Nonlinear propagation and shock waves in histotripsy ultrasound therapy.

Abstract

Histotripsy is a noninvasive therapy that applies short duration, highly focused pulses of ultrasound to mechanically break down targeted tissues by acoustic cavitation. These pulses become strongly distorted due to nonlinear propagation, and shock fronts are formed at the transducer focus. Focal pressure levels at which cavitation clouds are achieved can exceed 20 MPa peak negative pressure and 100 MPa peak positive pressure. Current methods for characterizing histotripsy pulses are discussed. Single‐mode and multi‐mode fiber optic hydrophones have been constructed in our laboratory for measuring histotripsy waveforms. Accurate measurement of acoustic waveform shape and amplitude is important because nonlinear distortion contributes greatly to generating cavitation in histotripsy. High‐speed photographic observations suggest that cavitation clouds are not formed directly by the negative pressure of the wave, but by scattering of shocks from single cavitation bubbles within the focal region. Experiments and simulations describing the role of shock waves in forming cavitation clouds during histotripsy are presented. [Work supported by NSF GRFP and NIH R01 EB008998 and S10 RR022425.]