Therapeutic ultrasound to noninvasively create intracardiac communications in an intact animal model

Revolutionizing cardiovascular care: the power of histotripsy

Histotripsy, an innovative ultrasonic technique, is poised to transform the landscape of cardiovascular disease management. This review explores the multifaceted applications of histotripsy across various domains of cardiovascular medicine. In thrombolysis, histotripsy presents a non-invasive, drug-free, and precise method for recanalizing blood vessels obstructed by clots, minimizing the risk of vessel damage and embolism. Additionally, histotripsy showcases its potential in congenital heart defect management, offering a promising alternative to invasive procedures by creating intracardiac communications noninvasively. For patients with calcified aortic stenosis, histotripsy demonstrates its effectiveness in softening calcified bioprosthetic valves, potentially revolutionizing valve interventions. In the realm of arrhythmias, histotripsy could play an important role in scar-based ventricular tachycardia ablation, eliminating channel-like isthmuses of slowly conducting myocardium. Histotripsy`s potential applications also extend to structural heart interventions, enabling the safe sectioning of basal chordae and potentially addressing mitral regurgitation. Furthermore, it showcases its versatility by safely generating ventricular septal defects, providing a non-invasive means of creating intracardiac communications in neonates with congenital heart disease. Yet, most supporting studies are in-vitro or animal studies and there are possible challenges in translating experimental data on cardiac histotripsy to the clinical level. As histotripsy continues to evolve and mature, its remarkable potential in cardiovascular disease management holds promise for improving patient outcomes and reducing the burden of invasive procedures in the field of cardiology.

Histotripsy - hype or hope? Review of innovation and future implications

BACKGROUND

Histotripsy is a novel, ultrasound-based ablative technique that was recently approved by the Food and Drug Administration for hepatic targets. It has several promising additional theoretical applications that need to be further investigated. Its basis as a nonthermal cavitational technology presents a unique advantage over existing thermal ablation techniques in maximizing local effects while minimizing adjacent tissue destruction. This review discusses the technical basis and current preclinical and clinical data surrounding histotripsy.

METHODS

This was a comprehensive review of the literature surrounding histotripsy and the clinical landscape of existing ablative techniques using the PubMed database. A technical summary of histotripsy's physics and cellular effect was described. Moreover, data from recent clinical trials, including Hope4Liver, and future implications regarding its application in various benign and malignant conditions were discussed.

RESULTS

Preclinical data demonstrated the efficacy of histotripsy ablation in various organ systems with minimal tissue destruction when examined at the histologic level. The first prospective clinical trial involving histotripsy in hepatocellular carcinoma and liver metastases, Hope4Liver, demonstrated a primary efficacy of 95.5% with minimal complications (6.8%). This efficacy was replicated in similar trials involving the treatment of benign prostatic hypertrophy.

DISCUSSION

In addition to the noninvasive ability to ablate lesions in the liver, histotripsy offers additional therapeutic potential. Early data suggest a potential complementary therapeutic effect when combining histotripsy with existing immunologic therapies because of the technology's theoretical ability to sensitize tumors to adaptive immunity. As with most novel therapies, the effect of histotripsy on the oncologic therapeutic landscape remains uncertain.

Histotripsy: A Method for Mechanical Tissue Ablation with Ultrasound

Histotripsy is a relatively new therapeutic ultrasound technology to mechanically liquefy tissue into subcellular debris using high-amplitude focused ultrasound pulses. In contrast to conventional high-intensity focused ultrasound thermal therapy, histotripsy has specific clinical advantages: the capacity for real-time monitoring using ultrasound imaging, diminished heat sink effects resulting in lesions with sharp margins, effective removal of the treated tissue, a tissue-selective feature to preserve crucial structures, and immunostimulation. The technology is being evaluated in small and large animal models for treating cancer, thrombosis, hematomas, abscesses, and biofilms; enhancing tumor-specific immune response; and neurological applications. Histotripsy has been recently approved by the US Food and Drug Administration to treat liver tumors, with clinical trials undertaken for benign prostatic hyperplasia and renal tumors. This review outlines the physical principles of various types of histotripsy; presents major parameters of the technology and corresponding hardware and software, imaging methods, and bioeffects; and discusses the most promising preclinical and clinical applications.

Insights from in vivo preclinical cancer studies with histotripsy

Histotripsy is the first noninvasive, non-ionizing, and non-thermal ablation technique that mechanically fractionates target tissue into acellular homogenate via controlled acoustic cavitation. Histotripsy has been evaluated for various preclinical applications requiring noninvasive tissue removal including cancer, brain surgery, blood clot and hematoma liquefaction, and correction of neonatal congenital heart defects. Promising preclinical results including local tumor suppression, improved survival outcomes, local and systemic anti-tumor immune responses, and histotripsy-induced abscopal effects have been reported in various animal tumor models. Histotripsy is also being investigated in veterinary patients with spontaneously arising tumors. Research is underway to combine histotripsy with immunotherapy and chemotherapy to improve therapeutic outcomes. In addition to preclinical cancer research, human clinical trials are ongoing for the treatment of liver tumors and renal tumors. Histotripsy has been recently approved by the FDA for noninvasive treatment of liver tumors. This review highlights key learnings from in vivo shock-scattering histotripsy, intrinsic threshold histotripsy, and boiling histotripsy cancer studies treating cancers of different anatomic locations and discusses the major considerations in planning in vivo histotripsy studies regarding instrumentation, tumor model, study design, treatment dose, and post-treatment tumor monitoring.

The histotripsy spectrum: differences and similarities in techniques and instrumentation

Since its inception about two decades ago, histotripsy - a non-thermal mechanical tissue ablation technique - has evolved into a spectrum of methods, each with distinct potentiating physical mechanisms: intrinsic threshold histotripsy, shock-scattering histotripsy, hybrid histotripsy, and boiling histotripsy. All methods utilize short, high-amplitude pulses of focused ultrasound delivered at a low duty cycle, and all involve excitation of violent bubble activity and acoustic streaming at the focus to fractionate tissue down to the subcellular level. The main differences are in pulse duration, which spans microseconds to milliseconds, and ultrasound waveform shape and corresponding peak acoustic pressures required to achieve the desired type of bubble activity. In addition, most types of histotripsy rely on the presence of high-amplitude shocks that develop in the pressure profile at the focus due to nonlinear propagation effects. Those requirements, in turn, dictate aspects of the instrument design, both in terms of driving electronics, transducer dimensions and intensity limitations at surface, shape (primarily, the F-number) and frequency. The combination of the optimized instrumentation and the bio-effects from bubble activity and streaming on different tissues, lead to target clinical applications for each histotripsy method. Here, the differences and similarities in the physical mechanisms and resulting bioeffects of each method are reviewed and tied to optimal instrumentation and clinical applications.

