title: Focusing and targeting of magnetic brain stimulation using multiple coils.
abstract: Neurones can be excited by an externally applied time-varying electromagnetic field. Focused magnetic brain stimulation is attained using multiple small coils instead of one large coil, the resultant induced electric field being a superposition of the fields from each coil. In multichannel magnetic brain stimulation, partial cancellation of fields from individual coils provides a significant improvement in the focusing of the stimulating field, and independent coil channels allow targeting of the stimuli on a given spot without moving the coils. The problem of shaping the stimulating field in multichannel stimulation is analysed, and a method is derived that yields the driving currents required to induce a field with a user-defined shape. The formulation makes use of lead fields and minimum-norm estimation from magneto-encephalography. Using these methods, some properties of multichannel coil arrays are examined. Computer-assisted multichannel stimulation of the cortex will enable several new studies, including quick determination of the cortical regions, the stimulation of which disrupts cortical processing required by a task.
authors.affiliation: BioMag Laboratory, Medical Engineering Centre, Helsinki University Central Hospital, Finland. jarmo@biomag.helsinki.fi
authors.forename: J
authors.initials: J
authors.lastname: Ruohonen
authors.forename: R J
authors.initials: RJ
authors.lastname: Ilmoniemi
mesh_terms: D001921:Brain / Q000502:physiology* / Q000528:radiation effects*; D004574:Electromagnetic Fields; D055590:Electromagnetic Phenomena / Q000295:instrumentation / Q000379:methods; D006801:Humans