| Magnetic resonance imaging (MRI) scanners are powerful medical diagnostic tools in hospital clinic. However, high levels of acoustic noise during scanning make the diagnostic process extremely uncomfortable. Many important factors, including the structural dynamic properties of gradient coils, contribute greatly to the vibro-acoustic problem. From the viewpoint of structural dynamics, the vibro-acoustic mechanism, the main noise sources, the source-path-response effects, and the dynamic behavior of gradient coils are comprehensively reviewed and investigated. All these analyses and investigations indicate that in-depth study on the dynamic behavior of gradient coils and building a dynamic model are desirable but have to date received relatively little attention.; Dynamic modeling of gradient coils were carried out in the dissertation to improve the understanding of the vibro-acoustic mechanism, to predict the vibration of the gradient coil, and to develop potential tools for designing "quiet" MRI machines in the future. To accomplish these goals, analytical, numerical and experimental modeling approaches are implemented concurrently. Three models, a thin-walled cylinder model, a single-layered gradient coil, and a multi-layered gradient coil were built and investigated based on the Love's equations and their modified forms. Their modal features and vibration patterns are completely disclosed. By means of finite element (FE) software, numerical models of the three models with different boundary conditions are explored as well. FE simulations verify these modal features, and show the different boundary conditions heavily affecting the modal features of the gradient coil. By using a state-of-the-art experimental apparatus, a laser vibrometer and a portable spectrum analyzer, modal testing and in-situ measuring of gradient coils in a MRI were carried out. The measured mode shapes and vibration patterns not only validate the analytical and numerical simulations, but also reveal the real vibration behavior during MRI scanning.; These analytical, numerical and experimental results mutually agree each other and provide the insight into the dynamic behavior of the gradient coil, which should provide significant guidance in the future design of "quiet" gradient coils. To our knowledge, this study is the first time the dynamic behavior of a gradient coil used in MRI scanning has been investigated. |