Effects of frequency on bubble-cloud behavior and ablation efficiency in intrinsic threshold histotripsy

Objective.Histotripsy is a non-thermal focused ultrasound ablation method that destroys tissue through the generation of a cavitation bubble cloud. Previous work studying intrinsic threshold histotripsy has shown that dense bubble clouds can be formed by a single-cycle pulse when the negative pressure exceeds an intrinsic threshold of ∼25-30 MPa, with the ablation efficiency dependent upon the size and density of bubbles within the cloud. This work investigates the effects of frequency on bubble-cloud behavior and ablation efficiency in intrinsic threshold histotripsy.Approach.A modular transducer was used to expose agarose tissue phantoms to 500 kHz, 1 MHz, or 3 MHz, histotripsy pulses. Optical imaging was used to measure the bubble-cloud dimensions, bubble density, and bubble size. The effects of frequency on ablation efficiency were also investigated by applying histotripsy to red blood cell (RBC) phantoms.Main results.Results revealed that the bubble-cloud size closely matched theoretical predictions for all frequencies. The bubble density, which is a measure of the number of bubbles per unit area, was shown to increase with increasing frequency while the size of individual bubbles within the cloud decreased at higher frequencies. Finally, RBC phantom experiments showed decreasing ablation efficiency with increasing frequency.Significance.Overall, results demonstrate the effects of frequency on histotripsy bubble-cloud behavior and show that lower frequency generates more efficient tissue ablation, primarily due to enhanced bubble expansion.

Histotripsy: the first noninvasive, non-ionizing, non-thermal ablation technique based on ultrasound

Histotripsy is the first noninvasive, non-ionizing, and non-thermal ablation technology guided by real-time imaging. Using focused ultrasound delivered from outside the body, histotripsy mechanically destroys tissue through cavitation, rendering the target into acellular debris. The material in the histotripsy ablation zone is absorbed by the body within 1-2 months, leaving a minimal remnant scar. Histotripsy has also been shown to stimulate an immune response and induce abscopal effects in animal models, which may have positive implications for future cancer treatment. Histotripsy has been investigated for a wide range of applications in preclinical studies, including the treatment of cancer, neurological diseases, and cardiovascular diseases. Three human clinical trials have been undertaken using histotripsy for the treatment of benign prostatic hyperplasia, liver cancer, and calcified valve stenosis. This review provides a comprehensive overview of histotripsy covering the origin, mechanism, bioeffects, parameters, instruments, and the latest results on preclinical and human studies.

Feasibility and safety of non-invasive ultrasound therapy (NIUT) on an porcine aortic valve

Calcific aortic stenosis (CAS) is associated with advanced age and comorbidities, therefore a non-invasive therapy for it would be beneficial. We previously demonstrated that ultrasound therapy improved calcified bioprosthetic valve function in an open chest model. For translational applications, we tested non-invasive ultrasound therapy (NIUT) transthoracically on swine aortic valves and investigated the need for antithrombotic treatment as a follow-up. Primary objective: feasibility and safety of NIUT. Secondary objectives: occurrence, severity and evolution of side effects during therapy and at 1 month follow-up. The device (Valvosoft, Cardiawave) consisted of an electronically steered multi-element transducer and a 2D echocardiographic probe. Three groups of swine received treatment on aortic valves: NIUT (group 1; n = 10); NIUT and 1 month antithrombotic treatment (group 2; n = 5); sham group (group 3; n = 4). Feasibility was successfully reached in all treated swine (n = 15) and no life-threatening arrhythmia were detected. Non-sustained ventricular tachycardia occurred during the procedure in seven swine. Decrease or interruption of NIUT ended arrhythmia. Histopathology revealed no valve or surrounding tissue damage and echocardiography revealed no valvular dysfunction. Only one animal had side effects [right ventricle (RV) dilatation], but the RV normalized after therapy cessation with no sequelae at follow-up. No disturbance in biological markers nor valve thrombosis were observed at follow-up. Antithrombotic treatment did not demonstrate any advantage. Survival at 30 d was 100%. We demonstrated, in vivo, the feasibility and safety of transthoracic NIUT on aortic valves in a swine model without serious adverse events. We expect this first-time transthoracic delivery of NIUT to pave the way towards a new non-invasive approach to valve softening in human CAS to restore valve function.

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.

Coalescence of residual histotripsy cavitation nuclei using low-gain regions of the therapy beam during electronic focal steering

Following collapse of a histotripsy cloud, residual microbubbles may persist for seconds, distributed throughout the focus. Their presence can attenuate and scatter subsequent pulses, hindering treatment speed and homogeneity. Previous studies have demonstrated use of separate low-amplitude (~1 MPa) pulses interleaved with histotripsy pulses to drive bubble coalescence (BC), significantly improving treatment speed without sacrificing homogeneity. We propose that by using electronic focal steering (EFS) to direct the therapy focus throughout specially-designed EFS sequences, it is possible to use low-gain regions of the therapy beam to accomplish BC during EFS without any additional acoustic sequence. First, to establish proof of principle for an isolated focus, a 50-foci EFS sequence was constructed with the first position isolated near the geometric focus and remaining positions distributed post-focally. EFS sequences were evaluated in tissue-mimicking phantoms with gas concentrations of 20% and 100% with respect to saturation. Results using an isolated focus demonstrated that at 20% gas concentration, 49 EFS pulses were sufficient to achieve BC in all samples for pulse repetition frequency (PRF)  ⩽  800 Hz and 84.1%  ±  3.0% of samples at 5 kHz PRF. For phantoms prepared with 100% gas concentration, BC was achieved by 49 EFS pulses in 39.2%  ±  4.7% of samples at 50 Hz PRF and 63.4%  ±  15.3% of samples at 5 kHz. To show feasibility of using the EFS-BC method to ablate a large volume quickly, a 1000-foci EFS sequence covering a volume of approximately 27 ml was tested. Results indicate that the BC effect was similarly present. A treatment rate of 27  ±  6 ml min^-1 was achieved, which is signficantly faster than standard histotripsy and ultrasound thermal ablation. This study demonstrates that histotripsy with EFS can achieve BC without employing a separate acoustic sequence which has the potential to accelerate large-volume ablation while minimizing energy deposition.

