Acoustic analysis of MRI scanners

MRI (Magnetic Resonance Imaging) scanners can generate a tremendous amount of noise during a scan. This acoustic noise is mainly caused by the vibration of the gradient coil system due to the Lorentz forces generated by the interaction of electrical current in the conductors of the gradient coil and the main static magnetic field. The main objective of this research is to develop computational models to describe the sound radiation characteristics of gradient coils and the simulation results will be used for the design of low noise gradient coils for MRI scanners.; By simplifying the gradient coils in MRI scanners to a finite axisymmetric cylindrical duct, the development of a baffled finite cylindrical duct model, a BEM (Boundary Element Method) model and the Green's function model are presented in this thesis. For the analytical models (the baffled finite duct model and Green's function model), the sound wave reflection at the duct open ends was described by the general radiation impedances. All of the acoustical models developed in this thesis showed similar results for the sound field of gradient coils. The observation of acoustic energy concentration around the cut-off frequencies makes it possible for MRI scanner designers to avoid designing inherently noisy gradient coils. However, the analytical models run much faster than the BEM model and can demonstrate the relationship between the MRI noise and the geometry (diameter) of the gradient coils more directly.; Acoustic measurements on a 4 Tesla MRI scanner verified the results obtained from the computational models that showed the acoustic energy concentration around the cut-off frequencies. Experiments on a gradient coil insert to investigate the noise attenuation efficiency of using acoustically absorptive materials showed that the noise reduction was significant when acoustic foam was applied on the inner wall of the insert.; Prediction of the acoustic performance of liners was performed based on the vibro-acoustic analyses of the coupling of the FE (Finite Element) structural model and BE (Boundary Element) acoustical model. The simulation results showed that MRI noise could be significantly reduced by using thicker liners or putting some acoustically absorptive materials on the inner wall of the liner.