Development of an excitation and reception system for primate imaging on a 7 Tesla ultra-high field human whole-body MRI system

Die Ultrahochfeld-Magnetresonanztomographie (MRT) stellt neuartige technische Anforderungen, vor allem an die Anregung und Detektion. Neben den in den letzten Jahren entwickelten Mehrkanal-Spulen wurde ein weiteres interessantes Prinzip entwickelt, das die Abschirmung des Gradientensystems (RF-Shield) als Wellenleiter für die Anregung nutzt. Das Hauptziel dieser Arbeit war die Konstruktion und Evaluierung eines Anregungs- und Empfangssystems zur Prima- ten-Bildgebung, das vergleichbar einem entfernbaren Insert für ein Human-Ganzkörper 7 Tesla Ultrahochfeld (UHF) Magnetresonanztomographie-System (nachfolgend 7 T MRT-System genannt) entwickelt werden sollte. Wichtige Anforderungen an das Anregungs- und Empfangssystem waren ein hohes Signal-Rausch-Verhältnis (SNR) und eine hohe Homogenität des eingestrahlten B1+-Feldes, das als Teil des B1-Feldes der Anregung dient. Insbesondere die funktionelle MRT (fMRT) benötigt für ihre Untersuchungen MR-HF-Spulen, welche bei der visuellen und auditorischen Stimulation für den hier anzuwendenden experimentellen Aufbau ausreichende Flexibilität bieten. Bei Systemen mit tieferen Feldern (1,5 T und 3 T) wird eine homogene B1+-Feldanregung in der Regel über Ganzkörper-Resonatoren erzeugt. Bei 7 T MRT-Systemen fehlen diese Ganzkörper-Resonatoren, da die Standard Birdcage-Ganzkörper-Resonatorarchitektur nur bedingt bei einer Systemfrequenz von 297,2 MHz einsetzbar ist. In dieser Arbeit wurde die Anwendung des Travelling-Wave (TW) Verfahrens als neuartiges Anregungssystem für ein 7 T MRT-System (Siemens) zur Primaten-Bildgebung untersucht. Das hierfür entwickelte Gesamt- konzept wird als Travelling-Wave Primatensystem bezeichnet. Das TW-Primatensystem benutzt das RF-Shield des Gradienten-Systems als Hohlwellenleiter, in welchem sich der TE11-Mode ausbreiten kann. Unter Verwendung von Feldsimulationssoftware wurde eine für das 7 T MRT- System angepasste 2-Port Patchantenne (Systemfrequenz 297,2 MHz) entwickelt, die ein zirkular polarisiertes B1-Feld generiert. Für den Empfang wurde eine 3-Elemente Phased-Array-Primatenkopfspule speziell für die Hirn-Bildgebung bei Makaken konstruiert, die eine auditorische Stimulation zulässt und ein hohes SNR gewährleistet. Um die Homogenität des erzeugten B1+-Feldes für einen größeren Bereich mit der entwickelten Patchantenne zu untersuchen und mit einer Birdcagearchitektur zu vergleichen, wurde zusätzlich eine 8-Elemente Phased-Array-Empfangsspule konstruiert. Eine Vergleichsmessung des generierten B1+-Feldes wurde unter Verwendung einer FLASH (Fast Low Angle Shot)-basierten B1+-Flipwinkelmap Sequenz mit einem Silikonöl-Kugelphantom als Beladung durchgeführt. Es konnte nachgewiesen werden, dass die Homogenität für Volumina bis ca. 10 cm Durchmesser vergleichbar ist zur Birdcagearchitektur. Zur Validierung des TW-Primatensystems wurden mehrere in vivo Messungen mit Makaken durchgeführt, wobei Turbo-Spin-Echo (TSE) und Echoplanar (EPI) Sequenzen verwendet wurden, um anatomische Datensätze mit hohem Kontrast und hoher nomineller Auflösung 0,46x0,46x0,5 mm3 (TSE) bzw. 0,64x0,58x0,2 mm3 (EPI) zu akquirieren. Hierdurch wurde erfolgreich die prinzipielle Anwendbarkeit von fMRT Messungen mit dem entwickelten TW-Primatensystem evaluiert. Damit bildet das TW-Primatensystem die weltweit erste Applikation für Makaken Messungen mit guten räumlichen Stimulationsmöglichkeiten an einem 7 T Human-Ganzkörper MRT-System.Ultra-high field magnetic resonance imaging (MRI) imposes novel technical requirements, especially for excitation and detection. In addition to the developing multi-channel RF coils in recent years, another interesting concept was developed that uses the RF shield of the gradient system as a waveguide for excitation. The main objective of this work was the design and evaluation of an excitation and receiver system for primate imaging, comparable to a removable insert for a human whole-body 7 Tesla ultra-high field (UHF) MRI system (7 T MRI system). Important requirements for this excitation and reception system were a high signal-to-noise ratio (SNR) and a high homogeneity of the irradiated B1+ field which serves for the excitation as a part of the B1 field. Especially functional MRI (fMRI) needs RF coils which provide sufficient flexibility for visual and auditory stimulation within the experimental setting. For systems with lower fields (1.5 T and 3 T) the homogeneous B1+ field excitation is usually provided by whole-body resonators. At 7 T whole-body MRI systems, these resonators are not available because the standard birdcage architecture is only limited usable at system frequency of 297.2 MHz. In this work the application of the traveling-wave (TW) approach as a novel excitation system for a 7 T MRI system (Siemens) was studied for primate imaging. The developed overall concept is called Traveling-Wave-Primate-System. The TW-Primate-System uses the RF shield of the gradient system as a waveguide in which the TE11-mode can propagate. A 2-port patch antenna (system frequency 297.2 MHz) which generates a circularly polarized B1 field was designed for the 7 T MRI system using field simulation software. For receive a 3-element phased array primate head coil primates was specifically designed for brain imaging of macaques which allowed an auditory stimulation and ensured a high SNR. To study the homogeneity of the B1+ field generated by the developed patch antenna for a larger range and comparing it with Birdcage architecture, an 8-element phased array receiving coil was additionally designed. A comparative measurement of the generated B1+ field was performed using a FLASH (fast low angle shot)-based B1+ flip angle map sequence with a silicone oil spherical phantom as a load. It was shown that the homogeneity for volumes up to approx. 10 cm diameter is comparable to the Birdcage architecture. To validate the TW-Primate-System, several in vivo measurements with macaques were performed. Turbo Spin Echo (TSE) and Echo Planar (EPI) sequences were used to acquire anatomical data with high contrast and high nominal resolution of 0.46x0.46x0.5 mm3 (TSE) respectively 0.64x0.58x0.2 mm3 (EPI). These results prove the feasibility of fMRI measurements was successfully evaluated for the developed TW-Primate-System. Thus, the TW-Primate-System is the world's first application of macaques measurements with good spatial stimulation options at a 7 T human whole-body MRI system