Histotripsy Using Fundamental and Second Harmonic Superposition Combined with Hundred-Microsecond Ultrasound Pulses

A novel histotripsy approach based on fundamental and second harmonic superposition and incorporating hundred-microsecond-long pulses and two-stage pulse protocol is proposed in this study to rapidly generate mechanically homogenized lesions. Two pulse stages were applied: stage 1, pulses with a pulse duration of 500-600 μs and pulse repetition frequency of 100 Hz, and stage 2, multiple periods, each composed of multiple pulses with the same pulse duration and pulse repetition frequency as those in stage 1, but with an off-time of 600 ms between periods. A custom-designed 1.1/2.2-MHz two-element confocal-annular array, with an f-number of 0.69, and lateral and axial full width at half-maximum pressure dimensions of approximately 1.0 and 6.0 mm, was used. The peak positive/negative pressures at the focus were +22/-7 MPa for 1.1 MHz and +56/-14 MPa with shock wave for 2.2 MHz. To investigate the feasibility of this approach, experiments were designed and performed in tissue-mimicking polyacrylamide gel phantoms with bovine serum albumin and in ex vivo porcine tissues. Cavitation and boiling activities were observed through high-speed photography, and the corresponding acoustic emissions were recorded through passive cavitation detection. Ex vivo experimental results revealed that complete tissue homogeneous regions with regular long tear shape and typical dimensions of 5.80 ± 0.19 mm in axial and 2.20 ± 0.26 mm in lateral were successfully generated in porcine kidney samples. The hematoxylin and eosin staining evidenced that the lesions were thoroughly homogenized and sharply demarcated from untreated regions. These results indicated that the histotripsy approach using fundamental and second harmonic superposition combined with hundred-microsecond pulses and two-stage pulse protocol can efficiently obtain a mechanical disruption of soft tissues with spatial precision, and this approach may have the potential to be developed as a useful tool for precise tumor treatment.

Integrated Histotripsy and Bubble Coalescence Transducer for Rapid Tissue Ablation

Residual bubbles produced after collapse of a cavitation cloud provide cavitation nuclei for subsequent cavitation events, causing cavitation to occur repeatedly at the same discrete set of sites. This effect, referred to as cavitation memory, limits the efficiency of histotripsy soft tissue fractionation. Besides passively mitigating cavitation memory by using a low pulse repetition frequency (~1 Hz), an active strategy was developed by our group. In this strategy, low-amplitude ultrasound sequences were used to stimulate coalescence of residual bubbles. The goal of this work is to remove cavitation memory and achieve rapid, homogeneous lesion formation using a single phased array transducer. A 1-MHz integrated histotripsy and bubble coalescing (BC) transducer system with a specialized electronic driving system was built in house. High-amplitude ( MPa) histotripsy pulses and subsequent low-amplitude (~1-2 MPa) BC sequences were applied to a red blood cell tissue-mimicking phantom at a single focal site. Significant reduction of the cavitation memory effect and increase in the fractionation rate were observed by introducing BC sequence. Effects of BC pulsing parameters were further studied. The optimal BC parameters were then utilized to homogenize a mm² region at high rate.

Bubble-Induced Color Doppler Feedback Correlates with Histotripsy-Induced Destruction of Structural Components in Liver Tissue

Bubble-induced color Doppler (BCD) is a histotripsy-therapy monitoring technique that uses Doppler ultrasound to track the motion of residual cavitation nuclei that persist after the collapse of the histotripsy bubble cloud. In this study, BCD is used to monitor tissue fractionation during histotripsy tissue therapy, and the BCD signal is correlated with the destruction of structural and non-structural components identified histologically to further understand how BCD monitors the extent of treatment. A 500-kHz, 112-element phased histotripsy array is used to generate approximately 6- × 6- × 7-mm lesions within ex vivo bovine liver tissue by scanning more than 219 locations with 30-1000 pulses per location. A 128-element L7-4 imaging probe is used to acquire BCD signals during all treatments. The BCD signal is then quantitatively analyzed using the time-to-peak rebound velocity (tprv) metric. Using the Pearson correlation coefficient, the tprv is compared with histologic analytics of lesions generated by various numbers of pulses using a significance level of 0.001. Histologic analytics in this study include viable cell count, reticulin-stained type III collagen area and trichrome-stained type I collagen area. It is found that the tprv metric has a statistically significant correlation with the change in reticulin-stained type III collagen area with a Pearson correlation coefficient of -0.94 (p <0.001), indicating that changes in BCD are more likely because of destruction of the structural components of tissue.

The response of MRI contrast parameters in in vitro tissues and tissue mimicking phantoms to fractionation by histotripsy

Histotripsy is a non-invasive, focused ultrasound lesioning technique that can ablate precise volumes of soft tissue using a novel mechanical fractionation mechanism. Previous research suggests that magnetic resonance imaging (MRI) may be a sensitive image-based feedback mechanism for histotripsy. However, there are insufficient data to form some unified understanding of the response of the MR contrast mechanisms in tissues to histotripsy. In this paper, we investigate the response of the MR contrast parameters R1, R2, and the apparent diffusion coefficient (ADC) to various treatment levels of histotripsy in in vitro porcine liver, kidney, muscle, and blood clot as well in formulations of bovine red blood cells suspended in agar gel. We also make a histological analysis of histotripsy lesions in porcine liver. We find that R2 and the ADC are both sensitive to ablation in all materials tested here, and the degree of response varies with tissue type. Correspondingly, under histologic analysis, the porcine liver exhibited various levels of mechanical disruption and necrotic debris that are characteristic of histotripsy. While the area of intact red blood cells and nuclei found within these lesions both decreased with increasing amounts of treatment, the area of red blood cells decreased much more rapidly than the area of intact nuclei. Additionally, the decrease in area of intact red blood cells saturated at the same treatment levels at which the response of the R2 saturated while the area of intact nuclei appeared to vary linearly with the response of the ADC.

Effects of f-number on the histotripsy intrinsic threshold and cavitation bubble cloud behavior

Histotripsy is an ultrasound ablation method that depends on the initiation of a cavitation bubble cloud to fractionate soft tissue. Although previous work has provided significant insight into the process of intrinsic threshold histotripsy, the majority of these studies have used highly focused (i.e. f-number  <  0.6) transducers. In this study, we investigate the effects of f-number on the histotripsy intrinsic threshold and cavitation bubble cloud behavior using a 500 kHz array transducer, with the effective f-number of the transducer varied from 0.51 to 0.89. The intrinsic threshold did not significantly change with f-number, with the threshold remaining ~27-30 MPa for all conditions. The predictability of intrinsic threshold histotripsy was further demonstrated by experiments comparing the predicted and experimentally measured bubble cloud dimensions, with results showing close agreement for all f-numbers. Finally, the effects of f-number on 'bubble density' and tissue fractionation efficiency were investigated, with results supporting the hypothesis that the density of the bubbles within the bubble cloud significantly decreases at higher f-numbers, resulting in decreased fractionation efficiency. Overall, this study provides significant insight into the effects of f-number on intrinsic threshold histotripsy that will help to guide the development of histotripsy for specific clinical applications.

