A THEORETICAL-MODEL FOR MAGNETOACOUSTIC IMAGING OF BIOELECTRIC CURRENTS

A theoretical model of magneto-acoustic current imaging is derived, ba sed on fundamental equations of continuum mechanics and electromagneti sm In electrically active tissue, the interaction between an applied m agnetic field, B, and action currents, J, creates a pressure distribut ion. In the near field limit, this pressure obeys Poisson's equation, with a source term (del x J) B. The displacement and pressure fields a re calculated for a dipole (q) oriented either parallel or perpendicul ar to the applied magnetic field (B), at the center of an elastic, con ducting sphere (radius a, shear modulus G). Surface displacements are on the order of qB/(4 pi Ga), which is about 1 nm for typical biologic al parameters. If the applied magnetic field is changing with time, ed dy currents induced in the tissue may be larger than the action curren ts themselves. The frequency of the pressure and displacement arising from these eddy currents, however, is twice the frequency of the appli ed magnetic field, so it may be possible to eliminate this artifact by filtering or lock-in techniques. Magneto-acoustic and biomagnetic mea surements both image del x J in a similar may, although magneto-acoust ic current imaging has the disadvantage that acoustic properties vary among tissues to a greater degree than do magnetic properties.