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​Within the past few decades MRI has become one of the most important imaging modalities in medicine. For a reliable diagnosis of pathologies further technological improvements are of primary importance. This study deals with a radically new approach of image encoding. Gradient linearity has ever since been an unquestioned technological design criterion. With the advent of parallel imaging, this approach may be questioned, making way of much a more flexible gradient hardware that uses encoding fields with an arbitrary geometry. The theoretical basis of this new imaging modality – PatLoc imaging – are comprehensively presented, suitable image reconstruction algorithms are developed for a variety of imaging sequences and imaging results – including in vivo data – are explored based on novel hardware designs.
Abstract: Magnetic Resonance Imaging (MRI) is a tomographic imaging modality commonly used for diagnosis in medicine. In an attempt to push the limits of MRI, quadratic magnetic fields were recently added to the image encoding process. While this concept was shown to aid the image acquisition, it is yet unclear, which properties such nonlinear fields should have in order to fully exploit their potential. Therefore in the recent past so-called matrix gradient coils, which consist of a large number of small coils, were introduced. The current in each such coil element can be adjusted individually, and the final field shape is given by the superposition of the fields of all coil elements. Such an approach on one hand allows for a wide range of different field shapes. On the other hand it necessitates as many amplifiers as coil elements, which can be expensive and technically challenging. The first part of this thesis introduces a method for overcoming the above-mentioned problem by driving the matrix gradient coil with fewer amplifiers than coil elements. This is achieved by first finding a so-called configuration, which defines a network of coils capable of approximating a desired field shape. Since most image encoding strategies in MRI require more than a single field, one configuration per target field is obtained. Then a switching circuit is optimized, which is able to switch between the set of configurations with a low number of switches. While nonlinear fields have shown to add additional degrees of freedom to the image acquisition process, it remained unclear how to utilize them for image encoding in MRI most efficiently. Therefore the second part this thesis introduces an algorithm, which obtains ways to drive the acquisition of the MR signal by efficiently utilizing the available hardware (gradient coils with arbitrary field geometries, radio-frequency receiver coils) such that the overall acquired information content is maximized. This approach can also be used as a means to investigate the interplay of spatial encoding steps and local radio-frequency receiver coils, which may help to find ways of driving the available hardware, such that imperfections of one component are compensated for by another component while reducing the number of required encoding steps. In the past, hardware components where typically designed independent of each other, but with the insights gained from this method, it may in the future be possible to design components in parallel while considering their interactions with each other. This may in the future lead to faster and higher quality image acquisition, which is beneficial for both the operation of the MRI as well as the patients
Magnetic resonance systems are used in almost every academic and industrial chemistry, physics and biochemistry department, as well as being one of the most important imaging modalities in clinical radiology. The design of such systems has become increasingly sophisticated over the years. Static magnetic fields increase continuously, large-scale arrays of receive elements are now ubiquitous in clinical MRI, cryogenic technology has become commonplace in high resolution NMR and is expanding rapidly in preclinical MRI, specialized high strength magnetic field gradients have been designed for studying the human connectome, and the commercial advent of ultra-high field human imaging has required new types of RF coils and static shim coils together with extensive electromagnetic simulations to ensure patient safety. This book covers the hardware and engineering that constitutes a magnetic resonance system, whether that be a high-resolution liquid or solid state system for NMR spectroscopy, a preclinical system for imaging animals or a clinical system used for human imaging. Written by a team of experts in the field, this book provides a comprehensive and instructional look at all aspects of current magnetic resonance technology, as well as outlooks for future developments.
Abstract: The goal of this thesis was to design, implement and test a shielded matrix gradient coil for magnetic resonance imaging (MRI). The design process addressed gradient strength, flexibility, magnetic shielding, cooling, electrical decoupling, balance of force and torque and patient safety. These demands were fulfilled by designing two different coil element types which form a cylindrical coil configuration containing two main current carrying surfaces and a shared shielding surface. For manufacturing and handling reasons the coil elements were designed such that each coil element can be manufactured and tested individually. Scaling the dimensions of a whole-body gradient coil to an insert coil led to a total of 7 rings with 12 elements each, summing up to a total of 84 elements. The resulting modular design led to a successful patent application. All 84 coil element channels were manufactured in-house using a powder bed ink-jet head 3D printing technology and assembled with the water cooling. Before integrating the realized matrix coil prototype into a 3T MRI environment, its electrical and thermal behavior was characterized experimentally. The gradient strength, eddy current behavior, acoustic response and the resulting field maps were characterized within the scanner environment. Established imaging methods were implemented and the resulting images prove the successful realization and integration of the coil. In vivo imaging experiments were performed after a satisfying safety assessment. The flexibility regarding the realizable nonlinear spatial encoding magnetic field (SEM) shapes of the realized coil prototype allows for novel imaging methodologies. This is demonstrated in this thesis by deploying such SEM for simultaneous multislice imaging. The simultaneous excitation of multiple slices with standard single-band radio frequency pulses was explored. Additionally frequency shifting of signals from different slices was demonstrated, which in principle allows for parallel imaging without additional information from radio frequency receiver array coils
