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10 MAGNETIC SOURCE IMAGING
Pages 147-156

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From page 147...
... The magnetic field at the surface is most strongly determined by current sources directed parallel to the surface, but the electrical potentials are determined by current sources directed perpendicular to the surface. Other than the signal distortion from the heterogeneity of tissue conductivity, there is no clear physical reason that the clinical information produced by biomagnetic measurements could not as well be obtained from electrical potential mapping.
From page 148...
... is that it can resolve events separated by milliseconds, whereas other methods such as functional magnetic resonance imaging (fMRI) , magnetic resonance spectroscopy (MRS)
From page 149...
... Since the body is moderately conductive, the current sources being imaged generate "volume currents" within the body that depend on the body geometry and the distribution of tissue conductivity. These volume currents generate their own magnetic fields and thereby modify the fields generated by the primary current sources themselves.
From page 150...
... A straightforward extension of the single (lipole model is the multidipole model, with two or more spatially separated equivalent current dipoles. Model simulations have shown that two simultaneously active dipoles can be separated only if the spacing of the dipoles is comparable to the distances between the dipoles and measurement locations.
From page 151...
... Since the fields depend nonlinearly on the dipole position and orientation, iterative optimization algorithms are required; such iterative algorithms are expensive in computer time and often find local rather than global minima. These methods work reasonably well for source models with one or perhaps two dipoles but rapidly become more expensive to compute and less reliable as the number of dipoles Increases.
From page 152...
... A current line of research is to improve the apparent spatial resolution of MST by using prior knowledge of the anatomy obtained from magnetic resonance imaging (MRT) or computed tomography (CT)
From page 153...
... The particular algorithms used can also incorporate prior information about the expected amplitude of the sources, expected amplitude of the detector noise, and spatial and temporal correlations between sources. The use of such prior knowledge has been demonstrated in computer simulation to improve the accuracy of the reconstructed source distribution.
From page 154...
... One question of particular interest is how accurate the forward model must be to yield accurate reconstructions from data with some specified noise level; since improved accuracy costs more computer time, it is preferable to use a forward model that is no more accurate than necessary. · Existing distributed source models can exploit prior information about the expected source amplitudes and both spatial and temporal correlations between sources.
From page 155...
... 9. Menninghaus, E., Lutkenhoner, B., Gonzalez, S.L., Hampson, S., and Pantev, C., Localization of a dipolar source in a skull phantom: Comparison between boundary element model and spherical model, in Proceedings of the Satellite Symposium on Neuroscience and Technology of the 14th International Conference of the IEEE Engineering in Medicine and Biology Society, A
From page 156...
... 15. Wang, Jia-Zhu, Williamson, Samuel I., and Kaufman, Lloyd, Magnetic source images determined by a lead-field analysis: The unique minimum-norm least-squares estimation, IEEE Trans.


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