Download or read book Applications of Parallel Radiofrequency Transmission to Ultra high field Magnetic Resonance Imaging written by Yan Tong and published by . This book was released on 2021 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Parallel Radiofrequency Transmission for 3 Tesla and 7 Tesla Magnetic Resonance Imaging written by Filiz Yetişir and published by . This book was released on 2017 with total page 156 pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic resonance imaging (MRI) is a noninvasive imaging technique with high soft tissue contrast. MR scanners are characterized by their main magnetic field strength. Commercially available clinical MR scanners commonly have main field strengths of 1.5 and 3 Tesla. Researchers increasingly explore clinical benefits of higher field strength scanners as they provide higher signal to noise ratio and higher resolution images. On the other hand, higher field strength imaging comes with increased image shading leading to non-uniform image contrast. Moreover, the tissue heating rate due to radiofrequency (RF) energy deposition (also called specific absorption rate or SAR) increases, limiting the imaging speed. Parallel RF transmission (pTx) was proposed to address both of these challenges by optimization of RF pulses transmitted from multiple independent channels simultaneously. However, both the RF pulse design and RF safety management become more complicated with pTx. In this work, a framework to apply pTx to 3T fetal and 7T brain imaging is developed to address the image shading and high SAR issues. Fetal imaging where a large pregnant torso is imaged rapidly to avoid fetal motion artifacts, suffers from similar levels of image shading and imaging limitations by SAR to 7T brain MRI. Hence the same techniques benefit both application domains. First, a SAR constrained pTx RF pulse design technique is developed for slice selective high flip angle imaging which is clinically the most common imaging technique. Next, the performance of the developed technique in reducing SAR and the image contrast non-uniformity is demonstrated through simulations and in phantom experiments for 7T brain imaging. Then, a comprehensive RF safety workflow for an 8 channel pTx system at 7T is developed. Finally, the potential of pTx for fetal imaging at 3T is demonstrated with simulation studies and a protected fetus mode of pTx was created using additional constraints in the RF pulse design. By addressing the two main RF transmission challenges associated with high and ultrahigh field MRI, this work aims to help bring the benefits of 7T brain imaging into routine clinical use and significantly improve the clinical experience for 3T fetal imaging.

Download or read book Non selective Refocusing Pulse Design in Parallel Transmission for Magnetic Resonance Imaging of the Human Brain at Ultra High Field written by Aurélien Massire and published by . This book was released on 2014 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: In Magnetic Resonance Imaging (MRI), the increase of the static magnetic field strength is used to provide in theory a higher signal-to-noise ratio, thereby improving the overall image quality. The purpose of ultra-high-field MRI is to achieve a spatial image resolution sufficiently high to be able to distinguish structures so fine that they are currently impossible to view in a non-invasive manner. However, at such static magnetic fields strengths, the wavelength of the electromagnetic waves sent to flip the water proton spins is of the same order of magnitude than the scanned object. Interference wave phenomena are then observed, which are caused by the radiofrequency (RF) field inhomogeneity within the object. These generate signal and/or contrast artifacts in MR images, making their exploitation difficult, if not impossible, in certain areas of the body. It is therefore crucial to provide solutions to mitigate the non-uniformity of the spins excitation. Failing this, these imaging systems with very high fields will not reach their full potential.For relevant high field clinical diagnosis, it is therefore necessary to create RF pulses homogenizing the excitation of all spins (here of the human brain), and optimized for each individual to be imaged. For this, an 8-channel parallel transmission system (pTX) was installed in our 7 Tesla scanner. While most clinical MRI systems only use a single transmission channel, the pTX extension allows to simultaneously playing various forms of RF pulses on all channels. The resulting sum of the interference must be optimized in order to reduce the non-uniformity typically seen.The objective of this thesis is to synthesize this type of tailored RF pulses, using parallel transmission. These pulses will have as an additional constraint the compliance with the international exposure limits for radiofrequency exposure, which induces a temperature rise in the tissue. In this sense, many electromagnetic and temperature simulations were carried out as an introduction of this thesis, in order to assess the relationship between the recommended RF exposure limits and the temperature rise actually predicted in tissues.This thesis focuses specifically on the design of all RF refocusing pulses used in non-selective MRI sequences based on the spin-echo. Initially, only one RF pulse was generated for a simple application: the reversal of spin dephasing in the transverse plane, as part of a classic spin echo sequence. In a second time, sequences with very long refocusing echo train applied to in vivo imaging are considered. In all cases, the mathematical operator acting on the magnetization, and not its final state as is done conventionally, is optimized. The gain in high field imaging is clearly visible, as the necessary mathematical operations (that is to say, the rotation of the spins) are performed with a much greater fidelity than with the methods of the state of the art. For this, the generation of RF pulses is combining a k-space-based spin excitation method, the kT-points, and an optimization algorithm, called Gradient Ascent Pulse Engineering (GRAPE), using optimal control.This design is relatively fast thanks to analytical calculations rather than finite difference methods. The inclusion of a large number of parameters requires the use of GPUs (Graphics Processing Units) to achieve computation times compatible with clinical examinations. This method of designing RF pulses has been experimentally validated successfully on the NeuroSpin 7 Tesla scanner, with a cohort of healthy volunteers. An imaging protocol was developed to assess the image quality improvement using these RF pulses compared to typically used non-optimized RF pulses. All methodological developments made during this thesis have contributed to improve the performance of ultra-high-field MRI in NeuroSpin, while increasing the number of MRI sequences compatible with parallel transmission.

