Localised and Distributed Source Solutions for the Biomagnetic Inverse Problem II

Until recently, equivalent current dipole sources have been used almost universally to describe the biomagnetic signals generated by ionic currents in conducting body tissues but, as the performance of multichannel systems improves, clinical applications will emerge which require PET-like pictures demonstrating increased activity (or the absence of activity) in relation to normal background levels. A vital step in this direction is the development of a reliable, efficient and mathematically well-founded method of studying continuous source densities. A number of attempts to treat biomagnetic data in a model-independent way have been reported [1,2,3,4]Starting from maximum entropy arguments [5] we have developed a very general method of inversion. In a certain limit, our method reduces to that of reference [1] but, in general, it is more powerful in its ability to take account of prior information about the source space and the noise spectrum of the magnetometers. It is implemented within a comprehensive computing environment that can deal with a wide range of experimental geometries and accommodate discrete or continuous sources, including both ionic flows and magnetic dipoles. In this paper we provide an outline of the method and computing environment and we discuss two simple examples. A more detailed description of the method will be given elsewhere, but its application to 3D inversions and the analysis of real MEG data is given in the next paper [6].