Visualizing the Histotripsy Process: Bubble Cloud-Cancer Cell Interactions in a Tissue-Mimicking Environment

Histotripsy is a non-invasive ultrasonic ablation method that uses cavitation to mechanically fractionate tissue into acellular debris. With a sufficient number of pulses, histotripsy can completely fractionate tissue into a liquid-appearing homogenate with no cellular structures. The location, shape and size of lesion formation closely match those of the cavitation cloud. Previous work has led to the hypothesis that the rapid expansion and collapse of histotripsy bubbles fractionate tissue by inducing large stress and strain on the tissue structures immediately adjacent to the bubbles. In the work described here, the histotripsy bulk tissue fractionation process is visualized at the cellular level for the first time using a custom-built 2-MHz transducer incorporated into a microscope stage. A layer of breast cancer cells were cultured within an optically transparent fibrin-based gel phantom to mimic cells inside a 3-D extracellular matrix. To test the hypothesis, the cellular response to single and multiple histotripsy pulses was investigated using high-speed optical imaging. Bubbles were always generated in the extracellular space, and significant cell displacement/deformation was observed for cells directly adjacent to the bubble during both bubble expansion and collapse. The largest displacements were observed during collapse for cells immediately adjacent to the bubble, with cells moving more than 150-300 μm in less than 100 μs. Cells often underwent multiple large deformations (>150% strain) over multiple pulses, resulting in the bisection of cells multiple times before complete removal. To provide theoretical support to the experimental observations, a numerical simulation was conducted using a single-bubble model, which indicated that histotripsy exerts the largest strains and cell displacements in the regions immediately adjacent to the bubble. The experimental and simulation results support our hypothesis, which helps to explain the formation of the sharp lesions formed in histotripsy therapy localized to the regions directly exposed to the bubbles.

Histotripsy Produced by Hundred-Microsecond-Long Focused Ultrasonic Pulses: A Preliminary Study

A new strategy is proposed in this study to rapidly generate mechanical homogenized lesions using hundred-microsecond-long pulses. The pulsing scheme was divided into two stages: generating sufficient bubble seed nuclei via acceleration by boiling bubbles and efficiently forming a mechanically homogenized and regularly shaped lesion with a homogenate inside via inertial cavitation. The duty cycle was set at 4.9%/3.9% in stage 1 and 1%/0.88% in stage 2 by changing the pulse duration (PD) and off-time independently. The pulse sequence was 500-μs/400-μs PD with a 100-Hz pulse repetition frequency (PRF) in stage 1, followed by 500-μs/400-μs PD with a 100-Hz PRF and 200-μs PD with a 200-Hz PRF in stage 2. Experiments were conducted on polyacrylamide phantoms with bovine serum albumin and on ex vivo porcine kidney tissues using a single-element 1.06-MHz transducer at an 8-MPa peak negative pressure with shock waves. The lesion evolution and dynamic elastic modulus variation in the phantoms and the histology in the tissue samples were investigated. The results indicate that the two-stage treatment using hundred-microsecond-long pulses can efficiently produce mechanically homogenized lesions with smooth borders, long tear shapes and the total homogenate inside. The time to generate a single mechanically homogenized lesion is shortened from >50 s to 17.1 s.

Effects of Temperature on the Histotripsy Intrinsic Threshold for Cavitation

Histotripsy is an ultrasound ablation method that depends on the initiation of a dense cavitation bubble cloud to fractionate soft tissue. Previous work has demonstrated that a cavitation cloud can be formed by a single acoustic pulse with one high amplitude negative cycle, when the negative pressure amplitude exceeds a threshold intrinsic to the medium. The intrinsic thresholds in soft tissues and tissue phantoms that are water-based are similar to the intrinsic threshold of water over an experimentally verified frequency range of 0.3-3 MHz. Previous work studying the histotripsy intrinsic threshold has been limited to experiments performed at room temperature (~20°C). In this study, we investigate the effects of temperature on the histotripsy intrinsic threshold in water, which is essential to accurately predict the intrinsic thresholds expected over the full range of in vivo therapeutic temperatures. Based on previous work studying the histotripsy intrinsic threshold and classical nucleation theory, we hypothesize that the intrinsic threshold will decrease with increasing temperature. To test this hypothesis, the intrinsic threshold in water was investigated both experimentally and theoretically. The probability of generating cavitation bubbles was measured by applying a single pulse with one high amplitude negative cycle at 1 MHz to distilled, degassed water at temperatures ranging from 10°C-90°C. Cavitation was detected and characterized by passive cavitation detection and high-speed photography, from which the probability of cavitation was measured vs. pressure amplitude. The results indicate that the intrinsic threshold (the negative pressure at which the cavitation probability=0.5) significantly decreases with increasing temperature, showing a nearly linear decreasing trend from 29.8±0.4 MPa at 10˚C to 14.9±1.4 MPa at 90˚C. Overall, the results of this study support our hypothesis that the intrinsic threshold is highly dependent upon the temperature of the medium, which may allow for better predictions of cavitation generation at body temperature in vivo and at the elevated temperatures commonly seen in high intensity focused ultrasound (HIFU) regimes.

Review on Lithotripsy and Cavitation in Urinary Stone Therapy

Cavitation is the sudden formation of vapor bubbles or voids in liquid media and occurs after rapid changes in pressure as a consequence of mechanical forces. It is mostly an undesirable phenomenon. Although the elimination of cavitation is a major topic in the study of fluid dynamics, its destructive nature could be exploited for therapeutic applications. Ultrasonic and hydrodynamic sources are two main origins for generating cavitation. The purpose of this review is to give the reader a general idea about the formation of cavitation phenomenon and existing biomedical applications of ultrasonic and hydrodynamic cavitation. Because of the high number of the studies on ultrasound cavitation in the literature, the main focus of this review is placed on the lithotripsy techniques, which have been widely used for the treatment of urinary stones. Accordingly, cavitation phenomenon and its basic concepts are presented in Section II. The significance of the ultrasound cavitation in the urinary stone treatment is discussed in Section III in detail and hydrodynamic cavitation as an important alternative for the ultrasound cavitation is included in Section IV. Finally, side effects of using both ultrasound and hydrodynamic cavitation in biomedical applications are presented in Section V.

