Novel MRI Technologies for Structural and Functional Imaging of Tissues with Ultra-short T₂ Values

Conventional MRI has several limitations such as long scan durations, motion artifacts, very loud acoustic noise, signal loss due to short relaxation times, and RF induced heating of electrically conducting objects. The goals of this work are to evaluate and improve the state-of-the-art methods for MRI of tissue with short T₂, to prove the feasibility of in vivo Concurrent Excitation and Acquisition, and to introduce simultaneous electroglottography measurement during functional lung MRI

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 Following the introduction, five different published and yet unpublished papers for improving the utility of MRI are shown.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

Novel MRI Technologies for Structural and Functional Imaging of Tissues with Ultra-short T2 Values

Conventional MRI has several limitations such as long scan durations, motion artifacts, very high acoustic noise levels, signal loss due to short relaxation times, and RF induced heating of electrically conducting objects. The goals of this thesis are to evaluate state-of-the-art methods for MRI of tissue with short relaxation times, to prove the feasibility of CEA in a clinical MRI system, and to introduce a new electrophysiological measurement unit applied simultaneously with lung MRI

Head Motion Correction in Magnetic Resonance Imaging Using NMR Field Probes

Magnetic Resonance Imaging (MRI) is a widely used imaging technology in medicine. Its advantages include good soft tissue contrast and the use of non-ionizing radiation in contrast to for example computed tomography (CT). One drawback are the long acquisition times that are needed. They depend on the diagnostic use case but are usually within the range of minutes. These long scan times make the images prone to patient motion during image acquisition which can lead to blurring or ghosting artifacts. Those artifacts might render the diagnostic value of the images useless which requires the image to be reacquired or the patient to be sedated before the scan to prevent motion artifacts. This is where motion correction comes into play. One can distinguish between retrospective and prospective motion correction (PMC) methods. Retrospective motion correction tries to improve image quality after the image acquisition by post-processing and possibly using additional motion tracking information, if available. Prospective motion correction relies on a motion tracking modality that is used to provide motion information to update imaging parameters during image acquisition. Both motion correction methods can also be used in combination with each other. This thesis, however, will focus on the implementation and validation of a system for prospective head motion correction. The system consisted of four nuclear magnetic resonance (NMR) field probes using. Those feld probes were attached to the head and used to measure the spatiotemporal evolution of magnetic felds. By switching spatially varying magnetic fields, this information can be used to track the field probes' positions and calculate the corresponding head motion in order to perform prospective motion correction.Die Magnetresonanztomographie (MRT) ist ein in der Medizin weitverbreitetes bildgebendes Verfahren. Ihre Vorteile sind unter anderem der gute Gewebekontrast und die Verwendung von nichtionisierender Strahlung im Gegensatz zur Computertomographie (CT). Ein Nachteil ist die Länge der Zeit, die notwendig ist um ein Bild aufzunehmen. Sie hängt natürlich vom jeweiligen diagnostischen Anwendungsfall ab, bewegt sich aber normalerweise im Bereich von Minuten. Diese langen Aufnahmezeiten machen die Bilder anfällig für Patientenbewegungen, welche zu unscharfen Bildern oder sogenannten Ghostingartefakten, bei denen sich Bildteile wiederholen, führen. Diese Artefakte können dazu führen, dass eine Diagnose nicht mehr möglich ist, was entweder eine erneute Aufnahme des Bildes notwendig macht oder eine Sedierung des Patienten, um Bewegung zu vermeiden. Hier kommen Bewegungskorrekturverfahren ins Spiel. Die sogenannte prospektive Bewegungskorrektur benötigt zusätzliche Bewegungsinformationen, die noch während der Bildaufnahme dazu verwendet werden, die Bildgebungsparameter so zu verändern, dass der Bildausschnitt der Bewegung folgt. Diese Arbeit beschäftigt sich mit der Entwicklung und Validierung eines Systems zur prospektiven Bewegungskorrektur. Das entwickelte System bestand aus vier Kernspinresonanz-Magnetfeldsensoren (NMR field probes). Diese Sensoren wurden am Kopf der Probanden befestigt und konnten die räumliche und zeitliche Veränderung des Magnetfeldes messen. Das Ziel war es, dadurch die Sensorpositionen zu bestimmen und die zugehörigen Kopfbewegungen zu berechnen, um mit diesen Informationen die prospektive Bewegungskorrektur zu implementieren. Dabei war der erste Schritt die Entwicklung eines eigenständigen Sende- und Empfangssystems zur Signalgeneration und -akquise der Sensoren. Dieses System bestand aus mikroelektronischen Komponenten und war nötig, um die Messungen der Sensoren unabhängig von der Hardware des Kernspintomographen durchführen zu können. Im zweiten Schritt sollte die Genauigkeit der Positionsbestimmung der Sensoren verbessert werden. Die Position der Sensoren wurde durch lineare Magnetfeldgradienten bestimmt, die nacheinander auf allen räumlichen Achsen geschaltet wurden. Echte Gradienten besitzen allerdings ein charakteristisches nichtlineares Verhalten, das ausgemessen werden musste, um das lineare Modell der Positionsbestimmung zu verbessern. Dazu wurden Messungen mit einem Sensor in verschiedenen bekannten Positionen durchgeführt sowie zusätzlich Messungen mit einer sogenannten Feldkamera, welche aus 16 dieser Sensoren besteht. Im letzten Schritt wurde dann das fertige System zur Bewegungskorrektur für verschiedene Bildgebungssequenzen getestet und schließlich mit einem anderen Bewegungskorrektursystem verglichen, welches auf einer optischen Kamera basiert

