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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
Magnetic resonance imaging (MRI) is a widely used non-invasive imaging technology for both clinical diagnosis and neuroscientific research. However, the imaging sensitivity and specificity of brain MRI are limited by the well-known technical challenge of MRI acquisition-low image encoding efficiency, leading to limited acquisition speed, spatial resolution and signal-to-noise ratio especially for in-vivo imaging. In order to address these challenges, this thesis presents newly developed spatiotemporal encoding methods, which are used to improve the sensitivity and specificity as well as provide time and cost savings for different MRI applications, including diffusion, quantitative relaxometry and functional imaging. The novel encoding strategies in high-dimensional space together with efficient data sampling schemes allow better use of radio-frequency pulse, modern receiver coil arrays and shared data correlation. The high imaging efficiency provided by these spatiotemporal acquisition methods was demonstrated to help overcome several long-standing challenges in brain MRI, which should help increase its diagnosis power and gain further understanding of the structural and functional organization of the human brain.
​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.
The first book to cover the groundbreaking development and clinical applications of Magnetic Resonance Elastography, this book is essential for all practitioners interested in this revolutionary diagnostic modality. The book is divided into three sections. The first covers the history of MRE. The second covers technique and clinical applications of MRE in the liver with respect to fibrosis, liver masses, and other diseases. Case descriptions are presented to give the reader a hands-on approach. The final section presents the techniques, sequence and preliminary results of applications in other areas of the body including muscle, brain, lung, heart, and breast.
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
This book is designed to introduce the reader to the field of NMR/MRI at very low magnetic fields, from milli-Tesla to micro-Tesla, the ultra-low field (ULF) regime. The book is focused on applications to imaging the human brain, and hardware methods primarily based upon pre-polarization methods and SQUID-based detection. The goal of the text is to provide insight and tools for the reader to better understand what applications are best served by ULF NMR/MRI approaches. A discussion of the hardware challenges, such as shielding, operation of SQUID sensors in a dynamic field environment, and pulsed magnetic field generation are presented. One goal of the text is to provide the reader a framework of understanding the approaches to estimation and mitigation of low signal-to-noise and long imaging time, which are the main challenges. Special attention is paid to the combination of MEG and ULF MRI, and the benefits and challenges presented by trying to accomplish both with the same hardware. The book discusses the origin of unique relaxation contrast at ULF, and special considerations for image artifacts and how to correct them (i.e. concomitant gradients, ghost artifacts). A general discussion of MRI, with special consideration to the challenges of imaging at ULF and unique opportunities in pulse sequences, is presented. The book also presents an overview of some of the primary applications of ULF NMR/MRI being pursued.