Human Brain Stimulation with Transcranial Ultrasound Potential Applications for Mental Health

Interest in noninvasive brain stimulation for therapeutic effects on mental health has increased in recent years. The ability to directly modulate brain activity in targeted or diffuse regions noninvasively, that is, from outside the skull, has enormous potential for the treatment of psychiatric and neurological disorders. Brain stimulation also holds promise for the functional mapping of brain systems by coupling stimulation with subjective report and imaging techniques such as functional magnetic resonance imaging (fMRI). A potential advantage of therapeutic brain stimulation over pharmacological intervention is that targeted stimulation of brain circuits implicated in psychiatric disorders might minimize global effects on the brain and body, potentially minimizing or eliminating side effects. Frequently used noninvasive brain stimulation methods, including transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS), have limitations (reviewed below), restricting their scientific and therapeutic applications. Ultrasound is a less frequently used brain stimulation technology, but may have advantages over other techniques. Ultrasound consists of mechanical vibrations above the threshold for human hearing (>20,000 Hz). Ultrasound can penetrate biological tissue, and echo off surfaces to give anatomical images, such as fetuses in the womb. Although primarily used for medical imaging, ultrasound can also modulate neural activity, both peripherally and in the brain. This chapter reviews the history of ultrasound neurostimulation and the first clinical trials in humans. Future directions in this emerging field are discussed.