Hybrid MEG-MRI: Geometry and time course of magnetic fields inside a magnetically shielded room
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1
Helsinki University of Technology, Department of Biomedical Engineering and Computational Science, Finland
In hybrid MEG-MRI, a magnetically shielded room (MSR) is often required to measure the weak magnetic fields (~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 the MR image quality and hindering the 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. Special attention is paid to a spherical MSR with a single shell composed of an inner mu-metal layer and an outer aluminium layer. First, an analytical expression is derived for the magnetization of the mu-metal layer. Second, it is shown that for a source with a square-wave time dependence, the induced eddy currents in the outer aluminium layer decay exponentially and are shielded by the inner mu-metal layer. On the other hand, eddy currents in the mu-metal layer decay nonexponentially. Analytical expressions are derived for the amplitudes and time constants of the decays. The amplitude of the magnetic field generated by eddy currents in the mu-metal layer is several orders of magnitude higher than that generated by eddy currents in the aluminium layer. However, the decay time for the mu-metal layer is only a few milliseconds while the corresponding time constant for the aluminium layer is around a second. The order of magnitude of the amplitudes and time constants of the decays match those measured experimentally in a rectangular MSR.
Conference:
Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010.
Presentation Type:
Poster Presentation
Topic:
Instrumentation and Multi-modal Integrations: MEG, Low-field MRI,EEG, fMRI,TMS,NIRS
Citation:
Vesanen
P and
Ilmoniemi
R
(2010). Hybrid MEG-MRI: Geometry and time course of magnetic fields inside a magnetically shielded room.
Front. Neurosci.
Conference Abstract:
Biomag 2010 - 17th International Conference on Biomagnetism .
doi: 10.3389/conf.fnins.2010.06.00414
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Received:
09 Apr 2010;
Published Online:
09 Apr 2010.
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Correspondence:
Panu Vesanen, Helsinki University of Technology, Department of Biomedical Engineering and Computational Science, Espoo, Finland, panu.vesanen@tkk.fi