Doctor of Philosophy

dissertationThis dissertation presents original research that improves the ability of magnetic resonance imaging (MRI) to measure temperature in aqueous tissue using the proton resonance frequency (PRF) shift and T1 measurements in fat tissue in order to monitor focused ultrasound (FUS) treatments. The inherent errors involved in measuring the longitudinal relaxation time T1 using the variable flip angle method with a two-dimensional (2D) acquisition are presented. The edges of the slice profile can contribute a significant amount of signal for large flip angles at steady state, which causes significant errors in the T1 estimate. Only a narrow range of flip angle combinations provided accurate T1 estimates. Respiration motion causes phase artifacts, which lead to errors when measuring temperature changes using the PRF method. A respiration correction method for 3D imaging temperature of the breast is presented. Free induction decay (FID) navigators were used to measure and correct phase offsets induced by respiration. The precision of PRF temperature measurements within the breast was improved by an average factor of 2.1 with final temperature precision of approximately 1 °C. Locating the position of the ultrasound focus in MR coordinates of an ultrasound transducer with multiple degrees of freedom can be difficult. A rapid method for predicting the position using 3 tracker coils with a special MRI pulse iv sequence is presented. The Euclidean transformation of the coil's current positions to their calibration positions was used to predict the current focus position. The focus position was predicted to within approximately 2.1 mm in less than 1 s. MRI typically has tradeoffs between imaging field of view and spatial and temporal resolution. A method for acquiring a large field of view with high spatial and temporal resolution is presented. This method used a multiecho pseudo-golden angle stack of stars imaging sequence to acquire the large field of view with high spatial resolution and k-space weighted image contrast (KWIC) to increase the temporal resolution. The pseudo-golden angle allowed for removal of artifacts introduced by the KWIC reconstruction algorithm. The multiple echoes allowed for high readout bandwidth to reduce blurring due to off resonance and chemical shift as well as provide separate water/fat images, estimates of the initial signal magnitude M(0), T2 * time constant, and combination of echo phases. The combined echo phases provided significant improvement to the PRF temperature precision, and ranged from ~0.3-1.0 °C within human breast. M(0) and T2 * values can possibly be used as a measure of temperature in fat

1-D broadside-radiating leaky-wave antenna based on a numerically synthesized impedance surface

A newly-developed deterministic numerical technique for the automated design of metasurface antennas is applied here for the first time to the design of a 1-D printed Leaky-Wave Antenna (LWA) for broadside radiation. The surface impedance synthesis process does not require any a priori knowledge on the impedance pattern, and starts from a mask constraint on the desired far-field and practical bounds on the unit cell impedance values. The designed reactance surface for broadside radiation exhibits a non conventional patterning; this highlights the merit of using an automated design process for a design well known to be challenging for analytical methods. The antenna is physically implemented with an array of metal strips with varying gap widths and simulation results show very good agreement with the predicted performance

Beam scanning by liquid-crystal biasing in a modified SIW structure

A fixed-frequency beam-scanning 1D antenna based on Liquid Crystals (LCs) is designed for application in 2D scanning with lateral alignment. The 2D array environment imposes full decoupling of adjacent 1D antennas, which often conflicts with the LC requirement of DC biasing: the proposed design accommodates both. The LC medium is placed inside a Substrate Integrated Waveguide (SIW) modified to work as a Groove Gap Waveguide, with radiating slots etched on the upper broad wall, that radiates as a Leaky-Wave Antenna (LWA). This allows effective application of the DC bias voltage needed for tuning the LCs. At the same time, the RF field remains laterally confined, enabling the possibility to lay several antennas in parallel and achieve 2D beam scanning. The design is validated by simulation employing the actual properties of a commercial LC medium

Autocalibration Region Extending Through Time: A Novel GRAPPA Reconstruction Algorithm to Accelerate 1H Magnetic Resonance Spectroscopic Imaging

Magnetic resonance spectroscopic imaging (MRSI) has the ability to noninvasively interrogate metabolism in vivo. However, excessively long scan times have thus far prevented its adoption into routine clinical practice. Generalized autocalibrating partially parallel acquisitions (GRAPPA) is a parallel imaging technique that allows one to reduce acquisition duration and use spatial sensitivity correlations to reconstruct the unsampled data points. The coil sensitivity weights are determined implicitly via a fully-sampled autocalibration region in k-space. In this dissertation, a novel GRAPPA-based algorithm is presented for the acceleration of 1H MRSI. Autocalibration Region extending Through Time (ARTT) GRAPPA instead extracts the coil weights from a region in k-t space, allowing for undersampling along each spatial dimension. This technique, by exploiting spatial-spectral correlations present in MRSI data, allows for a more accurate determination of the coil weights and subsequent parallel imaging reconstruction. This improved reconstruction accuracy can then be traded for more aggressive undersampling and a further reduction of acquisition duration. It is shown that the ARTT GRAPPA technique allows for approximately two-fold more aggressive undersampling than the conventional technique while achieving the same reconstruction accuracy. This accelerated protocol is then applied to acquire high-resolution brain metabolite maps in less than twenty minutes in three healthy volunteers at B0 = 7 T

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