The potential of transskull ultrasound therapy and surgery using the maximum available skull surface area

Based on previous studies,(1-3) the transskull ultrasound field is re-examined by utilizing the maximum available skull surface area. The source is assumed to be in direct contact with the skull outer surface, and phase correction is adopted to obtain a sharp focus at a desired location. A digitized skull profile was obtained from Magnetic Resonance (MR) scan images of a volunteer. Two driving frequencies (0.5 and 1.0 MHz) within the appropriate frequency range for transskull ultrasound therapy and surgery are investigated. With no phase correction, there is no apparent transskull focus. With phase correction, a sharp transskull focus is obtained at the desired location. Both pressure and specific absorption rate (SAR) gains (ratios of pressure amplitude and SAR at the focal point compared to those on the outer skull surface) are examined, and it is shown that the skull heating problem can be overcome by utilizing the maximum available skull surface area. By specifying the phase correction for different locations, the focus can be successfully moved inside the deep brain volume without significantly compromising the pressure and SAR gains; however the sidelobes may be of concern at superficial sites. (C) 1999 Acoustical Society of America. [S0001-4966(99)03404-4].