Simulation study of noise reduction methods for a split MRI system using a finite element method.

PURPOSE To evaluate the acoustic noise levels induced by the split-gradient coils of a MRI-LINAC system, and also propose a scheme for acoustic noise reduction. METHODS A finite element (FE) modeling method was used to simulate the acoustic environment of a MRI-LINAC system. By optimizing mounting supports between the split gradient assembly and the main magnet, the authors first altered the vibration modes of the acoustic system, smoothed the acoustic frequency responses, and relocated the acoustic field distribution in the central gap, cylindrical tunnels, and outside of the central gap. The acoustic noise was further reduced by means of applying damping treatment on the surfaces of the split-gradient assembly. RESULTS Compared with conventional bolts that were placed at the ends, when the near supports were fixed and the far adjustable supports were 40 cm away from the further (here near and far are relative to the central gap) ends of the split-gradient assembly, the overall average sound pressure level (SPL) reductions were about 6.3-9.9 dB both inside and outside the central gap for the transverse and longitudinal coils. After applying damping treatment, the average SPLs inside the central gap and outside it were further reduced by 8.5-12.5 and 6.3-8.8 dB for the transverse coils and longitudinal coils, respectively. CONCLUSIONS The authors' FE results showed that, for the cases of transverse coil and longitudinal coil switching, the overall average-SPL reduction quantities amounted to around 20 dB by applying the proposed noise reduction scheme, resulting in lower SPLs than the human hearing threshold.