Multipole analysis of biomagnetic signals from the human leg

Computationally tractable solutions of the "inverse problem," the calculation of source currents from magnetic field data, are an essential component of biomagnetic studies. A multipole analysis suitable for any system that may be modeled as having all currents oriented along a given axis but not depending on the value of position coordinate on that axis is described. The lowest nonvanishing term is dipolar, and we specify a "best fit" criterion that estimates from data the dipole's magnitude and orientation together with its location in the transverse plane. The dipolar term is seen to give good fits to fields measured outside the normal relaxed lower human leg, even though the field data varies with position along the leg-axis. Reasons for this success are considered. The analysis is not specific to biomagnetic systems and may be useful in electrical engineering or geophysical problems.