Effects of Droplet Composition on Nanodroplet-Mediated Histotripsy

Nanodroplet-mediated histotripsy (NMH) is a targeted ablation technique combining histotripsy with nanodroplets that can be selectively delivered to tumor cells. In two previous studies, polymer-encapsulated perfluoropentane nanodroplets were used to generate well-defined ablation similar to that obtained with histotripsy, but at significantly lower pressure, when NMH therapy was applied at a pulse repetition frequency (PRF) of 10 Hz. However, cavitation was not maintained over multiple pulses when ultrasound was applied at a lower PRF (i.e., 1-5 Hz). We hypothesized that nanodroplets with a higher-boiling-point perfluorocarbon core would provide sustainable cavitation nuclei, allowing cavitation to be maintained over multiple pulses, even at low PRF, which is needed for efficient and complete tissue fractionation via histotripsy. To test this hypothesis, we investigated the effects of droplet composition on NMH therapy by applying histotripsy at various frequencies (345 kHz, 500 kHz, 1.5 MHz, 3 MHz) to tissue phantoms containing perfluoropentane (PFP, boiling point ∼29°C, surface tension ∼9.5 mN/m) and perfluorohexane (PFH, boiling point ∼56°C, surface tension ∼11.9 mN/m) nanodroplets. First, the effects of droplet composition on the NMH cavitation threshold were investigated, with results revealing a significant decrease (>10 MPa) in the peak negative pressure (p-) cavitation threshold for both types of nanodroplets compared with controls. A slight decrease (∼1-3 MPa) in threshold was observed for PFP phantoms compared with PFH phantoms. Next, the ability of nanodroplets to function as sustainable cavitation nuclei over multiple pulses was investigated, with results revealing that PFH nanodroplets were sustainable cavitation nuclei over 1,000 pulses, whereas PFP nanodroplets were destroyed during the first few pulses (<50 pulses), likely because of the lower boiling point. Finally, tissue phantoms containing a layer of embedded red blood cells were used to compare the damage generated for NMH treatments using PFP and PFH droplets, with results indicating that PFH nanodroplets significantly improved NMH ablation, allowing for well-defined lesions to be generated at all frequencies and PRFs tested. Overall, the results of this study provide significant insight into the role of droplet composition in NMH therapy and provide a rational basis to tailor droplet parameters to improve NMH tissue fractionation.

Bubble-Induced Color Doppler Feedback for Histotripsy Tissue Fractionation

Histotripsy therapy produces cavitating bubble clouds to increasingly fractionate and eventually liquefy tissue using high-intensity ultrasound pulses. Following cavitation generated by each pulse, coherent motion of the cavitation residual nuclei can be detected using metrics formed from ultrasound color Doppler acquisitions. In this paper, three experiments were performed to investigate the characteristics of this motion as real-time feedback on histotripsy tissue fractionation. In the first experiment, bubble-induced color Doppler (BCD) and particle image velocimetry (PIV) analysis monitored the residual cavitation nuclei in the treatment region in an agarose tissue phantom treated with two-cycle histotripsy pulses at [Formula: see text] using a 500-kHz transducer. Both BCD and PIV results showed brief chaotic motion of the residual nuclei followed by coherent motion first moving away from the transducer and then rebounding back. Velocity measurements from both PIV and BCD agreed well, showing a monotonic increase in rebound time up to a saturation point for increased therapy dose. In a second experiment, a thin layer of red blood cells (RBC) was added to the phantom to allow quantification of the fractionation of the RBC layer to compare with BCD metrics. A strong linear correlation was observed between the fractionation level and the time to BCD peak rebound velocity over histotripsy treatment. Finally, the correlation between BCD feedback and histotripsy tissue fractionation was validated in ex vivo porcine liver evaluated histologically. BCD metrics showed strong linear correlation with fractionation progression, suggesting that BCD provides useful quantitative real-time feedback on histotripsy treatment progression.

Effects of Thermal Preconditioning on Tissue Susceptibility to Histotripsy

Histotripsy is a non-invasive ablation method that mechanically fractionates tissue by controlling acoustic cavitation. Previous work has revealed that tissue mechanical properties play a significant role in the histotripsy process, with stiffer tissues being more resistant to histotripsy-induced tissue damage. In this study, we propose a thermal pretreatment strategy to precondition tissues before histotripsy. We hypothesize that a thermal pretreatment can be used to alter tissue stiffness by modulating collagen composition, thus changing tissue susceptibility to histotripsy. More specifically, we hypothesize that tissues will soften and become more susceptible to histotripsy when preheated at ∼60°C because of collagen denaturation, but that tissues will rapidly stiffen and become less susceptible to histotripsy when preheated at ∼90°C because of collagen contraction. To test this hypothesis, a controlled temperature water bath was used to heat various ex vivo bovine tissues (tongue, artery, liver, kidney medulla, tendon and urethra). After heating, the Young's modulus of each tissue sample was measured using a tissue elastometer, and changes in tissue composition (i.e., collagen structure/density) were analyzed histologically. The susceptibility of tissues to histotripsy was investigated by treating the samples using a 750-kHz histotripsy transducer. Results revealed a decrease in stiffness and an increase in susceptibility to histotripsy for tissues (except urethra) preheated to 58°C. In contrast, preheating to 90°C increased tissue stiffness and reduced susceptibility to histotripsy for all tissues except tendon, which was significantly softened due to collagen hydrolysis into gelatin. On the basis of these results, a final set of experiments were conducted to determine the feasibility of using high-intensity focused ultrasound to provide the thermal pretreatment. Overall, the results of this study indicate the initial feasibility of a thermal pretreatment strategy to precondition tissue mechanical properties and alter tissue susceptibility to histotripsy. Future work will aim to optimize this thermal pretreatment strategy to determine if this approach is practical for specific clinical applications in vivo without causing unwanted damage to surrounding or overlying tissue.

Histotripsy Lesion Formation Using an Ultrasound Imaging Probe Enabled by a Low-Frequency Pump Transducer

When histotripsy pulses shorter than 2 cycles are applied, the formation of a dense bubble cloud relies only on the applied peak negative pressure (p-) exceeding the "intrinsic threshold" of the medium (absolute value of 26-30 MPa in most soft tissues). It has been found that a sub-threshold high-frequency probe pulse (3 MHz) can be enabled by a sub-threshold low-frequency pump pulse (500 kHz) where the sum exceeds the intrinsic threshold, thus generating lesion-producing dense bubble clouds ("dual-beam histotripsy"). Here, the feasibility of using an imaging transducer to provide the high-frequency probe pulse in the dual-beam histotripsy approach is investigated. More specifically, an ATL L7-4 imaging transducer (Philips Healthcare, Andover, MA, USA), pulsed by a V-1 Data Acquisition System (Verasonics, Redmond, WA, USA), was used to generate the high-frequency probe pulses. The low-frequency pump pulses were generated by a 20-element 345-kHz array transducer, driven by a custom high-voltage pulser. These dual-beam histotripsy pulses were applied to red blood cell tissue-mimicking phantoms at a pulse repetition frequency of 1 Hz, and optical imaging was used to visualize bubble clouds and lesions generated in the red blood cell phantoms. The results indicated that dense bubble clouds (and resulting lesions) were generated when the p- of the sub-threshold pump and probe pulses combined constructively to exceed the intrinsic threshold. The average size of the smallest reproducible lesions using the imaging probe pulse enabled by the sub-threshold pump pulse was 0.7 × 1.7 mm, whereas that using the supra-threshold pump pulse alone was 1.4 × 3.7 mm. When the imaging transducer was steered laterally, bubble clouds and lesions were steered correspondingly until the combined p- no longer exceeded the intrinsic threshold. These results were also validated with ex vivo porcine liver experiments. Using an imaging transducer for dual-beam histotripsy can have two advantages: (i) lesion steering can be achieved using the steering of the imaging transducer (implemented with the beamformer of the accompanying programmable ultrasound system), and (ii) treatment can be simultaneously monitored when the imaging transducer is used in conjunction with an ultrasound imaging system.