A 16-Channel Receive Array Insert for Magnetic Resonance Imaging of the Breast at 7T

Breast cancer is the second leading cause of cancer death among females in the United States. Magnetic resonance imaging (MRI) has emerged as a powerful tool for detecting and evaluating the disease, with notable advantages over other modalities, and the advent of ultra-high field strength scanners promises even more potential. In comparison to standard clinical MRI field strengths (1.5, 3.0 tesla), breast MRI at 7T provides increased signal-to-noise ratio (SNR) and spectral resolution. These benefits, however, are accompanied by significant challenges in hardware design, limiting the availability of commercial radiofrequency coils for 7T. The primary objective of this work is to enable the study of breast cancer at 7T with the development of a 16-channel receive array coil. The use of array coils to receive is standard in clinical MRI, as it provides higher SNR over a field of view than a single coil. In this case, when combined with the increased sensitivity provided by the high field strength, this will enable the ability to acquire images with higher resolution than could be achieved at 3T or 1.5T in clinically standard scan times. This has the potential to improve the morphological characterization of tumors and their involvement in the surrounding tissues. This thesis discusses the design and construction of a 16-channel receive array insert, characterization of its performance as an array, and comparison of the achievable SNR to a transmit-receive quadrature volume coil. With the 16-channel receive array insert, the results demonstrate a 6.5 times improvement in mean SNR and the ability to accelerate up to a reduction factor of 9 with a mean g-factor of 1.3. Finally, we present initial in vivo images acquired with the array, demonstrating the utility of the array coil through higher resolution imaging than the current protocols at lower field strengths

Engineering Parallel Transmit/Receive Radio-Frequency Coil Arrays for Human Brain MRI at 7 Tesla

Magnetic resonance imaging is widely used in medical diagnosis to obtain anatomical details of the human body in a non-invasive way. Clinical MR scanners typically operate at a static magnetic field strength (B0) of 1.5T or 3T. However, going to higher field is of great interest since the signal-to-noise ratio is proportional to B0. Therefore, higher image resolution and better contrast between the human tissues could be achieved. Nevertheless, new challenges arise when increasing B0. The wavelength associated with the radio-frequency field B1+ has smaller dimensions - approx. 12 cm for human brain tissues - than the human brain itself (20 cm in length), the organ of interest in this thesis. The main consequence is that the transmit field distribution pattern (B1+) is altered and the final MR images present bright and dark signal spots. These effects prevent the ultra-high field MR scanners (>= 7T) to be used for routine clinical diagnosis. Parallel-transmit is one approach to address these new challenges. Instead of using an RF coil connected to a single power input as it is commonly done at lower magnetic fields, multiple RF coils are used with independent power inputs. The subsequent distinct RF signals can be manipulated separately, which provides an additional degree of freedom to generate homogeneous B1+-field distributions over large or specific regions in the human body. A transmit/receive RF coil array optimized for whole-brain MR imaging was developed and is described in this thesis. Dipoles antennas were used since they could provide a large longitudinal (vertical axis-head to neck) coverage and high transmit field efficiency. Results demonstrated a complete coverage of the human brain, and particularly high homogeneity over the cerebellum. However, since the receive sensitivity over large field-of-views is related to the number of channels available to detect the NMR signal, the next work was to add a 32-channel receive loop coil array to the transmit coil array. The complete coverage of the human brain was assessed with a substantial increase in signal-to-noise compared to the transmit/receive dipole coil array alone. Moreover, acquisition time was shortened since higher acceleration factors could be used. To optimize the individual RF fields and generate an homogeneous B1+-field, a method was developed making use of the particle-swarm algorithm. A user-friendly graphical interface was implemented. Good homogeneity could be achieved over the whole-brain after optimization with the coil array built in this study. Moreover, the optimization was shown to be robust across multiple subjects. The last project was focused on the single transmit system. Local volume coils (single transmit) present pronounced transmit field inhomogeneities in specific regions of the human brain such as the temporal lobes. A widely used approach to address locally these challenges is to add dielectric pads inside the volume coils to enhance the local transmit field efficiency. It was shown in this thesis that constructing dedicated surface coils is a valuable alternative to the dielectric pads in terms of transmit field efficiency and MR spectroscopy results. Two RF coil setups were developed for the temporal and frontal lobes of the human brain, respectively. This thesis provides extensive insight on MR engineering of RF coils at ultra-high field and the potential of parallel-transmit to address the future needs in clinical applications