Download or read book Computational Methods and Optimization Strategies for Parallel Transmission in Ultra High Field MRI written by Mihir Rajendra Pendse and published by . This book was released on 2019 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic resonance imaging (MRI) is a powerful imaging modality that is widely used in medicine for both clinical and research purposes. Despite its success, there is still a demand for improved image quality in the form of higher SNR and resolution and a promising approach to achieve this is with higher static field strengths (7 Tesla and above) corresponding to the ultra high frequency (UHF) regime of the RF pulse. At these frequencies, wavelength effects and complex interactions with biological tissue become problematic leading to field inhomogeneity artifacts and tissue heating concerns quantified by the specific absorption rate (SAR). This dissertation will focus on the excitation portion of the imaging process with parallel transmission (pTx) that involves using a transmit RF coil with multiple independent transmit channels. pTx is an effective way to address the challenges of ultra high field MRI through optimization of the transmitted pulse in a patient-specific way. We introduce the Iterative Minimization Procedure with Uncompressed Local SAR Estimate (IMPULSE) which is a novel distributed optimization algorithm that has favorable scaling properties and eliminates the need for virtual observation points (VOPs) thus resulting in superior SAR performance and shorter computation time. The optimization problem is to minimize SAR over a pulse sequence consisting of multiple slice excitations while ensuring that the flip angle inhomogeneity (FAI) for each excited slice is within some user specified tolerance. IMPULSE uses the alternating direction method of mulitpliers (ADMM) to split the optimization into two subproblems, a SAR-update and a FAI-update, that are solved at each iteration until convergence. The SAR-update can be formulated as an unconstrained minimization of a piecewise quadratic function which can be solved efficiently by using a bundle method to build a piecewise linear surrogate that can easily be minimized. The computation time for the FAI-update can be reduced by exploiting parallelization and using an efficient algorithm for projection of a point onto an ellipsoid. IMPULSE achieves superior SAR performance and reduced computation time compared to a conventional approach using virtual observation points or compared to using a generic sequential quadratic programming (SQP) solver in MATLAB. Using the Duke head model consisting of over six million voxels, minimum SAR pTx pulses were designed for 120 slices within 45 seconds with an FAI tolerance of 5\% at each slice. IMPULSE combined with variable rate selective excitation (VERSE) can also be used to improve SAR performance and reduce computation time for simultaneous multislice (SMS) excitation with a pTx-SMS pulse. This method (IMPULSE-SMS) was used for the pTx-SMS task of the ISMRM RF Pulse Design competition in 2016 and resulted in a pulse that was about 20\% shorter than the second best submission and about 10 times shorter than a conventional approach (SAR-unaware pulse design without VERSE). Increasing the number of transmit channels in a coil can give more degrees of freedom to achieve flip angle uniformity and reduce SAR but also increases cost and complexity of the hardware. Studying the performance of massively parallel transmit arrays in simulation can help determine whether investment in these arrays is justified based on new applications that are enabled. An 84 channel loop array for 10.5T with 6 rows and 14 columns was simulated using the Ella body model and applied to two novel applications: power independent of number of slices (PINS) pulses combined with pTx for SMS excitation and SAR focusing for therapeutic hyperthermia. Using this coil in addition to an insertable head gradient (slew rate of 1500 T/m/s), a pulse duration of about 13ms for a 16 slice coronal excitation with 0.4mm slice thickness with 10\% FAI was achieved. SAR focusing is possible for a range of locations throughout the head (although focusing is better at the periphery than at the center). A solution to a simplified bioheat equation indicates that achievable temperature rise would be within acceptable range for some forms of hyperthermia (but not high enough to achieve for ablation). A significant concern in SAR-aware pTx is mismatch between the patient and the tissue model used for SAR estimation since running the optimization on a mismatched model can result in significantly higher SAR compared to a perfect match. One technique to introduce robustness to this mismatch is to use the SAR terms for voxels of multiple tissue models (rather than a single model) in the cost function of IMPULSE. Results indicate that using multiple poorly matched models can achieve similar SAR performance compared to using a single closely matched model indicating that the multiple model approach is a way to get by with a sparse model library that doesn't fully represent the entire human population. A more sophisticated approach is to use deep learning to predict the 3D SAR maps from measured magnetic field maps. An initial implementation of this concept shows promise but is still inconclusive.