Effects of Ultrasound Frequency on Nanodroplet-Mediated Histotripsy

Nanodroplet-mediated histotripsy (NMH) is a targeted ultrasound ablation technique combining histotripsy with nanodroplets that can be selectively delivered to tumor cells for targeted tumor ablation. In a previous study, it was reported that by use of extremely short, high-pressure pulses, histotripsy cavitation bubbles were generated in regions containing nanodroplets at significantly lower pressure (∼10.8 MPa) than without nanodroplets (∼28 MPa) at 500 kHz. Furthermore, it was hypothesized that lower frequency would improve the effectiveness of NMH by increasing the size of the focal region, increasing bubble expansion, and decreasing the cavitation threshold. In this study, we investigated the effects of ultrasound frequency (345 kHz, 500 kHz, 1.5 MHz, and 3 MHz) on NMH. First, the NMH cavitation threshold was measured in tissue phantoms with and without nanodroplets, with results indicating that the NMH threshold was significantly below the histotripsy intrinsic threshold at all frequencies. Results also indicated that the NMH threshold decreased at lower frequency, ranging from 7.4 MPa at 345 kHz to 13.2 MPa at 3 MHz. In the second part of this study, the effects of frequency on NMH bubble expansion were investigated, with results indicating larger expansion at lower frequency, even at a lower pressure. In the final part of this study, the ability of perfluoropentane-encapsulated nanodroplets to act as sustainable cavitation nuclei over multiple pulses was investigated, with results indicating that the nanodroplets are destroyed by the cavitation process and only function as cavitation nuclei for the first few pulses, with this effect being most pronounced at higher frequencies. Overall, the results of this study support our hypothesis that using a lower frequency will improve the effectiveness of NMH by increasing the size of the focal region, increasing bubble expansion and decreasing the cavitation threshold.

Mechanical high-intensity focused ultrasound destruction of soft tissue: working mechanisms and physiologic effects

The best known method of high-intensity focused ultrasound is thermal ablation, but interest in non-thermal, mechanical destruction is increasing. The advantages of mechanical ablation are that thermal protein denaturation remains limited and less damage is created to the surrounding tissue by thermal diffusion. The two main techniques for mechanical fragmentation of tissue are histotripsy and boiling histotripsy. These techniques can be used for complete liquefaction of tumor tissue into submicron fragments, after which the fragmented tissue can be easily removed by natural (immunologic) responses. Interestingly it seems that there is a correlation between the degree of destruction and tissue specific characteristics based on the treatment settings used. In this review article, the technical aspects of these two techniques are described, and an overview of the in vivo pathologic and immunologic responses is provided.

Hemodynamic and Hematologic Effects of Histotripsy of Free-Flowing Blood: Implications for Ultrasound-Mediated Thrombolysis

PURPOSE

To investigate the extent and consequences of histotripsy-induced hemolysis in vivo.

MATERIALS AND METHODS

Porcine femoral venous blood was treated with histotripsy in 11 animals with systemic heparinization and 11 without heparin. Serum and hemodynamic measurements were obtained at 0, 2, 5, 10, 15, and 30 minutes and 48-72 hours after the procedure. Fisher exact test was used to determine differences in mortality between heparinized and nonheparinized groups. A linear mixed effects model was used to test for differences in blood analytes and hemodynamic variables over time.

RESULTS

Of 11 animals in the nonheparinized group, 5 died during or immediately after histotripsy (45% nonheparin mortality vs 0% heparin mortality, P = .035). Serum hematocrit, free hemoglobin, lactate dehydrogenase (LDH), and right ventricular systolic pressure changed significantly (P < .001) over the treatment time. Serum hematocrit decreased slightly (from 32.5% ± 3.6% to 29.4% ± 4.2%), whereas increases were seen in free hemoglobin (from 6.2 mg/dL ± 4.6 to 348 mg/dL ± 100), LDH (from 365 U/L ± 67.8 ± to 722 U/L ± 84.7), and right ventricular systolic pressure (from 23.2 mm Hg ± 7.2 to 39.7 mm Hg ± 12.3). After 48-72 hours, hematocrit remained slightly decreased (P = .005), whereas LDH and free hemoglobin remained slightly increased compared with baseline (both P < .001).

CONCLUSIONS

Intravascular histotripsy applied to free-flowing venous blood is safe with systemic heparinization, causing only transient hemodynamic and metabolic disturbances, supporting its use as a future noninvasive thrombolytic therapy modality.

Imaging feedback for histotripsy by characterizing dynamics of acoustic radiation force impulse (ARFI)-induced shear waves excited in a treated volume

Our previous study indicated that shear waves decay and propagate at a lower speed as they propagate into a tissue volume mechanically fractionated by histotripsy. In this paper, we hypothesize that the change in the shear dynamics is related to the degree of tissue fractionation, and can be used to predict histotripsy treatment outcomes. To test this hypothesis, lesions with different degrees of tissue fractionation were created in agar-graphite tissue phantoms and ex vivo kidneys with increasing numbers of therapy pulses, from 0 to 2000 pulses per treatment location. The therapy pulses were 3-cycle 750-kHz focused ultrasound delivered at a peak negative/positive pressure of 17/108 MPa and a repetition rate of 50 Hz. The shear waves were excited by acoustic radiation force impulse (ARFI) focused at the center of the lesion. The spatial and temporal behavior of the propagating shear waves was measured with ultrasound plane wave imaging. The temporal displacement profile at a lateral location 10 mm offset to the shear excitation region was detected with M-mode imaging. The decay and delay of the shear waves were quantitatively characterized on the temporal displacement profile. Results showed significant changes in two characteristics on the temporal displacement profile: the peak-to-peak displacement decayed exponentially with increasing numbers of therapy pulses; the relative time-to-peak displacement increased with increasing numbers of therapy pulses, and appeared to saturate at higher numbers of pulses. Correspondingly, the degree of tissues fractionation, as indicated by the percentage of structurally intact cell nuclei, decreased exponentially with increasing numbers of therapy pulses. Strong linear correlations were found between the two characteristics and the degree of tissue fractionation. These results suggest that the characteristics of the shear temporal displacement profile may provide useful feedback information regarding the treatment outcomes.

