Development of Focused Transcranial Magnetic Stimulation for Rodents by Copper-Array Shields

Transcranial magnetic stimulation (TMS) is one of the most widely used noninvasive brain stimulation methods. It has been utilized for both treatment and diagnosis of many neural diseases, such as neuropathic pain and loss of function caused by stroke. Existing TMS tools cannot deliver focused electric field to targeted penetration depth even though many important neurological disorders are originated from there. A breakthrough is needed to achieve noninvasive, focused brain stimulation. We demonstrated using magnetic shield to achieve magnetic focusing without sacrificing significant amount of throughput. The shield is composed of multiple layers of copper ring arrays, which utilize induced current to generate counter magnetic fields. We experimentally set up a two-pole stimulator system to verify device simulation. A transient magnetic field probe was used for field measurements. The focusing effect highly depends on the geometric design of shield. A tight focal spot with a diameter of smaller than 5 mm (plotted in MATLAB contour map) can be achieved by using copper ring arrays. With properly designed array structures and ring locations, the combined original and induced counter fields can produce a tightly focused field distribution with enhanced field strength at a depth of 7.5 mm beyond the shield plane, which is sufficient to reach many deep and critical parts of a mouse brain.