Enhanced High-Rate Shockwave Lithotripsy Stone Comminution in an In Vivo Porcine Model Using Acoustic Bubble Coalescence

Investigation of interaction effects on dual-frequency driven cavitation dynamics in a two-bubble system

The cavitation dynamics of a two-bubble system in viscoelastic media excited by dual-frequency ultrasound is studied numerically with a focus on the effects of inter-bubble interactions. Compared to the isolated bubble cases, the enhancement or suppression effects can be exerted on the amplitude and nonlinearity of the bubble oscillations to different degrees. Moreover, the interaction effects are found to be highly sensitive to multiple paramount parameters related to the two-bubble system, the dual-frequency ultrasound and the medium viscoelasticity. Specifically, the larger bubble of a two-bubble system shows a stronger effect on the smaller one, and this effect becomes more pronounced when the larger bubble undergoes harmonic and/or subharmonic resonances as well as the two bubbles get closer (e.g., d0 < 100 μm). For the influences of the dual-frequency excitation, the results show that the bubbles can achieve enhanced harmonic and/or subharmonic oscillations as the frequency combinations with small frequency differences (e.g., Δf < 0.2 MHz) close to the corresponding resonance frequencies of bubbles, and the interaction effects are consequently intensified. Similarly, the bubble oscillations and the interaction effects can also be enhanced as the acoustic pressure amplitude of each frequency component is equal and the pressure amplitude pA increases. Above a pressure threshold (pA = 215 kPa), a larger bubble undergoes period 2 (P2) oscillations, which can force a smaller bubble to change its oscillation pattern from period 1 (P1) into P2 oscillations. In addition, it is found that the medium viscosity dampens the bubble oscillations while the medium elasticity affects the bubble resonances, accordingly exhibiting stronger interaction effects at smaller viscosities (e.g., μ < 4 mPa·s) or certain elasticities (approximately G = 70-120 kPa, G = 160-200 kPa and G = 640-780 kPa) at which the bubble resonances occur. The study can contribute to a better understanding of the complex dynamic behaviors of interacting cavitation bubbles in viscoelastic tissues for high efficient cavitation-mediated biomedical applications using dual-frequency ultrasound.

Enhanced shockwave lithotripsy with active cavitation mitigation

The goal of this study was to examine acoustical mechanisms that manipulate cavitation events in order to improve the efficacy of shockwave lithotripsy (SWL) at higher rates. Previous work has shown that applying low amplitude acoustic pulses immediately after each shockwave (SW) can force cavitation bubbles to coalesce and enhance SWL efficacy. In this study, the effects of applying low amplitude acoustic pulses at different time delays is investigated before and after each SW, which would result in different interactions among residual microbubbles producing forced coalescence and dispersion. Utilizing forced coalescence and dispersion was hypothesized to mitigate the shielding effect of residual bubbles, further improving efficacy particularly for higher SWL rates. A set of in vitro experiments was performed in a water tank so that the behavior of bubbles, coalescence and dispersion, could be observed with a high-speed camera. Model kidney stones were treated by a clinical Dornier lithotripter with firing rates of 30 shocks/min and 120 shocks/min, along with an in-house made transducer to generate low amplitude acoustic pulses fired at different pressures and time delays. The average percentage of untreated stone fragments greater than 2 mm was 15.81% for 120 shocks/min without mitigation and significantly reduced to 0.19% for the optimum mitigation protocol.

Assessment of histotripsy-induced liquefaction with diagnostic ultrasound and magnetic resonance imaging in vitro and ex vivo

Histotripsy is a therapeutic ultrasound modality under development to liquefy tissue mechanically via bubble clouds. Image guidance of histotripsy requires both quantification of the bubble cloud activity and accurate delineation of the treatment zone. In this study, magnetic resonance (MR) and diagnostic ultrasound imaging were combined to assess histotripsy treatment in vitro and ex vivo. Mechanically ablative histotripsy pulses were applied to agarose phantoms or porcine livers. Bubble cloud emissions were monitored with passive cavitation imaging (PCI), and hyperechogenicity via plane wave imaging. Changes in the medium structure due to bubble activity were assessed with diagnostic ultrasound using conventional B-mode imaging and T ₁-, T ₂-, and diffusion-weighted MR images acquired at 3 Tesla. Liquefaction zones were correlated with diagnostic ultrasound and MR imaging via receiver operating characteristic (ROC) analysis and Dice similarity coefficient (DSC) analysis. Diagnostic ultrasound indicated strong bubble activity for all samples. Histotripsy-induced changes in sample structure were evident on conventional B-mode and T ₂-weighted images for all samples, and were dependent on the sample type for T ₁- and diffusion-weighted imaging. The greatest changes observed on conventional B-mode or MR imaging relative to baseline in the samples did not necessarily indicate the regions of strongest bubble activity. Areas under the ROC curve for predicting phantom or liver liquefaction were significantly greater than 0.5 for PCI power, plane wave and conventional B-mode grayscale, T ₁, T ₂, and ADC. The acoustic power mapped via PCI provided a better prediction of liquefaction than assessment of the liquefaction zone via conventional B-mode or MR imaging for all samples. The DSC values for T ₂-weighted images were greater than those derived from conventional B-mode images. These results indicate diagnostic ultrasound and MR imaging provide complimentary sets of information, demonstrating that multimodal imaging is useful for assessment of histotripsy liquefaction.

