INTRODUCTIONFocused ultrasound (FUS

INTRODUCTIONFocused ultrasound (FUS) is a transformative tool that can be used to noninvasively create lesionsor temporarily modify the function of targeted brain tissue while minimally affecting all interveningtissues carrying the ultrasound energy. Because FUS can be used to create these lesions remotelyfrom the source, with well-defined margins and precise localization, this technology is an attractiveoption for noninvasive neurosurgery (1).The field of FUS was created in 1927 by Wood and Loomis, who first documented the effectsof ultrasound on living biological tissue (2). Therapeutic applications for high-intensity focused ultrasound (HIFU) often use a lower frequency (300 kHz – fewMHz) with maximum ultrasound intensity at a beam focus ofapproximately 1,500 W/cm2, whereas a high frequency range (2–15 MHz) and low intensity (0.1 W/cm2) are typical for diagnosticultrasound (3). A drawback of the early animal experiments in thefield of HIFU was the considerable tissue damage caused alongthe ultrasound pathway from the skin to the target. This damagewas partly due to the use of a single source, which providedno geometric gain when compared with current technology.A further complication in the creation of brain lesions wasthe presence of the skull, which is a source of sound wavereflection, scatter, and absorption; all of these factors reducepower deposition at the target. Although the source powercould be sufficiently increased to overcome this loss and createdeep brain lesions, the increased power would also increasecollateral damage to the scalp and skull. Thus, early animalwork necessitated removal of skull flaps, thereby limiting clinicalapplication of this technology.