Optimal Elevator Shaft Shielding Method to Reduce Magnetic Field Fluctuations Due to Movement of Elevator for MRI

Abstract

This paper describes an optimal shielding shape of elevator shafts for installations of MRI and Electron Beam Lithography System. The imaging of the MRI is mainly disturbed by the magnetic flux density fluctuations (MFDF) in the direction that the uniform flux is applied with amplitudes of 1.5 T and 3.0 T. Almost all MRIs are set horizontally, so the shielding method for the horizontal component of the MFDF was primarily investigated. A shielding shaft divided into upper and lower parts with a gap also enables the opening for going in and out from the elevator. Firstly, the MFDFs due to the movement of the elevator were measured. Then, the magnitudes and directions of the magnetic moments that can model the magnetic materials of the elevator were estimated using an inverse problem, so that the MFDFs obtained agree with those measured. Secondly, the relationship between shape of shielding shaft and the estimated fluctuations of leakage magnetic field in the horizontal and vertical directions due to the movement of the modeled elevator was investigated using a 3D finite linear magnetic-field analysis. Lastly, the optimum shape of the elevator-shielding shaft for preventing MFDFs due to the movement of the elevator was investigated by analysis.