Histotripsy beyond the intrinsic cavitation threshold using very short ultrasound pulses: microtripsy

Histotripsy produces tissue fractionation through dense energetic bubble clouds generated by short, high-pressure, ultrasound pulses. Conventional histotripsy treatments have used longer pulses from 3 to 10 cycles, wherein the lesion-producing bubble cloud generation depends on the pressure-release scattering of very high peak positive shock fronts from previously initiated, sparsely distributed bubbles (the shock-scattering mechanism). In our recent work, the peak negative pressure (P-) for generation of dense bubble clouds directly by a single negative half cycle, the intrinsic threshold, was measured. In this paper, the dense bubble clouds and resulting lesions (in red blood cell phantoms and canine tissues) generated by these supra-intrinsic threshold pulses were studied. A 32-element, PZT-8, 500-kHz therapy transducer was used to generate very short (<2 cycles) histotripsy pulses at a pulse repetition frequency (PRF) of 1 Hz and P- from 24.5 to 80.7 MPa. The results showed that the spatial extent of the histotripsy-induced lesions increased as the applied P- increased, and the sizes of these lesions corresponded well to the estimates of the focal regions above the intrinsic cavitation threshold, at least in the lower pressure regime (P- = 26 to 35 MPa). The average sizes for the smallest reproducible lesions were approximately 0.9 × 1.7 mm (lateral × axial), significantly smaller than the -6-dB beamwidth of the transducer (1.8 × 4.0 mm). These results suggest that, using the intrinsic threshold mechanism, well-confined and microscopic lesions can be precisely generated and their spatial extent can be estimated based on the fraction of the focal region exceeding the intrinsic cavitation threshold. Because the supra-threshold portion of the negative half cycle can be precisely controlled, lesions considerably less than a wavelength are easily produced, hence the term microtripsy.

Dual-beam histotripsy: a low-frequency pump enabling a high-frequency probe for precise lesion formation

Histotripsy produces tissue fractionation through dense energetic bubble clouds generated by short, high-pressure, ultrasound pulses. When using pulses shorter than 2 cycles, the generation of these energetic bubble clouds only depends on where the peak negative pressure (P-) exceeds the intrinsic threshold of the medium (26 to 30 MPa in soft tissue with high water content). This paper investigates a strategic method for precise lesion generation in which a low-frequency pump pulse is applied to enable a sub-threshold high-frequency probe pulse to exceed the intrinsic threshold. This pump-probe method of controlling a supra-threshold volume can be called dual-beam histotripsy. A 20-element dual-frequency (500-kHz and 3-MHz elements confocally aligned) array transducer was used to generate dual-beam histotripsy pulses in red blood cell phantoms and porcine hepatic tissue specimens. The results showed that when sub-intrinsic-threshold pump (500-kHz) and probe (3-MHz) pulses were applied together, dense bubble clouds (and resulting lesions) were only generated when their peak negative pressures combined constructively to exceed the intrinsic threshold. The smallest reproducible lesion varied with the relative amplitude between the pump and probe pulses, and, with a higher proportion of the probe pulse, smaller lesions could be generated. When the propagation direction of the probe pulse relative to the pump pulse was altered, the shape of the produced lesion changed based on the region that exceeded intrinsic threshold. Because the low-frequency pump pulse is more immune to attenuation and aberrations, and the high-frequency probe pulse can provide precision in lesion formation, this dual-beam histotripsy approach would be very useful in situations in which precise lesion formation is required through a highly attenuative and aberrative medium, such as transcranial therapy. This is particularly true if a small low-attenuation acoustic window is available for the high-frequency probe transducer.

Effects of tissue mechanical properties on susceptibility to histotripsy-induced tissue damage

Histotripsy is a non-invasive tissue ablation method capable of fractionating tissue by controlling acoustic cavitation. To determine the fractionation susceptibility of various tissues, we investigated histotripsy-induced damage on tissue phantoms and ex vivo tissues with different mechanical strengths. A histotripsy bubble cloud was formed at tissue phantom surfaces using 5-cycle long ultrasound pulses with peak negative pressure of 18 MPa and PRFs of 10, 100, and 1000 Hz. Results showed significantly smaller lesions were generated in tissue phantoms of higher mechanical strength. Histotripsy was also applied to 43 different ex vivo porcine tissues with a wide range of mechanical properties. Gross morphology demonstrated stronger tissues with higher ultimate stress, higher density, and lower water content were more resistant to histotripsy damage in comparison to weaker tissues. Based on these results, a self-limiting vessel-sparing treatment strategy was developed in an attempt to preserve major vessels while fractionating the surrounding target tissue. This strategy was tested in porcine liver in vivo. After treatment, major hepatic blood vessels and bile ducts remained intact within a completely fractionated liver volume. These results identify varying susceptibilities of tissues to histotripsy therapy and provide a rational basis to optimize histotripsy parameters for treatment of specific tissues.

Cardiovascular applications of therapeutic ultrasound

Ultrasound (US) has gained widespread use in diagnostic cardiovascular applications. At amplitudes and frequencies typical of diagnostic use, its biomechanical effects on tissue are largely negligible. However, these parameters can be altered to harness US's thermal and non-thermal effects for therapeutic indications. High-intensity focused ultrasound (HIFU) and extracorporeal shock wave therapy (ECWT) are two therapeutic US modalities which have been investigated for treating cardiac arrhythmias and ischemic heart disease, respectively. Here, we review the biomechanical effects of HIFU and ECWT, their potential therapeutic mechanisms, and pre-clinical and clinical studies demonstrating their efficacy and safety limitations. Furthermore, we discuss other potential clinical applications of therapeutic US and areas in which future research is needed.

Histotripsy cardiac therapy system integrated with real-time motion correction

Histotripsy has shown promise in non-invasive cardiac therapy for neonatal and fetal applications. However, for cardiac applications in general, and especially in the adult heart, cardiac and respiratory motion may affect treatment accuracy and efficacy. In this article, we describe a histotripsy-mediated cardiac therapy system integrated with a fast motion tracking algorithm and treatment monitoring using ultrasound imaging. Motion tracking is performed by diamond search block matching in real-time ultrasound images using a reference image of the moving target, refined by Kalman filtering. As proof of feasibility, this algorithm was configured to track 2-D target motion and then electronically adjust the focus of a 1-MHz annular therapy array to correct for axial motion. This integrated motion tracking system is capable of sub-millimeter accuracy for displacements of 0-15 mm and velocities of 0-80 mm/s, with a maximum error less than 3 mm. Tissue phantom tests indicated that treatment efficiency and lesion size using motion tracking over displacements of 0-15 mm and velocities of 0-42 mm/s are comparable to those achieved when treating stationary targets. In vivo validation was conducted in an open-chest canine model, where the system provided 24 min of motion-corrected histotripsy therapy in the live beating heart, generating a targeted lesion on the atrial septum. Based on this proof of feasibility and the natural extension of these techniques to three dimensions, we anticipate a full motion correction system would be feasible and beneficial for non-invasive cardiac therapy.

