Inception of cavitation clouds by scattered shockwaves

Abstract

Previous observations of cavitation clouds forming during a single, multi-cycle (<;20 cycles) shocked ultrasound pulse (histotripsy pulse) suggested that cloud formation is due to scattering of shock fronts from a single cavitation bubble in the focus. This scattering creates a transient rarefaction wave behind the bubble, which interacts with the incident negative pressure, generating layers of microbubbles through cavitation. This hypothesis is supported by high speed images of cloud formation and experimental results showing cloud initiation is suppressed by reducing the positive shocks. To further evaluate this hypothesis and investigate the properties of the single bubble and incident waveform necessary to incite the cavitation cloud, scattering of a histotripsy pulse from a single bubble was simulated. Measurements of the histotripsy cavitation cloud threshold were compared with simulation results. A threshold in water and gelatin tissue phantom at a negative pressure of about 30 MPa (without Shockwave scattering) for the cavitation cloud to form within a pulse. As high speed images show that the single bubble is deformed to hemispherical shape prior to cloud initiation, the scattered pressure field from a hemisphere as well as a sphere was simulated. The simulation results show the following. 1) Scattering from a hemispherical bubble >; 80 μm in diameter results in negative pressure above the 30 MPa threshold, while scattering from a spherical bubble <; 200 μm in diameter did not result in sufficient pressure to exceed the threshold in response to a highly nonlinear 1 MHz incident plane wave. 2) When the shock rise time was increased from 10 ns to 100 ns, the scattered pressure was reduced by >;80%. 3) The region over which high negative pressure occurred in simulation coincided with the region cavitation is typically observed with high speed imaging. These results further support our hypothesis and suggest that a sufficiently large deformed bubble, short shock rise time, and high frequency harmonics are necessary to initiate a cloud cavitation within a pulse.