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

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 Following the introduction, five different published and yet unpublished papers for improving the utility of MRI are shown.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

Homogene und heterogene Resonatoren in der Ultrahochfeld-MRT

The main objective of this study was to construct, to improve and to evaluate the performance of different radiofrequency (RF) coils for ultrahigh-field (UHF) at 7T magnetic resonance imaging (MRI) with respect to signal-to-noise ratio (SNR), B1 homogeneity, and effective flip angle (FA) maps. Best magnetic resonance (MR) images are obtained if RF coils fulfill the two fundamental requirements: the B1 field in the excited volume respective region of interest (ROI) has to be homogeneous while the received SNR has to be as high as possible. However, the major problems associated at UHF conditions are amplified imaging artifacts, the shortened transversal relaxation times of tissue, the inhomogeneous B1 field in the ROI, and the increased absorption of RF energy. Although all RF coils in UHF MRI intrinsically generate inhomogeneous B1 fields due to the dielectric effect rather than the RF coil geometry, each RF coil may exhibit special characteristics, which makes them optimized for special MRI applications. They should therefore be designed according to the desired field of operation to obtain optimal performance MRI. The developed UHF RF coils were categorized according to their architecture, i.e., into homogeneous and heterogeneous coils. Four homogeneous transmit/receive (Tx/Rx) circular polarized (CP) volume coils - hybrid birdcage (HBC), hybrid spiral birdcage (HSBC), dual Helmholtz (DH), and slotted tube (ST) - were constructed, compared and evaluated at a magnetic field strength of 7T. An evaluation procedure for comparing the B1 field inhomogeneity and SNR is proposed. In particular, B1 excitation inhomogeneity has to be considered and compared between the different RF coils to improve the differences in the contrast and the spatially varying image signal in the ROI. Heterogeneous coils such as surface coils are usually much smaller than homogeneous coils. They are located nearer the object and hence usually have higher SNR because they receive higher signal from the ROI. However, they have a relatively poor B1 field homogeneity, and thus, they are mainly utilized as Rx-only coils. In an additional stage, a 16 element parallel Tx (pTx) coil was designed, constructed and evaluated for use as a head-array. The RF coil was connected to a 16 × 16 Butler matrix network operated by an eight-channel parallel Tx system to investigate the improvement of the B1 excitation. The transmit profiles were evaluated for various clockwise CP modes with respect to homogeneity. Compared to the CP coil architectures the 16 element pTx coil provides much more potential to improve excitation characteristics.Das Hauptziel dieser Arbeit war die Konstruktion, Evaluierung und Verbesserung verschiedener Hochfrequenz (HF) Spulen für Ultra-Hochfeld (UHF) Magnetresonanztomographie (MRT) bei 7T unter Berücksichtigung des Signal-zu-Rauschabstands (SNR), der B1 Homogenität und des effektiven Flip-Winkels (FA). Die besten MR-Bilder erhält man, wenn zwei fundamentale Bedingungen erfüllt sind: das B1 Feld im angeregten Volumen bzw. im betrachteten Bereich (ROI) sollte möglichst homogen und die SNR im Empfangsfall möglichst hoch sein. Dennoch kommt es unter Hochfeld-Bedingungen oft zu verstärkten Bildartefakten durch verkürzte transversale Relaxationszeiten im Gewebe, B1-Feldinhomogenitäten in der ROI und einer erhöhten Energieabsorption im untersuchten Objekt. Obwohl alle HF-Spulen eher durch dielektrische Effekte als durch die Spulengeometrie ein inhomogenes B1-Feld generieren, kann jede Spule spezielle Charakteristiken aufweisen, so dass sie für spezielle Anwendungen optimiert werden kann. Um die gewünschte Feldverteilung zu erhalten, sollte das Design der Spule entsprechend angepasst werden. Die entwickelten UHF HF Spulen unterscheidet man nach homogenen und heterogenen Spulentypen. Es wurden 4 zirkular polarisierte Sende/Empfangsspulen - Hybrid Birdcage (HBC), Hybrid spiral Birdcage (HSBC), dual Helmholtz (DH) und slotted tube (ST) - konstruiert, verglichen und mit Hilfe eines 7T MRT-Systems evaluiert. Für den Vergleich verschiedener Spulen hinsichtlich SNR und B1-Feld wird in dieser Arbeit ein Konzept vorgeschlagen. Für den Vergleich und zur Verbesserung der Kontrastdifferenzen und des örtlich variierenden Bildsignals müssen insbesondere die B1-Feldinhomogenitäten betrachtet und für die verschiedenen Spulen verglichen werden. Heterogene Spulen wie Oberflächenspulen sind gewöhnlich deutlich kleiner als homogene Spulen und sind nahe am Objekt lokalisiert. Daher haben sie in der Regel ein höheres SNR. Andererseits haben sie eine relativ schlechte B1-Feldhomogenität und werden daher in der Regel als reine Empfangsspulen genutzt. In einem weiteren Abschnitt wurde eine 16 Element Parallel-Transmit (pTx) Spule entwickelt, gebaut und evaluiert, mit der Zielstellung, sie als Kopfspule einzusetzen. Die Spule wurde mittels einer 16 × 16 Butler-Matrix an ein 8 Kanal Parallel-Transmit-System angeschlossen, um die Butler-Matrix anzusteuern und die Optimierung der B1 Anregung zu untersuchen. Die Transmit Profile wurden evaluiert für verschiedene CP Moden im Uhrzeigersinn hinsichtlich ihrer Homogenität. Verglichen mit der CP Spulenarchitektur besitzt die 16-Element-pTx-Spule ein höheres Potential bei der Verbesserung der Anregungscharakteristik

