| Higher frequency components, which are induced by a nonlinear response of tissue under high pressures, have long been known to enhance thermal ablation due to the increased absorption of the higher frequencies. It has been proposed that aspects of nonlinear propagation could be used to control power deposition. This study describes an approach designed to exploit the enhanced heating effect of nonlinear ultrasound while utilizing yet another property of a properly designed nonlinear field. By using a relatively low driving frequency to propagate through aberration layers close to the source before the buildup of higher harmonics, the less distorted low frequency wave then acts as a virtual source, creating a higher frequency beam of harmonics of the driving source. Simulation and experimental studies have established the feasibility of the proposed method. The results identify parameters that exhibit reduced spatial distortion of HIFU focal fields when the 2nd harmonic is analyzed. This was shown to be the case for both single-source and multiple-source focused ultrasound, and experimentally demonstrated with ultrasound transmission through both bone phantom and ex vivo human skull. In a highly inhomogeneous nearfield, as may be created by bone or fat layers, the resulting field is considerably less distorted than direct linear propagation of the same frequency. Future challenges include the optimization of this effect by designing a beam that minimizes the fundamental frequency and maximizes the harmonics at the target. |