Real-time magnetic resonance imaging (MRI) provides superior imaging environments for diagnosing and assessing heart disease due to its imaging speed and interactivity. This dissertation presents novel gradient design methods for robust MR flow imaging and low-power selective excitation where the interactivity permits the use of more efficient gradient waveforms. The peak velocity of stenotic blood flow can be measured more reliably by adopting more robust readout k-space trajectories against the artifacts from non-constant velocity and off-resonance. Peak RF power of selective excitation can be reduced by controlling the traversing speed in excitation k space. The peak velocity of transvalvular blood flow is a widely used metric in evaluating the severity of valvular stenosis. Doppler ultrasound (US) has been routinely used clinically due to the ability to provide velocity spectrum in real time. MR Doppler is the MR equivalent of Doppler US. It has been developed to provide a real-time velocity distribution of valvular blood flow. When compared to US, MRI has the advantage of providing unrestricted access to structures throughout the chest where US performance is degraded by poor acoustic window conditions caused by air or bone in the pathway. Moreover, MRI can assess various types of heart disease such as coronary disease, heart muscle abnormalities, tumors, and valve disease. In the context of comprehensive cardiac MRI, MR Doppler has been developed to provide a real-time velocity distribution of valvular blood flow. To better assess stenotic flow, both spatial excitation and data acquisition methods of the MR Doppler are improved through efficient k-space schemes. In particular, peak velocity detection capability can be improved by adopting an echo-shifted interleaved readout with a variable-density and circular k-space trajectory. The artifacts from non-constant velocity and off-resonance are reduced by the shorter echo and readout times of the echo-shifted interleaved acquisitions and temporal and spatial resolutions are improved through the variable-density and circular k-space sampling approach. A novel multipoint-traversing algorithm is introduced to achieve flexible gradient-waveform design. MR Doppler uses 2-D RF pulses. These are limited by the gradient and RF systems. A pulse needs to be either reshaped or redesigned when the peak RF power exceeds hardware or safety limits. Such RF power adjustment needs to be done online when RF waveforms are designed to reflect subject dependent main and RF field inhomogeneities as in parallel transmit. Variable-rate selective excitation (VERSE) technique can be used to limit the peak RF power without disrupting the on-resonance profile while minimizing the amount of reshaping via a local-only RF and gradient scaling. A simple and robust VERSE-guided RF pulse design framework is developed as an online RF reshaping tool in controlling peak RF power. VERSE principle is first generalized to a broader domain of multidimensional and multichannel excitation, where the conditions of identical spin rotation are formulated in excitation k space. Then, a noniterative time-optimal design method for VERSE is developed by translating peak RF limits into a gradient upper bounds in s domain, where the gradient upper bounds are used for a time-optimal gradient waveform designs. Implementation considerations are discussed to improve the fidelity of VERSE operation and an iterative approach is introduced to resolve the potential deviation of peak RF magnitude from the target value.
New edition explores contemporary MRI principles and practices Thoroughly revised, updated and expanded, the second edition of Magnetic Resonance Imaging: Physical Principles and Sequence Design remains the preeminent text in its field. Using consistent nomenclature and mathematical notations throughout all the chapters, this new edition carefully explains the physical principles of magnetic resonance imaging design and implementation. In addition, detailed figures and MR images enable readers to better grasp core concepts, methods, and applications. Magnetic Resonance Imaging, Second Edition begins with an introduction to fundamental principles, with coverage of magnetization, relaxation, quantum mechanics, signal detection and acquisition, Fourier imaging, image reconstruction, contrast, signal, and noise. The second part of the text explores MRI methods and applications, including fast imaging, water-fat separation, steady state gradient echo imaging, echo planar imaging, diffusion-weighted imaging, and induced magnetism. Lastly, the text discusses important hardware issues and parallel imaging. Readers familiar with the first edition will find much new material, including: New chapter dedicated to parallel imaging New sections examining off-resonance excitation principles, contrast optimization in fast steady-state incoherent imaging, and efficient lower-dimension analogues for discrete Fourier transforms in echo planar imaging applications Enhanced sections pertaining to Fourier transforms, filter effects on image resolution, and Bloch equation solutions when both rf pulse and slice select gradient fields are present Valuable improvements throughout with respect to equations, formulas, and text New and updated problems to test further the readers' grasp of core concepts Three appendices at the end of the text offer review material for basic electromagnetism and statistics as well as a list of acquisition parameters for the images in the book. Acclaimed by both students and instructors, the second edition of Magnetic Resonance Imaging offers the most comprehensive and approachable introduction to the physics and the applications of magnetic resonance imaging.
In 1971 Dr. Paul C. Lauterbur pioneered spatial information encoding principles that made image formation possible by using magnetic resonance signals. Now Lauterbur, "father of the MRI", and Dr. Zhi-Pei Liang have co-authored the first engineering textbook on magnetic resonance imaging. This long-awaited, definitive text will help undergraduate and graduate students of biomedical engineering, biomedical imaging scientists, radiologists, and electrical engineers gain an in-depth understanding of MRI principles. The authors use a signal processing approach to describe the fundamentals of magnetic resonance imaging. You will find a clear and rigorous discussion of these carefully selected essential topics: Mathematical fundamentals Signal generation and detection principles Signal characteristics Signal localization principles Image reconstruction techniques Image contrast mechanisms Image resolution, noise, and artifacts Fast-scan imaging Constrained reconstruction Complete with a comprehensive set of examples and homework problems, Principles of Magnetic Resonance Imaging is the must-read book to improve your knowledge of this revolutionary technique.