History and physical principles of MRI

International audienceThe first chapter of the three-volume Magnetic Resonance Imaging Handbook describes the historical and physical background of modern nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) methods and techniques

SAR Prediction and SAR Management for Parallel Transmit MRI

Parallel transmission enables control of the RF field in high-field Magnetic Resonance Imaging (MRI). However, the approach has also caused concerns about the specific absorption rate (SAR) in the patient body. The present work provides new concepts for SAR prediction. A novel approach for generating human body models is proposed, based on a water-fat separated MRI pre-scan. Furthermore, this work explores various approaches for SAR reduction

Radio Frequency Antenna Designs and Methodologies for Human Brain Computer Interface and Ultrahigh Field Magnetic Resonance Imaging

Brain Computer Interface (BCI) and Magnetic Resonance Imaging (MRI) are two powerful medical diagnostic techniques used for human brain studies. However, wired power connection is a huge impediment for the clinical application of BCI, and most current BCIs have only been designed for immobile users in a carefully controlled environment. For the ultrahigh field (≥7T) MRI, limitations such as inhomogeneous distribution of the transmit field (B1+) and potential high power deposition inside the human tissues have not yet been fully combated by existing methods and are central in making ultrahigh field MRI practical for clinical use. In this dissertation, radio frequency (RF) methods are applied and RF antennas/coils are designed and optimized in order to overcome these barriers. These methods include: 1) designing implanted miniature antennas to transmit power wirelessly for implanted BCIs; 2) optimizing a new 20-channel transmit array design for 7 Tesla MRI neuroimaging applications; and 3) developing and implementing a dual-optimization method to design the RF shielding for fast MRI imaging methods. First, three miniaturized implanted antennas are designed and results obtained using finite difference time domain (FDTD) simulations demonstrate that a maximum RF power of up to 1.8 miliwatts can be received at 2 GHz when the antennas are implanted at the dura, without violating the government safety regulations. Second, Eigenmode arrangement of the 20-channel transmit coil allows control of RF excitation not only at the XY plane but also along the Z direction. The presented results show the optimized eigenmode could generate 3D uniform transmit B1+ excitations. The optimization results have been verified by in-vivo experiments, and they are applied with different protocol sequences on a Siemens 7 Tesla MRI human whole body scanner equipped with 8 parallel transmit channels. Third, echo planar imaging (EPI), B1+ maps and S matrix measurements are used to verify that the proposed RF shielding can suppress the eddy currents while maintaining the RF characteristics of the transmit coil. The contributions presented here will provide a long-term and safer power transmission path compared to the wire-connected implanted BCIs and will bring ultrahigh field MRI technology closer to clinical applications

Hyperpolarized Xenon-129 Magnetic Resonance Imaging of Functional Lung Microstructure

Hyperpolarized 129Xe (HXe) is a non-invasive contrast agent for lung magnetic resonance imaging (MRI), which upon inhalation follows the functional pathway of oxygen in the lung by dissolving into lung tissue structures and entering the blood stream. HXe MRI therefore provides unique opportunities for functional lung imaging of gas exchange which occurs from alveolar air spaces across the air-blood boundary into parenchymal tissue. However challenges in acquisition speed and signal-to-noise ratio have limited the development of a HXe imaging biomarker to diagnose lung disease. This thesis addresses these challenges by introducing parallel imaging to HXe MRI. Parallel imaging requires dedicated hardware. This work describes design, implementation, and characterization of a 32-channel phased-array chest receive coil with an integrated asymmetric birdcage transmit coil tuned to the HXe resonance on a 3 Tesla MRI system. Using the newly developed human chest coil, a functional HXe imaging method, multiple exchange time xenon magnetization transfer contrast (MXTC) is implemented. MXTC dynamically encodes HXe gas exchange into the image contrast. This permits two parameters to be derived regionally which are related to gas-exchange functionality by characterizing tissue-to-alveolar-volume ratio and alveolar wall thickness in the lung parenchyma. Initial results in healthy subjects demonstrate the sensitivity of MXTC by quantifying the subtle changes in lung microstructure in response to orientation and lung inflation. Our results in subjects with lung disease show that the MXTC-derived functional tissue density parameter exhibits excellent agreement with established imaging techniques. The newly developed dynamic parameter, which characterizes the alveolar wall, was elevated in subjects with lung disease, most likely indicating parenchymal inflammation. In light of these observations we believe that MXTC has potential as a biomarker for the regional quantification of 1) emphysematous tissue destruction in chronic obstructive pulmonary disease (using the tissue density parameter) and 2) parenchymal inflammation or thickening (using the wall thickness parameter). By simultaneously quantifying two lung function parameters, MXTC provides a more comprehensive picture of lung microstructure than existing lung imaging techniques and could become an important non-invasive and quantitative tool to characterize pulmonary disease