Observation of a cavitation cloud in tissue using correlation between ultrafast ultrasound images

Experimental Evaluation of an Ultrasound-Guided High-Intensity-Focused Ultrasound Probe for Sonication of Artery

OBJECTIVES

This study aimed to develop an ultrasound-guided high-intensity-focused ultrasound (USgHIFU) probe for arterial sonication and to evaluate vascular contraction.

METHODS

The USgHIFU probe comprised two confocal spherical transducers for sonication and a US color Doppler flow imaging probe for guidance. A vessel-mimicking phantom was sonicated in two directions. In the vascular radial direction, an isolated rabbit aorta embedded in ex vivo pork liver was sonicated at different acoustic powers (245 and 519 W), flow rates (25, 30, and 50 mL/minute), and sonication energies (519, 980, and 1038 J). Changes in the postsonication vessels were evaluated using US imaging, microscopic observation, and histopathological analysis.

RESULTS

Beam focusing along the vascular radial direction caused significant deformation of both tube walls (n = 4), whereas focusing along the axial direction only affected the contraction of the anterior wall (n = 4). The contraction index (Dc) of the vessel sonicated at 245 W and 980 J was 56.2 ± 9.7% (n = 12) with 25 mL/minute. The Dc of the vessel sonicated at 519 W and 1038 J was 56.5 ± 7.8% (n = 17). The Dc of the vessel sonicated at 519 J total energy was 18.3 ± 5.1% (n = 12).

CONCLUSION

The developed USgHIFU probe induced greater vascular contractions by covering a larger area of the vessel wall in the radial direction. Sonication energy affects vascular contraction through temperature elevation of the vessel wall. When the acoustic power was high, an increase in acoustic power, even with comparable sonication energy, did not result in greater vessel contraction.

Ultrasound imaging guided precision histotripsy: Effects of pulse settings on ablation properties in rat brain

A high-frequency 6 MHz miniature handheld histotripsy device with an endoscopic form factor and co-registered high-resolution ultrasound imaging was developed. This device could allow precision histotripsy ablation during minimally invasive brain tumor surgeries with real-time image guidance. This study characterized the outcome of acute histotripsy in the normal in vivo rat brain using the device with a range of histotripsy pulse settings, including number of cycles, pulse repetition frequency, and pressure, as well as other experimental factors. The stability and shape of the bubble cloud were measured during ablations, as well as the post-histotripsy ablation shape in ultrasound B-mode and histology. The results were compared between histological images and the ultrasound imaging data to determine how well ultrasound data reflected observable damage in histology. The results indicated that while pulse settings can have some influence on ablation shape, sample-to-sample variation had a larger influence on ablation shape. This suggests that real-time ablation monitoring is essential for accurate knowledge of outcomes. Ultrasound imaging provided an accurate real-time indication of ablation shape both during ablation and post-ablation.

Effect of Overpressure on Acoustic Emissions and Treated Tissue Histology in ex Vivo Bulk Ultrasound Ablation

Bulk ultrasound ablation is a thermal therapy approach in which tissue is heated by unfocused or weakly focused sonication (average intensities on the order of 100 W/cm²) to achieve coagulative necrosis within a few minutes exposure time. Assessing the role of bubble activity, including acoustic cavitation and tissue vaporization, in bulk ultrasound ablation may help in making bulk ultrasound ablation safer and more effective for clinical applications. Here, two series of ex vivo ablation trials were conducted to investigate the role of bubble activity and tissue vaporization in bulk ultrasound ablation. Fresh bovine liver tissue was ablated with unfocused, continuous-wave ultrasound using ultrasound image-ablate arrays sonicating at 31 W/cm² (0.9 MPa amplitude) for either 20 min at a frequency of 3.1 MHz or 10 min at 4.8 MHz. Tissue specimens were maintained at a static overpressure of either 0.52 or 1.2 MPa to suppress bubble activity and tissue vaporization or at atmospheric pressure for control groups. A passive cavitation detector was used to record subharmonic (1.55 or 2.4 MHz), broadband (1.2-1.5 MHz) and low-frequency (5-20 kHz) acoustic emissions. Treated tissue was stained with 2% triphenyl tetrazolium chloride to evaluate thermal lesion dimensions. Subharmonic emissions were significantly reduced in overpressure groups compared with control groups. Correlations observed between acoustic emissions and lesion dimensions were significant and positive for the 3.1-MHz series, but significant and negative for the 4.8-MHz series. The results indicate that for bulk ultrasound ablation, where both acoustic cavitation and tissue vaporization are possible, bubble activity can enhance ablation in the absence of tissue vaporization, but can reduce thermal lesion dimensions in the presence of vaporization.