Download or read book Ultra High Field Neuro MRI written by Karin Markenroth Bloch and published by Elsevier. This book was released on 2023-08-21 with total page 628 pages. Available in PDF, EPUB and Kindle. Book excerpt: Ultra-High Field Neuro MRI is a comprehensive reference and educational resource on the current state of neuroimaging at ultra-high field (UHF), with an emphasis on 7T. Sections cover the MR physics aspects of UHF, including the technical challenges and practical solutions that have enabled the rapid growth of 7T MRI. Individual chapters are dedicated to the different techniques that most strongly benefit from UHF, as well as chapters with a focus on different application areas in anatomical, functional and metabolic imaging. Finally, several chapters highlight the neurological and psychiatric applications for which 7T has shown benefits. The book is aimed at scientists who develop MR technologies and support clinical and neuroscience research, as well as users who want to benefit from UHF neuro MR techniques in their work. It also provides a comprehensive introduction to the field. Presents the opportunities and technical challenges presented by MRI at ultra-high field Describes advanced ultra-high field neuro MR techniques for clinical and neuroscience applications Enables the reader to critically assess the specific UHF advantages over currently available techniques at clinical field strengths

Download or read book Understanding and Overcoming Head Motion in Ultra high Field Magnetic Resonance Imaging with Parallel Radio frequency Transmission written by Alix Plumley and published by . This book was released on 2022 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Parallel Transmission for Magnetic Resonance Imaging of the Human Brain at Ultra High Field written by Martijn Anton Hendrik Cloos and published by . This book was released on 2012 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The focus of this thesis lies on the development, and implementation, of parallel transmission (pTx) techniques in magnetic resonance imaging for flip-angle homogenization throughout the human brain at ultra-high field. In order to allow in-vivo demonstrations, a conservative yet viable safety concept is introduced to control the absorbed radiofrequency (RF) power . Subsequently, novel methods for local SAR control and non-selective RF pulse-design are investigated. The impact of these short and energy-efficient waveforms, referred to as kT-points, is first demonstrated in the context of the small-tip-angle domain. Targeting a larger scope of applications, the kT-points design is then generalized to encompass large flip angle excitations and inversions. This concept is applied to one of the most commonly used T1-weighted sequences in neuroimaging. Results thus obtained at 7 Tesla are compared to images acquired with a clinical setup at 3 Tesla, validating the principles of the kT-points method and demonstrating that pTx-enabled ultra-high field systems can also be competitive in the context of T1-weighted imaging. Finally, simplifications in the global design of the pTx-implementation are studied in order to obtain a more cost-effective solution.

Download or read book Parallel Imaging in Clinical MR Applications written by Stefan O. Schönberg and published by Springer Science & Business Media. This book was released on 2007-01-11 with total page 548 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book presents the first in-depth introduction to parallel imaging techniques and, in particular, to the application of parallel imaging in clinical MRI. It will provide readers with a broader understanding of the fundamental principles of parallel imaging and of the advantages and disadvantages of specific MR protocols in clinical applications in all parts of the body at 1.5 and 3 Tesla.

