A simple physical phantom to demonstrate the importance of volume currents
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1
Cleveland Clinic, United States
We present three nearly identical physical phantoms, each the same tangential current dipole in a sphere, but one that generates EEG and MEG fields, one that generates MEG only, and one that generates EEG only. Each emphasizes the importance of primary and secondary currents in the generation of external fields. We consider a classic tangential current dipole in a spherically symmetric conducting medium, i.e. the dipole in a sphere problem. For MEG sensors oriented radial to the sphere, the MEG solution is simple, comprising only the primary current. For non-radial sensors, the volume currents must be considered, and [1] demonstrated a simple physical model, unusual in that it generates no equivalent EEG signal on the surface of the sphere, yet is otherwise equivalent to the dipole in the sphere. This physical MEG-only phantom is in wide use with the Elekta Neuromag systems. Using the insight of [2], we demonstrate a simple variation of [1] that generates an EEG signal for a tangential current dipole, but no external magnetic field. The EEG/MEG phantom is simply a shielded pair of wires to a point R from the center, where they split into tangential directions for a few millimeters, say 5 mm. From [1], the MEG-only phantom is an isosceles triangle with sides R and end 5 mm at the same location, with its opposing vertex at the sphere center; the vertex is fed by a shielded pair of wires. The proposed EEG-only phantom is the same isosceles triangle, but with the end now removed. The current in this phantom flows as a volume current between the tips of the radial wires. From [2], we thus have a physical phantom with no external magnetic field, yet it generates the EEG pattern of a tangential current dipole. All three phantoms are effectively the same 5 mm tangential current dipole. Insight drawn from these simple phantoms should prove useful in proposed micro, meso, and macro scale neuronal circuit models and their impact on the EEG or MEG fields.
References
1. Ilmoniemi RJ, Hamalainen MS, Knuutila J, The forward and inverse problems in the spherical model, in Biomagnetism: Applications and Theory, eds. Weinberg, Stroink, Katil, 1985.
2. Tripp, JH, Physical Concepts and Mathematical Models, in Biomagnetism, An Interdisciplinary Approach, eds. Williamson, Romani, Kaufman, Modena. 1983
Conference:
Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010.
Presentation Type:
Poster Presentation
Topic:
MEG Modeling
Citation:
Mosher
J
(2010). A simple physical phantom to demonstrate the importance of volume currents.
Front. Neurosci.
Conference Abstract:
Biomag 2010 - 17th International Conference on Biomagnetism .
doi: 10.3389/conf.fnins.2010.06.00081
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Received:
22 Mar 2010;
Published Online:
22 Mar 2010.
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Correspondence:
John Mosher, Cleveland Clinic, Cleveland, United States, jcmosher@gmail.com