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• W2981102023 abstract "Magnetic Resonance Imaging (MRI) is one of the most utilized imaging modalities in medical diagnosis. Key components of common clinical MRI systems are the superconducting main coils and the so-called ’gradient coils’. The former generate a strong static magnetic field (several Tesla of field strength) while the latter can be operated dynamically, producing linear magnetic field gradients. For maintaining the superconductive state, the main coils are cooled with liquid helium, which is enclosed in several metallic cylinders. This so-called cryostat is to some extent assembled with materials possessing high electrical and thermal conductivity. The dynamic fields generated by the gradient coils will induce eddy currents within these materials. Lorentz forces from the interaction of these currents with the static main magnetic field lead to mechanical vibrations of the conductive parts. These vibrations in turn induce eddy currents according to Lenz’s law, as the conductors move within a magnetic field. The fields emitted by these additional eddy currents initiate the described magnetomechanical interaction process on other conductive parts in the MRI system. Consequences of this magnetomechanical coupling are phenomena like image artifacts or heat loads caused by eddy current losses which show a strong resonant behavior. Whereas a dependency of eddy current losses on magnetomechanical interactions was already reported and could also be simulated for simple configurations, imaging artifacts as a cause of magnetomechanical effects were so far not described. Purpose of this work is to demonstrate this connection for the example of so-called ’ghosting artifacts’ in echo planar imaging and to provide methods for analyzing and mitigating negative magnetomechanical implications.Since a general analytical solution of the complex (non-linear) coupled mechanisms is not available, numerical simulations via finite elements are employed to describe and analyze the system behavior. Former investigations of this kind have been limited to axial symmetric gradient coils reducing the problem to two dimensions. This thesis expands the analysis to coil shapes demanding a complete three-dimensional approach. The developed framework is successfully applied to reproduce and reduce cryostatresonances excited by the magnetomechanical coupling that leads to excessive eddy current losses. Capabilities, limitations and the reliability of the presented simulation setup are discussed.The system understanding obtained from the finite element simulations is used to develop a fast and drastically simplified algorithm to calculate estimations concerning the magnetomechanical behavior of the MRI system. Therefore, it is assumed that the main characteristics of the system are described by modal participation factors of the cryogenic shielding, reacting to the excitation generated by the gradient coil stray field. This method allows the identification of crucial mode shapes concerning eddy current losses as well as ghosting artifacts. Measurements verify that a reduction of the ascribed participation factors leads to a clear mitigation of the related effects.An experimental evidence of the imaging relevant influence of magnetomechanical interactions is provided with a series of measurements using magnetic resonance techniques combined with direct magnetic flux measurements utilizing a pickup coil. The results allow to establish a dependency between the ghosting provoking magnetic fields in the imaging volume of the MRI system and fields emitted from movements of the cryostat." @default.
• W2981102023 created "2019-10-25" @default.
• W2981102023 creator A5062346644 @default.
• W2981102023 date "2019-09-04" @default.
• W2981102023 modified "2023-09-27" @default.
• W2981102023 title "Magnetomechanical interactions in clinical MRI cryostats" @default.
• W2981102023 hasPublicationYear "2019" @default.
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