Download or read book Ultra high Field Magnetic Resonance Imaging Mri Instrumentation And Clinical Implementation written by Shaileshkumar B. Raval, Ph.D. and published by Index of Sciences Ltd. This book was released on 2021-04-06 with total page 340 pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic Resonance Imaging“Magnetic Resonance Imaging” (MRI) is the most widely clinically used diagnostic tool for soft tissue imaging. This advanced technology and its applications are under continuous research and development, ranging from lower fields to ultra-high fields to the highest possible fields for preclinical (animal) and human imaging. Formerly known as Nuclear Magnetic Resonance Imaging (NMR), with the rising demands of clinical diagnosis requirements, it is under constant development and innovation in hospitals for populations around the world because of constant desire to go to higher fields that lead to unique research and clinical applications that aren’t achievable with other commercially and or research technologies. The basics of MRIThe human body is rich in hydrogen, when a human body is subjected to a large magnetic field, many of the free hydrogen nuclei align themselves with the direction of the magnetic field. MRI works on the principle of the directional magnetic field associated with charged particles in motion. MRI is also known as nuclear magnetic resonance imaging, a technique used to create images of parts of the human body based on the resonance of nuclei in motion under the effect of a magnetic field. Overview of the bookThis book’s lucid style makes it an easy read. It is written in a simple and comprehensible way, making it easy to followand readfor a large audience ranging from students to researchers. The areas covered include an overview of the theories and practical aspects of High-Field MRIwith each chapter Introduction, Challenges, Objectives, Methods(Materials), Results, Discussion, FutureWworks,including basic concepts, along with research-oriented and clinical concepts, technologies that are researched and developed, and implemented clinically, and published nationally and internationally recognized conferences, and publications with global awards recognition from ISMRM, TTS, and many other academic and industry organizationsthat are recognized worldwide. In this book, unexplored research theories are described along with a list of products, project developments, and completion of major and unattempted theories, which are considered to be challenging in high-field MRI. These unexplored research theories are further delved into to emerge with practical and translational products, as described in various chapters. These products are deemed to be of potential research and clinical use if implemented in clinical and hospital settings, to help thus could the patients as well as healthy populations to improve the standard of their lives. Advances in extremities and musculoskeletal imaging in patients undergoing transplants, including first-ever(never been implemented)technologies such as Ultra high field upper extremity RF coils, research publications, and intellectual properties have been explored in detail. Another major advancement discussed in this book is the Whole-body MRI RF high density transmit coil and receiver array designs(first evevr application of antenna design), published in national and international journals as intellectual properties. Various other aspects of these 4intellectual properties have been discussed such as instrumentation developed, design procedures, Electromagnetic Simulations (simulated versions), Novel whole head(Brain) MRI RF array, Innovative Visualization Techniques, Neuro and vascular flow imaging, Segmentation methods. Regenerative Imaging, Pre and post-operative (surgical) imaging, clinical implementations, pulse sequence developments and optimizations, imaging resultswith 3D volume Texture and Visualizations, also peer research and references from around the world, plus future works, and more have been entailed. This is a rather different book in terms of depth and detail in which the subject is dealt with in this book. The data is well represented with tables, equations, and nearly three hundred figures. Combining technologies, research, and clinical applications of innovations in the field of MRI, it is one of a kind and a treat for curious minds. The content is mainly focused on whole head imaging, whole-body imaging, and extremity imaging, describing their clinical applications and their implementation for high risk and high demand patient populations, healthy populations for enhanced human anatomical, biological, functional and physiological performances in a detailed manner. The research has been utilized by peers in their studies, research, publications, and learning as part of the research and clinical developments, and implementations. This book presents the author’s original research works and their applications in the real world to offer advanced innovations to the healthcare sector and improve quality and standard of life for the masses around the world and beyond as future goals as there are many aviations,Biomedical Applications and projects are in demand. The author’s research works have been publishedand awarded in various nationalllyand internationallyrecognized journals and presented in numerous conferences as well. The chapters of this book are each one of the many research publications by the author

Download or read book Ultra High Field Magnetic Resonance Imaging written by Pierre-Marie Robitaille and published by Springer Science & Business Media. This book was released on 2007-12-31 with total page 487 pages. Available in PDF, EPUB and Kindle. Book excerpt: The foundation for understanding the function and dynamics of biological systems is not only knowledge of their structure, but the new methodologies and applications used to determine that structure. This volume in Biological Magnetic Resonance emphasizes the methods that involve Ultra High Field Magnetic Resonance Imaging. It will interest researchers working in the field of imaging.

