Forward model for the superconducting imaging-surface meg system
- Andrei N.
- Michelle A.
- Keeran
- Petr
We have recently completed a novel whole-head MEG system based on the Superconducting Imaging-Surface (SIS) concept originally proposed by van Hulsteyn, et al.[l]. The SIS concept is generally described as a source near a superconducting surface. The source field induces Meissner currents in the superconductor equivalent to a source image 'behind' the surface. A sensor (SQUIDS in our system) placed on the source-side of the SIS will measure the superposed fields from the real and image sources. A second consequence of the Meissner effect is to shield the SQUIDS sensors near the SIS from external or background fields. The shape of the SIS used in our MEG system is a hemisphere with two cut-outs at the nominal ear-locations. A brim is added around the entire periphery with a smooth 0.5 cm radius transition between brim and hemisphere. Benefits of the SIS concept over existing systems include significantly enhanced signal-to-noise as a consequence of the SIS shielding and inherently generating pseudo-first order gradient fields at the sensors. One of the most significant challenges in realizing this system has been to accurately describe how the SIS system impacts the forward physics of any source model. Two approaches have been examined. The first is a hybrid analytical and empirical model using the analytic formalism to describe the hemisphere [1] and a correction matrix derived from empirical measurements to correct for edge effects. This approach proved overly complex and difficult in practice to obtain sufficient empirical data to derive a well-conditioned correction matrix. The second approach, reported here, was to develop a boundary element model (BEM) description of the SIS using the exact as-built geometry. Each element is described by a uniform magnetization arising from a distribution of Meissner currents in the superconductor such that B{perpendicular} = 0 at the surface. B{sub i} at each element is a superposition of the source field and the fields resulting from currents in all other elements. A precision phantom was developed to test the model. Modeled and measured magnetic field distributions agreed with typically less than 1% (< 0.1% in most cases) discrepancy at all SQUID sensors for more than 60 phantom coil positions. The attached figure shows modeled and measured magnetic field distributions for 25 such phantom coils.
- Research Organization:
- Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 975284
- Report Number(s):
- LA-UR-01-1915; TRN: US201008%%156
- Resource Relation:
- Conference: "Submitted to: "3rd International Syposium on Noninvasive Functional Source Imaging, NFSI 2001 September 6-9, 2001, Innsbruck, Austria".
- Country of Publication:
- United States
- Language:
- English
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