Biomagnetic field measurements and their interpretation using the dipole in a sphere model.

Janday, Balwinder S (1988). Biomagnetic field measurements and their interpretation using the dipole in a sphere model. PhD thesis The Open University.



Although biomagnetism has advanced considerably in the last twenty years in terms of instrumentation and application, the fundamental requirement of relating the detected field to the underlying sources still has to rely on simple models. Most often the current dipole in a conducting sphere model is used. The exact geometry of the system under study is often ignored and clinically crucial decisions may be taken without understanding the limitations of this model.

The work described in this thesis assesses those limitations, first by analysis of physical model generated data using the dipole in a sphere model. Dipole location, orientation, strength and sphere centre location were parameters used to fit calculated data to real data iteratively. These calculations were fully corrected for detector coil size and geometry. The systems investigated included whole and partial spheres and gel filled skulls. Dipoles placed in close proximity to boundaries were more difficult to fit to the dipole in a sphere model than those further away. As the boundary itself became more distinctly non- spherical, this difficulty increased. A goodness of fit parameter (R) was defined indicating accuracy of signal fitting. Surprisingly, in some examples, a low K value could be obtained for a poor set of predicted dipole parameters. However, if the fitting procedure incorporates the model sphere centre position as additional fitting parameters and a search is made through model sphere centre parameter space the effect of these (fortuitously) low K values could be avoided.

The second half of the work applies the lessons of the first half to real biological systems. Magnetic fields emanating from a developing chick egg change during development. The egg provides a convenient system to investigate ionic fluxes involved with development and to test the appropriateness of the single dipole in a conducting sphere model in determining the source generators.

Finally the human visual evoked response was used to Investigate the retinotopic representation of the cortex. The source locations suggested by this work using magnetic fields support earlier work which used electrical potential measurements, though a significant temporal difference of peak power between the two sets of sources is predicted by analysis of the data available from the two techniques.

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