Download or read book Rf Pulse Design for Parallel Excitation in Magnetic Resonance Imaging written by Yinan Liu and published by . This book was released on 2012 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Parallel excitation is an emerging technique to improve or accelerate multi-dimensional spatially selective excitations in magnetic resonance imaging (MRI) using multi-channel transmit arrays. The technique has potential in many applications, such as accelerating imaging speed, mitigating field inhomogeneity in high-field MRI, and alleviating the susceptibility artifact in functional MRI (fMRI). In these applications, controlling radiofrequency (RF) power deposition (quantified by Specific Absorption Rate, or SAR) under safe limit is a critical issue, particularly in high-field MRI. This \dissertation will start with a review of multidimensional spatially selective excitation in MRI and current parallel excitation techniques. Then it will present two new RF pulse design methods to achieve reduced local/global SAR for parallel excitation while preserving the time duration and excitation pattern quality. Simulations incorporating human-model based tissue density and dielectric property were performed. Results have show that the proposed methods can achieve significant SAR reductions without enlonging the pulse duration at high-fields.

Download or read book Contrast Enhanced Clinical Magnetic Resonance Imaging written by Val M. Runge and published by University Press of Kentucky. This book was released on 1997 with total page 200 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book Novel Radio Frequency Resonators for in Vivo Magnetic Resonance Imaging and Spectroscopy at Very High Magnetic Fields written by Xiaoliang Zhang and published by . This book was released on 2002 with total page 560 pages. Available in PDF, EPUB and Kindle. Book excerpt:

Download or read book A Flexible Coil Array for High Resolution Magnetic Resonance Imaging at 7 Tesla written by Roberta Kriegl and published by . This book was released on 2015 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic resonance imaging (MRI), among other imaging techniques, has become a major backbone of modern medical diagnostics. MRI enables the non-invasive combined, identification of anatomical structures, functional and chemical properties, especially in soft tissues. Nonetheless, applications requiring very high spatial and/or temporal resolution are often limited by the available signal-to-noise ratio (SNR) in MR experiments. Since first clinical applications, image quality in MRI has been constantly improved by applying one or several of the following strategies: increasing the static magnetic field strength, improvement of the radiofrequency (RF) detection system, development of specialized acquisition sequences and optimization of image reconstruction techniques. This work is concerned with the development of highly sensitive RF detection systems for biomedical ultra-high field MRI. In particular, auto-resonant RF coils based on transmission line technology are investigated. These resonators may be fabricated on flexible substrate which enables form-fitting of the RF detector to the target anatomy, leading to a significant SNR gain. The main objective of this work is the development of a flexible RF coil array for high-resolution MRI on a human whole-body 7 T MR scanner. With coil arrays, the intrinsically high SNR of small surface coils may be exploited for an extended field of view. Further, parallel imaging techniques are accessible with RF array technology, allowing acceleration of the image acquisition. Secondly, in this PhD project a novel design for transmission line resonators is developed, that brings an additional degree of freedom in geometric design and enables the fabrication of large multi-turn resonators for high field MR applications. This thesis describes the development, successful implementation and evaluation of novel, mechanically flexible RF devices by analytical and 3D electromagnetic simulations, in bench measurements and in MRI experiments.

Download or read book Applications of Gallium Nitride FETs to RF Arrays for Magnetic Resonance Imaging written by Michael D. Twieg and published by . This book was released on 2016 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Magnetic resonance imaging (MRI) is an imaging technique with incredible utility for medical applications, along with incredible technical complexity. Two of the primary subsystems of an MRI scanner are the RF transmit and receive chains, containing coils, switches, and amplifiers. The RF chains' fundamental architecture has changed little since its inception, with Silicon transistors dominating the roles of switches and amplifiers. Recent trends have pushed the RF chains toward parallelized design in order to increase image quality, reduce scan time, and/or improve patient safety. Parallel RF chains face technical challenges due to the density of electronics and cables required. For receive chains the challenge is modest; clinical scanners with 128 receive channels are already available. For transmit chains, the challenge is severe due to the large amount of power dissipation and transmission required. Currently commercial systems are limited to 8 transmit channels, and at very high cost. Here we advocate for the replacement of older circuits with more efficient versions based on high performance transistors. In particular, we demonstrate eGaN FETs as a replacement for PIN diodes for active detuning circuits in receive coils. Unlike PIN diodes, FETs can be controlled with voltage, and require almost no bias current, which drastically simplifies the design and construction of large receive arrays. We also demonstrate eGaN FETs for RF power amplifier (RFPA) modules aimed towards parallel transmit (pTX) chains with high channel counts. The eGaN RFPA modules demonstrate far higher power efficiency and density than linear RFPAs based on Si LDMOS FETs, and can operate within the scanner bore. Since our RFPAs are built on switchmode topologies, we explore the nonlinear and time-varying behavior of coupled amplifiers using experimental and numerical methods. Our results show that pTX chains based on these modules have higher efficiency and lower coupling than pTX chains built with linear RFPAs terminated with isolators, and that these benefits increase with the number of channels in the array. We anticipate that the low cost and high performance of the modules will drastically reduce the cost of pTX chains, greatly increasing the value of MRI to millions of patients.