Image-guided non-invasive ultrasound liver ablation using histotripsy: feasibility study in an in vivo porcine model

Hepatocellular carcinoma (HCC), or liver cancer, is one of the fastest growing cancers in the United States. Current liver ablation methods are thermal based and share limitations resulting from the heat sink effect of blood flow through the highly vascular liver. In this study, we explore the feasibility of using histotripsy for non-invasive liver ablation in the treatment of liver cancer. Histotripsy is a non-thermal ablation method that fractionates soft tissue through the control of acoustic cavitation. Twelve histotripsy lesions ∼1 cm(3) were created in the livers of six pigs through an intact abdomen and chest in vivo. Histotripsy pulses of 10 cycles, 500-Hz pulse repetition frequency (PRF), and 14- to 17-MPa estimated in situ peak negative pressure were applied to the liver using a 1-MHz therapy transducer. Treatments were performed through 4-6 cm of overlying tissue, with 30%-50% of the ultrasound pathway covered by the rib cage. Complete fractionation of liver parenchyma was observed, with sharp boundaries after 16.7-min treatments. In addition, two larger volumes of 18 and 60 cm(3) were generated within 60 min in two additional pigs. As major vessels and gallbladder have higher mechanical strength and are more resistant to histotripsy, these remained intact while the liver surrounding these structures was completely fractionated. This work shows that histotripsy is capable of non-invasively fractionating liver tissue while preserving critical anatomic structures within the liver. Results suggest histotripsy has potential for the non-invasive ablation of liver tumors.

Non-invasive embolus trap using histotripsy-an acoustic parameter study

Free-flowing particles in a blood vessel were observed to be attracted, trapped and eroded by a histotripsy bubble cloud. This phenomenon may be used to develop a non-invasive embolus trap (NET) to prevent embolization. This study investigates the effect of acoustic parameters on the trapping ability of the NET generated by a focused 1.063 MHz transducer. The maximum trapping velocity, defined by the maximum mean fluid velocity at which a 3-4 mm particle trapped in a 6 mm diameter vessel phantom, increased linearly with peak negative pressure (P-) and increased as the square root of pulse length and pulse repetition frequency (PRF). At 19.9 MPa P-, 1000 Hz PRF and 10 cycle pulse length, a 3 mm clot-mimicking particle could remain trapped under a background velocity of 9.7 cm/s. Clot fragments treated by NET resulted in debris particles <75 μm. These results will guide the appropriate selection of NET parameters.

Imaging feedback of histotripsy treatments using ultrasound shear wave elastography

Histotripsy is a cavitation-based ultrasound therapy that mechanically fractionates soft solid tissues into fluid-like homogenates. This paper investigates the feasibility of imaging the tissue elasticity change during the histotripsy process as a tool to provide feedback for the treatments. The treatments were performed on agar tissue phantoms and ex vivo kidneys using 3-cycle ultrasound pulses delivered by a 750-kHz therapeutic array at peak negative/positive pressure of 17/108 MPa and a repetition rate of 50 Hz. Lesions with different degrees of damage were created with increasing numbers of therapy pulses from 0 to 2000 pulses per treatment location. The elasticity of the lesions was measured with ultrasound shear wave elastography, in which a quasi-planar shear wave was induced by acoustic radiation force generated by the therapeutic array, and tracked with ultrasound imaging at 3000 frames per second. Based on the shear wave velocity calculated from the sequentially captured frames, the Young's modulus was reconstructed. Results showed that the lesions were more easily identified on the shear wave velocity images than on B-mode images. As the number of therapy pulses increased from 0 to 2000 pulses/location, the Young's modulus decreased exponentially from 22.1 ± 2.7 to 2.1 ± 1.1 kPa in the tissue phantoms (R2 = 0.99, N = 9 each), and from 33.0 ± 7.1 to 4.0 ± 2.5 kPa in the ex vivo kidneys (R2 = 0.99, N = 8 each). Correspondingly, the tissues transformed from completely intact to completely fractionated as examined via histology. A good correlation existed between the lesions' Young's modulus and the degree of tissue fractionation as examined with the percentage of remaining structurally intact cell nuclei (R2 = 0.91, N = 8 each). These results indicate that lesions produced by histotripsy can be detected with high sensitivity using shear wave elastography. Because the decrease in the tissue elasticity corresponded well with the morphological and histological change, this study provides a basis for predicting the local treatment outcomes from tissue elasticity change.

An efficient treatment strategy for histotripsy by removing cavitation memory

Cavitation memory effects occur when remnants of cavitation bubbles (nuclei) persist in the host medium and act as seeds for subsequent events. In pulsed cavitational ultrasound therapy, or histotripsy, this effect may cause cavitation to repeatedly occur at these seeded locations within a target volume, producing inhomogeneous tissue fractionation or requiring an excess number of pulses to completely homogenize the target volume. We hypothesized that by removing the cavitation memory, i.e., the persistent nuclei, the cavitation bubbles could be induced at random locations in response to each pulse; therefore, complete disruption of a tissue volume may be achieved with fewer pulses. To test the hypothesis, the cavitation memory was passively removed by increasing the intervals between successive pulses, ∆t, from 2, 10, 20, 50 and 100, to 200 ms. Histotripsy treatments were performed in red blood cell tissue phantoms and ex vivo livers using 1-MHz ultrasound pulses of 10 cycles at P-/P+ pressure of 21/59 MPa. The phantom study allowed for direct visualization of the cavitation patterns and the lesion development process in real time using high-speed photography; the ex vivo tissue study provided validation of the memory effect in real tissues. Results of the phantom study showed an exponential decrease in the correlation coefficient between cavitation patterns in successive pulses from 0.5 ± 0.1 to 0.1 ± 0.1 as ∆t increased from 2-200 ms; correspondingly, the lesion was completely fractionated with significantly fewer pulses for longer ∆ts. In the tissue study, given the same number of therapy pulses, complete and homogeneous tissue fractionation with well-defined lesion boundaries was achieved only for ∆t ≥ 100 ms. These results indicated that the removal of the cavitation memory resulted in more efficient treatments and homogeneous lesions.