Suppressing the surface field during transcranial magnetic stimulation.

Transcranial magnetic stimulation (TMS) is used commonly as both a diagnostic tool and as an alternative to electric shock therapy for the treatment of clinical depression. Among the clinical issues encountered in its use is the mitigation of accompanying pain. The objective becomes one of minimizing the induced surface field while still achieving the target field objective. Three techniques discussed for realizing this end are 1) placing a conducting shield over a portion of the central target region, 2) using supplementary coils of opposite polarity in tandem with the primary field, and 3) opening the core angle to distribute the field. Option (3) shows the greatest promise for reducing the ratio of the maximum surface field to the induced target field.