Download or read book Electromagnetics in Magnetic Resonance Imaging written by Christopher M. Collins and published by Morgan & Claypool Publishers. This book was released on 2016-03-01 with total page 82 pages. Available in PDF, EPUB and Kindle. Book excerpt: In the past few decades, Magnetic Resonance Imaging (MRI) has become an indispensable tool in modern medicine, with MRI systems now available at every major hospital in the developed world. But for all its utility and prevalence, it is much less commonly understood and less readily explained than other common medical imaging techniques. Unlike optical, ultrasonic, X-ray (including CT), and nuclear medicine-based imaging, MRI does not rely primarily on simple transmission and/or reflection of energy, and the highest achievable resolution in MRI is orders of magnitude smaller that the smallest wavelength involved. In this book, MRI will be explained with emphasis on the magnetic fields required, their generation, their concomitant electric fields, the various interactions of all these fields with the subject being imaged, and the implications of these interactions to image quality and patient safety. Classical electromagnetics will be used to describe aspects from the fundamental phenomenon of nuclear precession through signal detection and MRI safety. Simple explanations and Illustrations combined with pertinent equations are designed to help the reader rapidly gain a fundamental understanding and an appreciation of this technology as it is used today, as well as ongoing advances that will increase its value in the future. Numerous references are included to facilitate further study with an emphasis on areas most directly related to electromagnetics.

Download or read book Microstrip Radio Frequency Coil and Array Design for Magnetic Resonance Imaging written by Bing Wu and published by Open Dissertation Press. This book was released on 2017-01-27 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: This dissertation, "Microstrip Radio-frequency Coil and Array Design for Magnetic Resonance Imaging" by Bing, Wu, 吳冰, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled Microstrip Radio-frequency Coil and Array Design for Magnetic Resonance Imaging submitted by Wu Bing For the degree of Doctor of Philosophy at The University of Hong Kong in September 2006 In the past five years, microstrip RF coils have been developed and widely applied for MR applications at high magnetic fields. In contrast to conventional surface coils, the microstrip coil uses the microstrip transmission line which in its simplest form consists of a thin strip conductor and a ground plane separated by a low-loss dielectric substrate. The semi-open, unbalanced structure microstrip generates a unique B field distribution mainly on one side of the coil where a sample is located. This results in several unique features: reduced losses, higher Q- factor, reduced coupling among multiple microstrip coils. In addition, the microstrip RF coil's unbalanced nature obviates the need of a matching balun. This dissertation presents the author's investigates in microstrip RF coil and array designs at the fields higher than 1.5 Tesla. Firstly, a novel tunable loop microstrip (TLM) RF coil based on the ring resonant circuit has been presented. SNR comparison between the TLM coil and conventional surface coil has been performed at various magnetic field strengths from 1.5 Tesla to 11.1 Tesla. Our study has demonstrated that utilization of the TLM coil can substantially reduce radiation loss and deliver better SNR performance than a conventional coil at ultra high fields. Results also indicate a trend to superior performance of the TLM coil as the field increases. Secondly, several current decoupling techniques have been utilized to the TLM planar array, which consists of two identical TLM coils elements. Simulation, bench test and MRI experiments have been carried out to provide a quantitative analysis of those decoupling schemes. Thirdly, the commonly used capacitive decoupling method for microstrip arrays has been analyzed. It appears that the decoupling capacitance is usually quite small at ultra-high fields and difficult to finely tune. A capacitively decoupled TLM array has been proposed, fabricated and tested at 7 Tesla. Using the TLM array, the capacitive decoupling method can be easily applied with reasonable decoupling capacitance. Bench test and MRI experiments at 7 Tesla show that excellent isolations (-37 dB) between the adjacent elements can be achieved and this TLM array is appropriate for SENSE imaging. Lastly, a new inductive decoupling approach for microstrip arrays has been presented at fields higher than 7 Tesla. In contrast to the capacitive decoupling methods, the decoupling inductance is independent of the resonant frequency, making this method much easier to be implemented. An inductively decoupled eight- channel microstrip array has been implemented and tested at 9.4 Tesla. This decoupling approach should enable more elements to be packed into microstrip arrays for the purpose of parallel imaging at ultrahigh fields. Number of words: 425 Signature: DOI: 10.5353/th_b3704672 Subjects: Magnetic resonance imaging