Observation and modulation of the dissolution of histotripsy-induced bubble clouds with high-frame rate plane wave imaging

Focused ultrasound therapies are a noninvasive means to ablate tissue. Histotripsy utilizes short ultrasound pulses with sufficient tension to nucleate bubble clouds that impart lethal strain to the surrounding tissues. Tracking bubble cloud dissolution between the application of histotripsy pulses is critical to ensure treatment efficacy. In this study, plane wave B-mode imaging was employed to monitor bubble cloud motion and grayscale at frame rates up to 11.25 kHz. Minimal changes in the area or position of the bubble clouds were observed 50 ms post excitation. The bubble cloud grayscale was observed to decrease with the square root of time, indicating a diffusion-driven process. These results were qualitatively consistent with an analytic model of gas diffusion during the histotripsy process. Finally, the rate of bubble cloud dissolution was found to be dependent on the output of the imaging pulse, indicating an interaction between the bubble cloud and imaging parameters. Overall, these results highlight the utility of plane wave B-mode imaging for monitoring histotripsy bubble clouds.

SVD-Based Separation of Stable and Inertial Cavitation Signals Applied to Passive Cavitation Mapping During HIFU

Detection of inertial and stable cavitation is important for guiding high-intensity focused ultrasound (HIFU). Acoustic transducers can passively detect broadband noise from inertial cavitation and the scattering of HIFU harmonics from stable cavitation bubbles. Conventional approaches to cavitation noise diagnostics typically involve computing the Fourier transform of the time-domain noise signal, applying a custom comb filter to isolate the frequency components of interest, followed by an inverse Fourier transform. We present an alternative technique based on singular value decomposition (SVD) that efficiently separates the broadband emissions and HIFU harmonics. Spatiotemporally resolved cavitation detection was achieved using a 128-element, 5-MHz linear-array ultrasound imaging system operating in the receive mode at 15 frames/s. A 1.1-MHz transducer delivered HIFU to tissue-mimicking phantoms and excised liver tissue for a duration of 5 s. Beamformed radio frequency signals corresponding to each scan line in a frame were assembled into a matrix, and SVD was performed. Spectra of the singular vectors obtained from a tissue-mimicking gel phantom were analyzed by computing the peak ratio ( R ), defined as the ratio of the peak of its fifth-order polynomial fit and the maximum spectral peak. Singular vectors that produced an were classified as those representing stable cavitation, i.e., predominantly containing harmonics of HIFU. The projection of data onto this singular base reproduced stable cavitation signals. Similarly, singular vectors that produced an were classified as those predominantly containing broadband noise associated with inertial cavitation. These singular vectors were used to isolate the inertial cavitation signal. The R -value thresholds determined using gel data were then employed to analyze cavitation data obtained from bovine liver ex vivo. The SVD-based method faithfully reproduced the structural details in the spatiotemporal cavitation maps produced using the more cumbersome comb-filter approach with a maximum root-mean-squared error of 10%.

For Whom the Bubble Grows: Physical Principles of Bubble Nucleation and Dynamics in Histotripsy Ultrasound Therapy

Histotripsy is a focused ultrasound therapy for non-invasive tissue ablation. Unlike thermally ablative forms of therapeutic ultrasound, histotripsy relies on the mechanical action of bubble clouds for tissue destruction. Although acoustic bubble activity is often characterized as chaotic, the short-duration histotripsy pulses produce a unique and consistent type of cavitation for tissue destruction. In this review, the action of histotripsy-induced bubbles is discussed. Sources of bubble nuclei are reviewed, and bubble activity over the course of single and multiple pulses is outlined. Recent innovations in terms of novel acoustic excitations, exogenous nuclei for targeted ablation and histotripsy-enhanced drug delivery and image guidance metrics are discussed. Finally, gaps in knowledge of the histotripsy process are highlighted, along with suggested means to expedite widespread clinical utilization of histotripsy.

The influence of gas diffusion on bubble persistence in shock-scattering histotripsy

Bubble cloud persistence reduces the efficacy of mechanical liquefaction with shock-scattering histotripsy. In this study, the contribution of gas transfer to bubble longevity was investigated in silico by solving the equations for bubble oscillations and diffusion in parallel. The bubble gas content increased more than 5 orders of magnitude during the expansion phase, arresting the inertial collapse. The residual gas bubble required more than 15 ms for passive dissolution post excitation, consistent with experimental observation. These results demonstrate gas diffusion is an important factor in the persistence of histotripsy-induced cavitation.

Cavitation-threshold Determination and Rheological-parameters Estimation of Albumin-stabilized Nanobubbles

Nanobubbles (NBs) are of high interest for ultrasound (US) imaging as contrast agents and therapy as cavitation nuclei. Because of their instability (Laplace pressure bubble catastrophe) and low sensitivity to US, reducing the size of commonly used microbubbles to submicron-size is not trivial. We introduce stabilized NBs in the 100-250-nm size range, manufactured by agitating human serum albumin and perfluoro-propane. These NBs were exposed to 3.34- and 5.39-MHz US, and their sensitivity to US was proven by detecting inertial cavitation. The cavitation-threshold information was used to run a numerical parametric study based on a modified Rayleigh-Plesset equation (with a Newtonian rheology model). The determined values of surface tension ranged from 0 N/m to 0.06 N/m. The corresponding values of dilatational viscosity ranged from 5.10-10 Ns/m to 1.10-9 Ns/m. These parameters were reported to be 0.6 N/m and 1.10-8 Ns/m for the reference microbubble contrast agent. This result suggests the possibility of using albumin as a stabilizer for the nanobubbles that could be maintained in circulation and presenting satisfying US sensitivity, even in the 3-5-MHz range.

