In hybrid MEG-MRI, a magnetically shielded room (MSR) is often required to measure the weak magnetic fields (similar to 0.1 pT) in MEG. On the other hand, to generate an MR image, relatively strong magnetic fields (<1 mt) need to be applied inside the MSR. These time-varying fields induce eddy currents and magnetize the walls of the room, possibly deteriorating MR image quality and hindering MEG data acquisition. In this study, static and dynamic magnetic fields are inspected inside a spherical MSR made of linear materials. An analytical expression is derived for the field generated by an arbitrary magnetic time dependent source inside the MSR, including the expressions for the amplitudes and time constants of the various eddy-current-associated fields. For the special case of a magnetic dipole in the centre of the room, it is shown that the response of the room is spatially homogeneous. The order of magnitude of the derived fields match those measured experimentally in a rectangular MSR.
