Transcranial magnetic stimulation of small animals: a modeling study of the influence of coil geometry, size and orientation.

Several recent studies have investigated the mechanisms of repetitive transcranial magnetic stimulation (rTMS) using small animals. However, there is still limited knowledge about the distribution of the induced electric field, and its dependence on coil size, geometry and orientation. In this work we calculate the electric field induced in a realistically shaped homogeneous mouse model by commercially available coils in several different orientations. The results show that the secondary field, resulting from charge accumulation at the skin - air interface, drastically changes the magnitude, decay and focality of the primary field induced by the coil. Accurate knowledge about the distribution of the field is invaluable in designing experimental protocols and new coils for small animal stimulation.