Magnetic localisation of intracranial dipoles: simulation with a physical model.

Current dipoles energized by isolated sources were located in known positions inside a human skull filled with an electrically conductive medium. Maps of the measured electrical and magnetic fields confirmed the predicted relationships between those fields for both single and multiple dipoles. Two methods of dipole localisation were compared: the peak-location method which used only the locations of the maximum and minimum recorded values, and a least-squares iterative method which found the parameters for a dipole such that the sum of squared differences between the recorded and predicted data was minimized. Also, in an attempt to account for some of the error due to the non-sphericity of the head, the measured distance from the centre of the skull to each recording position was used in the dipole calculations. This last technique resulted in the smallest 3-dimensional location error (averaging 3.5 mm) for the least-squares method, even when no recording positions were near the actual field extrema and the peak-location method therefore produced much greater error. Also investigated were combinations of two dipoles for which the magnetic field maps appeared similar to those for a single dipole and comparisons were made to determine how well single and double dipole models could account for the recorded data.