MRI-guided transurethral insonation of silica-shell phase-shift emulsions in the prostate with an advanced navigation platform

PURPOSE

In this study, the efficacy of transurethral prostate ablation in the presence of silica-shell ultrasound-triggered phase-shift emulsions (sUPEs) doped with MR contrast was evaluated. The influence of sUPEs on MR imaging assessment of the ablation zone was also investigated.

METHODS

sUPEs were doped with a magnetic resonance (MR) contrast agent, Gd2 O3 , to assess ultrasound transition. Injections of saline (sham), saline and sUPEs alone, and saline and sUPEs with Optison microbubbles were performed under guidance of a prototype interventional MRI navigation platform in a healthy canine prostate. Treatment arms were evaluated for differences in lesion size, T1  contrast, and temperature. In addition, non-perfused areas (NPAs) on dynamic contrast-enhanced (DCE) MRI, 55°C isotherms, and areas of 240 cumulative equivalent minutes at 43°C (CEM43 ) dose or greater computed from MR thermometry were measured and correlated with ablated areas indicated by histology.

RESULTS

For treatment arms including sUPEs, the computed correlation coefficients between the histological ablation zone and the NPA, 55°C isotherm, and 240 CEM43 area ranged from 0.96-0.99, 0.98-0.99, and 0.91-0.99, respectively. In the absence of sUPEs, the computed correlation coefficients between the histological ablation zone and the NPA, 55°C isotherm, and 240 CEM43 area were 0.69, 0.54, and 0.50, respectively. Across all treatment arms, the areas of thermal tissue damage and NPAs were not significantly different (P = 0.47). Areas denoted by 55°C isotherms and 240 CEM43 dose boundaries were significantly larger than the areas of thermal damage, again for all treatment arms (P = 0.009 and 0.003, respectively). No significant differences in lesion size, T1 contrast, or temperature were observed between any of the treatment arms (P > 0.0167). Lesions exhibiting thermal fixation on histological analysis were present in six of nine insonations involving sUPE injections and one of five insonations involving saline sham injections. Significantly larger areas (P = 0.002), higher temperatures (P = 0.004), and more frequent ring patterns of restricted diffusion on ex vivo diffusion-weighted imaging (P = 0.005) were apparent in lesions with thermal fixation.

CONCLUSIONS

T1 contrast suggesting sUPE transition was not evident in sUPE treatment arms. The use of MR imaging metrics to predict prostate ablation was not diminished by the presence of sUPEs. Lesions generated in the presence of sUPEs exhibited more frequent thermal fixation, though there were no significant changes in the ablation areas when comparing arms with and without sUPEs. Thermal fixation corresponded to some qualitative imaging features.

Acoustic Methods for Increasing the Cavitation Initiation Pressure Threshold

Histotripsy is a tissue ablation method that utilizes focused, high-amplitude ultrasound to generate a cavitation bubble cloud that mechanically fractionates tissue. Effective histotripsy depends on the initiation, control, and maintenance of cavitation bubble clouds in the targeted area. In this study, we hypothesized that a low-pressure acoustic pulse sequence applied before and/or during histotripsy therapy would increase the cavitation initiation pressure threshold and the growth of cavitation bubble clouds. This technique could shrink or "sharpen" the focal zone during histotripsy to produce more precise and well-defined lesions with minimal collateral damage. It may also be a way to actively protect the soft tissue from cavitation damage during lithotripsy by increasing the pressure threshold for bubble cloud initiation. We applied these low-amplitude acoustic pulse sequences before and during histotripsy treatments with the pulse repetition frequency of 1 and 100 Hz, in three different mediums: water, tissue phantom agarose gel, and bovine liver in vitro. Acoustic backscatter signals and optical imaging were used to detect and monitor the initiation, maintenance, and growth of the resulting cavitation bubble cloud. The results demonstrated that the use of low-amplitude acoustic pulse sequences could increase the cavitation pressure amplitude threshold by 20% in the targeted area.