Design of Radio-Frequency Arrays for Ultra-High Field MRI

Magnetic Resonance Imaging (MRI) is an indispensable, non-invasive diagnostic tool for the assessment of disease and function. As an investigational device, MRI has found routine use in both basic science research and medicine for both human and non-human subjects. Due to the potential increase in spatial resolution, signal-to-noise ratio (SNR), and the ability to exploit novel tissue contrasts, the main magnetic field strength of human MRI scanners has steadily increased since inception. Beginning in the early 1980’s, 0.15 T human MRI scanners have steadily risen in main magnetic field strength with ultra-high field (UHF) 8 T MRI systems deemed to be insignificant risk by the FDA (as of 2016). However, at UHF the electromagnetic fields describing the collective behaviour of spin dynamics in human tissue assume ‘wave-like’ behaviour due to an increase in the processional frequency of nuclei at UHF. At these frequencies, the electromagnetic interactions transition from purely near-field interactions to a mixture of near- and far-field mechanisms. Due to this, the transmission field at UHF can produce areas of localized power deposition – leading to tissue heating – as well as tissue-independent contrast in the reconstructed images. Correcting for these difficulties is typically achieved via multi-channel radio-frequency (RF) arrays. This technology allows multiple transmitting elements to synthesize a more uniform field that can selectively minimize areas of local power deposition and remove transmission field weighting from the final reconstructed image. This thesis provides several advancements in the design and construction of these arrays. First, in Chapter 2 a general framework for modeling the electromagnetic interactions occurring inside an RF array is adopted from multiply-coupled waveguide filters and applied to a subset of decoupling problems encountered when constructing RF arrays. It is demonstrated that using classic filter synthesis, RF arrays of arbitrary size and geometry can be decoupled via coupling matrix synthesis. Secondly, in Chapters 3 and 4 this framework is extended for designing distributed filters for simple decoupling of RF arrays and removing the iterative tuning portion of utilizing decoupling circuits when constructing RF arrays. Lastly, in Chapter 5 the coupling matrix synthesis framework is applied to the construction of a conformal transmit/receive RF array that is shape optimized to minimize power deposition in the human head during any routine MRI examination

Parallel transit methods for arterial spin labelling magnetic resonance imaging

Vessel selective arterial spin labelling (ASL) is a magnetic resonance imaging technique which permits the visualisation and assessment of the perfusion territory of a specific set of feeding arteries. It is of clinical importance in both acute and chronic cerebrovascular disease, and the mapping of blood supplied to tumours. Continuous ASL is capable of providing the highest signal-to-noise (SNR) ratio of the various ASL methods. However on clinical systems it suffers from high hardware demands, and the control of systematic errors decreases perfusion sensitivity. A separate labelling coil avoids these problems, enabling high labelling efficiency and subsequent high SNR, and vessel specificity can be localised to one carotid artery. However this relies on the careful and accurate positioning of the labelling coil over the common carotid arteries in the neck. It is proposed to combine parallel transmission (multiple transmit coils, each transmitting with different amplitudes and phases) to spatially tailor the labelling field, removing the reliance on coil location for optimal labelling efficiency, and enabling robust vessel selective labelling with a high degree of specificity. Presented is the application of parallel transmission methods to continuous ASL, requiring the development of an ASL labelling coil array, and a two channel transmitter system. Coil safety testing was performed using a novel MRI temperature mapping technique to accurately measure small temperature changes on the order of 0.1 ⁰C. A perfusion phantom with distinct vascular territories was constructed for sequence testing and development. Phantom and in-vivo testing of parallel transmit CASL using a 3D-GRASE acquisition showed an improvement of up to 35% in vessel specificity when compared with using a single labelling coil, whilst retaining the high labelling efficiency and associated SNR of separate coil CASL methods