On the Employment of Matrix Gradient Coils for Novel Spatial Encoding and Magnetic Field Shimming in Magnetic Resonance Imaging PDF Download
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Author: Stefan Kroboth
Publisher:
ISBN:
Category : Magnetic resonance imaging
Languages : en
Pages :
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Book Description
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
Author: Stefan Kroboth
Publisher:
ISBN:
Category : Magnetic resonance imaging
Languages : en
Pages :
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Book Description
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
Author: Yoseph Shiferaw
Publisher:
ISBN:
Category :
Languages : en
Pages : 174
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Book Description
Author:
Publisher:
ISBN:
Category : Magnetic resonance imaging
Languages : en
Pages : 328
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Book Description
Abstract: This research concentrates on two major engineering areas associated with biomedical instrumentation that have recently gained significant academic and industrial interest: the gradient coil design for Magnetic Resonance Imaging (MRI) and the high frequency full-wave field simulations with the Method of Moments (MoM). A new computational approach to the design of gradient coils for magnetic resonance imaging is introduced. The theoretical formulation involves a constrained cost function between the desired field in a particular region of interest in space and the current-carrying coil plane. Based on Biot-Savart's integral equation, an appropriate weight function is introduced in conjunction with linear approximation functions. This permits the transformation of the problem formulation into a linear matrix equation whose solution yields discrete current elements in terms of magnitude and direction within a specified coil plane. These current elements can be synthesized into practical wire configuration by suitably combining the individual wire loops. Numerical predictions and measurements underscore the success of this approach in terms of achieving a highly linear field while maintaining low parasitic fields, low inductance and a sufficient degree of shielding. Experimental results confirm the field predictions of the computational approach. Extending the numerical modeling efforts to dynamic phenomena, a novel MoM formulation permits the computation of electromagnetic fields in conductive surfaces and in three-dimensional biological bodies. The excitation can be provided with current loops, voltage sources, or an incident electromagnetic wave. This method enables us to solve a broad spectrum of problems arising in MRI: full-wave RF coil simulations, eddy currents predictions in the magnet bore, and induced currents in the biological body. Surfaces are represented as triangles and the three-dimensional bodies are subdivided into tetrahedra. This numerical discretization methodology makes the approach very flexible to handle a wide range of practical coil geometries. Specifically, in this thesis the MoM is employed to study the effect of switching gradient coils in the presence of a biological load.
Author: Silvia Hidalgo-Tobon
Publisher: LAP Lambert Academic Publishing
ISBN: 9783659610257
Category :
Languages : en
Pages : 172
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Book Description
This work describes three new developments in the design and use of gradient coils in MRI. A novel gradient coil, consisting of folded loop current path has been developed. In this design, the current return paths for the inner coil are folded over onto the outer screening coil and both coils have the same length. This sort of design will potentially allow more efficient short coils with better screening to be designed. Mathematical expressions for the magnetic field produced by such coils have been developed and tested by comparison with the results of field calculations based on the elemental form of the Biot-Savart expression. A small prototype coil has been built and tested. The field variation which it produces is in good agreement with that calculated from the wire paths.
Author: Donald W. McRobbie
Publisher: Cambridge University Press
ISBN: 1316688259
Category : Medical
Languages : en
Pages : 405
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Book Description
MR is a powerful modality. At its most advanced, it can be used not just to image anatomy and pathology, but to investigate organ function, to probe in vivo chemistry, and even to visualise the brain thinking. However, clinicians, technologists and scientists struggle with the study of the subject. The result is sometimes an obscurity of understanding, or a dilution of scientific truth, resulting in misconceptions. This is why MRI from Picture to Proton has achieved its reputation for practical clarity. MR is introduced as a tool, with coverage starting from the images, equipment and scanning protocols and traced back towards the underlying physics theory. With new content on quantitative MRI, MR safety, multi-band excitation, Dixon imaging, MR elastography and advanced pulse sequences, and with additional supportive materials available on the book's website, this new edition is completely revised and updated to reflect the best use of modern MR technology.
Author: Christopher M. Collins
Publisher: Morgan & Claypool Publishers
ISBN: 1681740834
Category : Medical
Languages : en
Pages : 82
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Book Description
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.
Author: B. J. Fisher
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Author: Robert W. Brown
Publisher: John Wiley & Sons
ISBN: 0471720852
Category : Medical
Languages : en
Pages : 976
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Book Description
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.
Author: John Stuart Beeteson
Publisher: Academic Press
ISBN: 0120847310
Category : Computers
Languages : en
Pages : 171
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Book Description
CD-ROM contains: Application programs with sample data files providing worked examples -- Full source code for visualisation application program.
Author: Chih-Wei Chang
Publisher:
ISBN:
Category : Electric coils
Languages : en
Pages : 140
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Book Description
