Magnetic coupling of mechanical modes in MRI systems

Purpose The purpose of this paper is to investigate magneto-mechanical coupling occurring in magnetic resonance imaging (MRI) systems. The authors study influence of the strength of the background field on the coupling of mechanically isolated, conductive cylindrical structures and the so-called shields. This coupling has a strong impact on frequency-dependent thermal losses occurring in the shield structures which are of high importance in MRI systems. Design/methodology/approach In the investigations, numerical methods are applied. First, finite element methods taking into account the full magneto-mechanical coupling are used to investigate the coupled physical phenomena. As these calculations may be time-consuming, several approximate predictive methods are derived. Modal expansion factors and participation factors are based on combinations of structural eigenmode calculations and eddy current calculations using Biot-Savart representations of the dynamic gradient field. In addition, a parallelism factor expressed in terms of the shield vibrations is defined to measure the coupling between the distinct cylinders. Findings It is found that the strength of the background field strongly influences the coupling of the distinct shields, which strongly increases the parallelism of the shield vibrations. Furthermore, modal expansion and participation factors are significantly influenced, caused by frequency shifts due to magnetic stiffening and increased magnetic coupling. Research limitations/implications - The current work is limited to the modal expansions of a single shield. This needs to be extended in the future as comparison of modal expansion factors and finite element simulation indicate. Originality/value The defined factors estimating parallelism and modal participation in magneto-mechanical coupling are original work and studied for the first time.