WHOLE BODY AND UPPER EXTREMITY ULTRA-HIGH FIELD MAGNETIC RESONANCE IMAGING: COIL DEVELOPMENT AND CLINICAL IMPLEMENTATION

Since Magnetic Resonance Imaging (MRI)’s introduction into the clinical imaging application arena, MRI has become one of the most promising non-invasive methods for evaluating and identifying body organs in normal and diseased conditions. In the last two decades, a few research groups have been working on addressing the challenges to Ultra-High Field (UHF) imaging (≥ 7 Tesla), such as magnetic field inhomogeneities and elevated Radiofrequency (RF) power absorption through technological developments. In recent years, imaging at 7 Tesla has shown an inherent ability to improve scan time and anatomical resolution. To address the current challenges associated with UHF imaging, this thesis presents the development of innovative whole body and extremity RF coil systems for 7 Tesla imaging. For body imaging, the transmit (Tx) coil is based on the innovative Tic-Tac-Toe (TTT) design, which possesses a load insensitive characteristic in terms of magnetic and electric field distributions. 7 Tesla homogenous whole-body in-vivo imaging with and without a receive (Rx) only insert array is demonstrated showing excellent anatomical detail. As a part of upper extremity imaging, we have developed a transverse electromagnetic (TEM) coil as a transmitter in conjunction with an eight channel receive only insert for 7 Tesla hand/forearm imaging. We have acquired a wide variety of different sequences and used post-processing methods to extract specific anatomy from high resolution scans (i.e. nerve and vessels), which in turn has helped in exploring new clinical applications, such as arm transplantation, and has added knowledge to existing ones. The developed RF coil systems and methodologies not only enhance the fundamental scientific knowledge of RF coil design approaches at high frequencies but they also add to the realm of clinical applications of UHF human imaging

A comparison study of different RF shields for an 8-element transceive small animal array at 9.4T

In this study, three types of radio-frequency shields are studied and compared in the context of ultra-high field small-animal magnetic resonance imaging. It has been demonstrated that the coil penetration depth and mutual coupling between the coils depend heavily on the type of shield employed. The results were used to guide the design of a 9.4T 8-element transceive small animal array, which provides high overall coil penetration

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

Diagnostic Significance of Exosomal miRNAs in the Plasma of Breast Cancer Patients

Poster Session AbstractsBackground and Aims: Emerging evidence that microRNAs (miRNAs) play an important role in cancer development has opened up new opportunities for cancer diagnosis. Recent studies demonstrated that released exosomes which contain a subset of both cellular mRNA and miRNA could be a useful source of biomarkers for cancer detection. Here, we aim to develop a novel biomarker for breast cancer diagnosis using exosomal miRNAs in plasma. Methods: We have developed a rapid and novel isolation protocol to enrich tumor-associated exosomes from plasma samples by capturing tumor specific surface markers containing exosomes. After enrichment, we performed miRNA profiling on four sample sets; (1) Ep-CAM marker enriched plasma exosomes of breast cancer patients; (2) breast tumors of the same patients; (3) adjacent non-cancerous tissues of the same patients; (4) Ep-CAM marker enriched plasma exosomes of normal control subjects. Profiling is performed using PCR-based array with human microRNA panels that contain more than 700 miRNAs. Results: Our profiling data showed that 15 miRNAs are concordantly up-regulated and 13 miRNAs are concordantly down-regulated in both plasma exosomes and corresponding tumors. These account for 25% (up-regulation) and 15% (down-regulation) of all miRNAs detectable in plasma exosomes. Our findings demonstrate that miRNA profile in EpCAM-enriched plasma exosomes from breast cancer patients exhibit certain similar pattern to that in the corresponding tumors. Based on our profiling results, plasma signatures that differentiated breast cancer from control are generated and some of the well-known breast cancer related miRNAs such as miR-10b, miR-21, miR-155 and miR-145 are included in our panel list. The putative miRNA biomarkers are validated on plasma samples from an independent cohort from more than 100 cancer patients. Further validation of the selected markers is likely to offer an accurate, noninvasive and specific diagnostic assay for breast cancer. Conclusions: These results suggest that exosomal miRNAs in plasma may be a novel biomarker for breast cancer diagnosis.link_to_OA_fulltex