In addition, isolating preamps complicates the designs of coils for transmit SENSE since they do not decouple during transmitting. Therefore, this thesis reexamines a "tried and true" method for decoupling coil elements. In this work five different coils for mouse imaging at 200MHz are presented: a 16 leg trombone design quadrature birdcage coil and four geometrically decoupled volume phased array coils. The first mouse array coil is a two saddle quadrature coil with a circularly polarized field. The second coil is a four channel transmit/receive volume array coil that is decoupled purely geometrically, without the need for other forms of decoupling. The third array coil is a modified 'open' configuration to facilitate the loading of animals. The fourth coil presented is a 'tunable' decoupling coil, where the geometric decoupling between elements is 'tunable', in order to compensate for different loading conditions of the coil. Tunable decoupling between elements was achieved using two mechanisms, a decoupling paddle for isolation of top to bottom elements, with a variable overlap mechanism for decoupling diagonal elements. Bench measurements demonstrate good decoupling (better than -20dB) of the coil elements and 'tunability' of both mechanisms. Phantom images from all coils are presented

Assessment of Post-Treatment Imaging Changes Following Radiotherapy using Magnetic Susceptibility Techniques

Radiation therapy (RT) is a common treatment for brain neoplasms and is used alone or in combination with other therapies. The use of RT has been found to be successful in controlling tumors and extending the overall survival of patients; however, there are many unanswered questions regarding radiotherapy effects in the normal brain surrounding or infiltrated by tumor. Changes to the vascular and parenchyma have been documented, and more recently inflammatory mechanisms have been postulated to play a role in radiation injury. Traditional imaging techniques used within the clinic (CT and MRI) are often lacking in their ability to differentiate between recurrent tumor, transient treatment effects, or radiation necrosis. The primary goal of this thesis is to demonstrate an MRI acquisition method that has been shown to be sensitive to deoxygenated blood and iron content as a potential biomarker of radiation effect on the normal brain. Specifically, post-processing techniques are used to determine the applicability of qualitative images such as Susceptibility-Weighted Imaging (SWI) and quantitative methods such as Quantitative Susceptibility Mapping (QSM) and apparent traverse relaxation (R2*) using the same sequence. These methods are potential surrogate markers for vascular changes and neuroinflammatory components that could predict sub-acute and long-term radiation effects. Within this thesis, R2* is shown to be a promising marker for the prediction of radiation necrosis, whereas SWI and QSM are shown to be excellent modalities for detecting longterm effects such as microbleeds. Additionally, R2 * is shown to be a potentially useful technique in identifying post-imaging treatment changes (pseudoprogression) following chemoradiotherapy for malignant glioma. Finally, the use of this non-contrast method shows promise for integration within a clinical setting and the potential for expansion to multicenter clinical trials