Bubble dynamics in boiling histotripsy

Abstract

Boiling histotripsy is a non-invasive, cavitation-based ultrasonic technique which uses a number of millisecond pulses to mechanically fractionate tissue. Though a number of studies have demonstrated the efficacy of boiling histotripsy for fractionation of solid tumours, treatment monitoring by cavitation measurement is not well studied because of the limited understanding of the dynamics of bubbles induced by boiling histotripsy. The main objectives of this work are to (a) extract qualitative and quantitative features of bubbles excited by shockwaves and (b) distinguish between the different types of cavitation activity for either a thermally or a mechanically induced lesion in the liver. A numerical bubble model based on the Gilmore equation accounting for heat and mass transfer (gas and water vapour) was developed to investigate the dynamics of a single bubble in tissue exposed to different High Intensity Focused Ultrasound fields as a function of temperature variation in the fluid. Furthermore, ex vivo liver experiments were performed with a passive cavitation detection system to obtain acoustic emissions. The numerical simulations showed that the asymmetry in a shockwave and water vapour transport are the key parameters which lead the bubble to undergo rectified growth at a boiling temperature of 100°C. The onset of rectified radial bubble motion manifested itself as (a) an increase in the radiated pressure and (b) the sudden appearance of higher order multiple harmonics in the corresponding spectrogram. Examining the frequency spectra produced by the thermal ablation and the boiling histotripsy exposures, it was observed that higher order multiple harmonics as well as higher levels of broadband emissions occurred during the boiling histotripsy insonation. These unique features in the emitted acoustic signals were consistent with the experimental measurements. These features can, therefore, be used to monitor (a) the different types of acoustic cavitation activity for either a thermal ablation or a mechanical fractionation process and (b) the onset of the formation of a boiling bubble at the focus in the course of HIFU exposure.

Mechanical damage induced by the appearance of rectified bubble growth in a viscoelastic medium during boiling histotripsy exposure

“…8a) and the corresponding spectrogram (Fig. 8b) are indications of the production of a boiling bubble during boiling histotripsy [21,39]. Assuming the speed of sound is constant (1482 m/s), a time delay between the PCD signal and the high speed camera results for the formation of a boiling bubble can simply be calculated using the geometric focal length of the PCD transducer of 6.4 cm (provided by the manufacturer, Sonic…”

“…Water vapour is the main component of the bubble content at the end of the growth phase at t = 50 μs (i.e., Fig 4d). This overall trend is likely to be due to the increased number of available water vapour molecules in the surrounding medium that can transport into a bubble [39], which increases vapour pressure inside the bubble resulting in an enhanced bubble growth ( Figs. 4d, e).…”

“…The Gilmore-Zener bubble model employed in the present study accounted for water vapour transport, non-condensable gas transport and heat transfer at the bubble wall using the same formulas used in [39]. A brief explanation of the main components of the model developed is only presented in the following sections, whereas its detailed description can be found in [39].…”

“…The density, speed of sound, viscosity and the surface tension of the liver as a function of temperature (independent of acoustic pressure fields) were assumed to follow similar trends to those of water (72.8% of adult human liver tissue constitute is water [55]), as it is acknowledged that information regarding this is not readily available [56]. The same equations (33)-(36) used in [39] were employed to initially calculate the temperature dependences of the physical properties of water…”

Histotripsy: The Next Generation of High‐Intensity Focused Ultrasound for Focal Prostate Cancer Therapy

“…Operator dependence and the absence of clear feedback on the completeness of ablation beyond echogenicity changes that may or may not be accurate are limitations of US‐guided HIFU. Sophisticated mathematical techniques as well as advanced US technology such as that reported by Lyka et al, in which prostate motion registration optimizes targeting, have been used for the performance of fused and transrectal US examinations at biopsies and could be applied to HIFU monitoring as well. Algorithms have been produced that are clinically available, such as the Tissue Change Monitoring system on the Sonoblate system, which allows real‐time US imaging to monitor HIFU treatment .…”