Dipole separability in a neuromagnetic source analysis

By studying the ability of a one-dipole model to explain the magnetic field actually resulting from two dipoles, minimum requirements for a successful separation of two dipoles were explored. Two dipoles in different depths generally require a much higher signal-to-noise ratio (SNR) than two dipoles in the same depth. For the latter condition, the dipole distance as well as the angles between the moments and the line connecting the dipole locations (connecting line) were systematically varied. A perpendicular orientation of the two dipoles turned out to be the most favorable condition: the minimum distance required for a separation of two dipoles was more than four times smaller than for a configuration with both moments oriented parallel to the connecting line. Separability of parallel dipoles was moderately enhanced if both moments assumed an orientation perpendicular to the connecting line. The separability of two antiparallel dipoles is not limited by concurrence with a one-dipole model, but by the low signal amplitudes resulting from a mutual cancelation of the fields arising from the two dipoles, and by concurrence with a quadrupole model. The results are presented so that quantitative conclusions about dipole separability can be derived for arbitrary SNR's. The study does not generally disprove the common belief that magnetoencephalography has a relatively poor spatial resolution, but it qualifies this view by suggesting that under favorable conditions two sources with a distance of only 1 cm may be resolvable.