Evaluation of a Three Hydrophones Method for 2-Dimensional Cavitation Localization

Cavitation is a critical parameter in various therapeutic applications involving ultrasound (US) such as histotrispy, lithothripsy, drug delivery, and cavitation-enhanced hyperthermia. A cavitation exposure outside the region of interest may lead to suboptimal treatment efficacy or in a worse case, to safety issues. Current methods of localizing cavitation are based on imaging approaches, such as beamforming the cavitation signals received passively by a US imager. These methods, although efficient, require expensive equipment, which may discourage potential future developments. We propose a threehydrophone method to localize the cavitation cloud source. Firstly, the delays between the three receptors are measured by detecting the maximum of their inter-correlations. Then, the position of the source is calculated by either minimizing a cost function or solving hyperbolic equations. After a numerical validation, the method was assessed experimentally. This method was able to track a source displacement with accuracy similar to the size of the cavitation cloud (2-4 millimeters). This light and versatile method provides interesting perspectives since localization can be executed in real time and the extension to three-dimensional localization seems straightforward.

Post Hoc Analysis of Passive Cavitation Imaging for Classification of Histotripsy-Induced Liquefaction in Vitro

Histotripsy utilizes focused ultrasound to generate bubble clouds for transcutaneous tissue liquefaction. Bubble activity maps are under development to provide image guidance and monitor treatment progress. The aim of this paper was to investigate the feasibility of using plane wave B-mode and passive cavitation images to be used as binary classifiers of histotripsy-induced liquefaction. Prostate tissue phantoms were exposed to histotripsy pulses over a range of pulse durations (5- ) and peak negative pressures (12-23 MPa). Acoustic emissions were recorded during the insonation and beamformed to form passive cavitation images. Plane wave B-mode images were acquired following the insonation to detect the hyperechoic bubble cloud. Phantom samples were sectioned and stained to delineate the liquefaction zone. Correlation between passive cavitation and plane wave B-mode images and the liquefaction zone was assessed using receiver operating characteristic (ROC) curve analysis. Liquefaction of the phantom was observed for all the insonation conditions. The area under the ROC (0.94 versus 0.82), accuracy (0.90 versus 0.83), and sensitivity (0.81 versus 0.49) was greater for passive cavitation images relative to B-mode images ( ) along the azimuth of the liquefaction zone. The specificity was greater than 0.9 for both imaging modalities. These results demonstrate a stronger correlation between histotripsy-induced liquefaction and passive cavitation imaging compared with the plane wave B-mode imaging, albeit with limited passive cavitation image range resolution.

Numerical study of a confocal ultrasonic setup for cavitation creation

Acoustic cavitation has found a wide range of applications in the last few decades. For potential applications involving cavitation, the acoustic characteristics of a confocal ultrasonic setup are studied: two high-intensity focused ultrasound transducers are mounted so that their focal points overlap. A mathematical simulator is developed that takes into account nonlinear propagation, absorption, and diffraction. Each one of these physical effects is solved in the frequency domain for successive planes. Comparing the confocal setup with equivalent single transducer setups, it is shown that, with the confocal configuration, nonlinear distortion of the waveform is reduced, resulting in a greater peak rarefactional pressure and a lower peak positive pressure. Furthermore, additional features are investigated for confocal configurations such as a greater spatial stability for the focal point, which can be maintained while increasing the pressure level, and a focal region consisting of interference acting as an acoustic trap.

Enhancement of Fluorescent Probe Penetration into Tumors In Vivo Using Unseeded Inertial Cavitation

Ultrasound-induced cavitation has found many applications in the field of cancer therapy. One of its beneficial effects is the enhancement of drug intake by tumor cells. Our group has developed a device that can create and control unseeded cavitation in tissue using ultrasound. We conducted experiments on tumor-bearing mice using our device to assess the impact of sonication on the penetration of fluorescent probes into tumor cells. We studied the influence of pressure level, timing of sonication and sonication duration on treatment efficiency. Our results indicate that fluorescent probes penetrate better into tumors exposed to ultrasound. The best results revealed an increase in penetration of 61% and were obtained when sonicating the tumor in presence of the probes with a peak negative pressure at focus of 19 MPa. At this pressure level, the treatment generated only minor skin damage. Treatments could be significantly accelerated as equivalent enhanced penetration of probes was achieved when multiplying the initial raster scan speed by a factor of four.