Ultrasound as a noninvasive neuromodulation tool

Electrical stimulation has been the most common approach for the treatment of some neurological diseases and disorders. However, due to the risk of intrusive surgical procedure, the use of electrical stimulation is limited only to the subjects with profound symptoms even though much more can benefit from neurostimulation. To circumvent this limitation, several noninvasive neurostimulation methods have been proposed including transcranial magnetic stimulation (TMS) or ultrasonic stimulation. Even though the TMS is currently used for human applications, it still has the limitation of low focusing capacity and high attenuation though brain tissue. On the other hand, ultrasound is becoming a promising modality as a future neuromodulation approach due to its high spatial selectivity and high penetrating power into the brain. Recently several studies on the ultrasonic neurostimulation are being performed from different aspects including the mechanism underlying ultrasonic modulation, the realization of focused ultrasonic stimulation, the effectiveness in animal model, and so forth. In this paper, the current stage of the ultrasonic neural stimulation technique is reviewed and the future direction is discussed.