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.

Development of an Automated Ultrasound Signal Indicator of Lung Interstitial Syndrome.

OBJECTIVES: The number and distribution of lung ultrasound (LUS) imaging artifacts termed B-lines correlate with the presence of acute lung disease such as infection, acute respiratory distress syndrome (ARDS), and pulmonary edema. Detection and interpretation of B-lines require dedicated training and is machine and operator-dependent. The goal of this study was to identify radio frequency (RF) signal features associated with B-lines in a cohort of patients with cardiogenic pulmonary edema. A quantitative signal indicator could then be used in a single-element, non-imaging, wearable, automated lung ultrasound sensor (LUSS) for continuous hands-free monitoring of lung fluid. METHODS: In this prospective study a 10-zone LUS exam was performed in 16 participants, including 12 patients admitted with acute cardiogenic pulmonary edema (mean age 60 ± 12 years) and 4 healthy controls (mean age 44 ± 21). Overall,160 individual LUS video clips were recorded. The LUS exams were performed with a phased array probe driven by an open-platform ultrasound system with simultaneous RF signal collection. RF data were analyzed offline for candidate B-line indicators based on signal amplitude, temporal variability, and frequency spectrum; blinded independent review of LUS images for the presence or absence of B-lines served as ground truth. Predictive performance of the signal indicators was determined with receiving operator characteristic (ROC) analysis with k-fold cross-validation. RESULTS: Two RF signal features-temporal variability of signal amplitude at large depths and at the pleural line-were strongly associated with B-line presence. The sensitivity and specificity of a combinatorial indicator were 93.2 and 58.5%, respectively, with cross-validated area under the ROC curve (AUC) of 0.91 (95% CI = 0.80-0.94). CONCLUSION: A combinatorial signal indicator for use with single-element non-imaging LUSS was developed to facilitate continuous monitoring of lung fluid in patients with respiratory illness.

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.

Robust and durable aberrative and absorptive phantom for therapeutic ultrasound applications.

Phase aberration induced by soft tissue inhomogeneities often complicates high-intensity focused ultrasound (HIFU) therapies by distorting the field and, previously, we designed and fabricated a bilayer gel phantom to reproducibly mimic that effect. A surface pattern containing size scales relevant to inhomogeneities of a porcine body wall was introduced between gel materials with fat- and muscle-like acoustic properties-ballistic and polyvinyl alcohol gels. Here, the phantom design was refined to achieve relevant values of ultrasound absorption and scattering and make it more robust, facilitating frequent handling and use in various experimental arrangements. The fidelity of the interfacial surface of the fabricated phantom to the design was confirmed by three-dimensional ultrasound imaging. The HIFU field distortions-displacement of the focus, enlargement of the focal region, and reduction of focal pressure-produced by the phantom were characterized using hydrophone measurements with a 1.5 MHz 256-element HIFU array and found to be similar to those induced by an ex vivo porcine body wall. A phase correction approach was used to mitigate the aberration effect on nonlinear focal waveforms and enable boiling histotripsy treatments through the phantom or body wall. The refined phantom represents a practical tool to explore HIFU therapy systems capabilities.

Bilayer aberration-inducing gel phantom for high intensity focused ultrasound applications.

Aberrations induced by soft tissue inhomogeneities often complicate high-intensity focused ultrasound (HIFU) therapies. In this work, a bilayer phantom made from polyvinyl alcohol hydrogel and ballistic gel was built to mimic alternating layers of water-based and lipid tissues characteristic of an abdominal body wall and to reproducibly distort HIFU fields. The density, sound speed, and attenuation coefficient of each material were measured using a homogeneous gel layer. A surface with random topographical features was designed as an interface between gel layers using a 2D Fourier spectrum approach and replicating different spatial scales of tissue inhomogeneities. Distortion of the field of a 256-element 1.5 MHz HIFU array by the phantom was characterized through hydrophone measurements for linear and nonlinear beam focusing and compared to the corresponding distortion induced by an ex vivo porcine body wall of the same thickness. Both spatial shift and widening of the focal lobe were observed, as well as dramatic reduction in focal pressures caused by aberrations. The results suggest that the phantom produced levels of aberration that are similar to a real body wall and can serve as a research tool for studying HIFU effects as well as for developing algorithms for aberration correction.

Dependence of inertial cavitation induced by high intensity focused ultrasound on transducer F-number and nonlinear waveform distortion.

Pulsed high intensity focused ultrasound was shown to enhance chemotherapeutic drug uptake in tumor tissue through inertial cavitation, which is commonly assumed to require peak rarefactional pressures to exceed a certain threshold. However, recent studies have indicated that inertial cavitation activity also correlates with the presence of shocks at the focus. The shock front amplitude and corresponding peak negative pressure (p -) in the focal waveform are primarily determined by the transducer F-number: less focused transducers produce shocks at lower p -. Here, the dependence of inertial cavitation activity on the transducer F-number was investigated in agarose gel by monitoring broadband noise emissions with a coaxial passive cavitation detector (PCD) during pulsed exposures (pulse duration 1 ms, pulse repetition frequency 1 Hz) with p- varying within 1-15 MPa. Three 1.5 MHz transducers with the same aperture, but different focal distances (F-numbers 0.77, 1.02, 1.52) were used. PCD signals were processed to extract cavitation probability, persistence, and mean noise level. At the same p -, all metrics indicated enhanced cavitation activity at higher F-numbers; specifically, cavitation probability reached 100% when shocks formed at the focus. These results provide further evidence supporting the excitation of inertial cavitation at reduced p - by waveforms with nonlinear distortion and shocks.

Release of Cell-free MicroRNA Tumor Biomarkers into the Blood Circulation with Pulsed Focused Ultrasound: A Noninvasive, Anatomically Localized, Molecular Liquid Biopsy.

Purpose To compare the abilities of three pulsed focused ultrasound regimes (that cause tissue liquefaction, permeabilization, or mild heating) to release tumor-derived microRNA into the circulation in vivo and to evaluate release dynamics. Materials and Methods All rat experiments were approved by the University of Washington Institutional Animal Care and Use Committee. Reverse-transcription quantitative polymerase chain reaction array profiling was used to identify candidate microRNA biomarkers in a rat solid tumor cell line. Rats subcutaneously grafted with these cells were randomly assigned among three pulsed focused ultrasound treatment groups: (a) local tissue liquefaction via boiling histotripsy, (b) tissue permeabilization via inertial cavitation, and (c) mild (<10°C) heating of tissue, as well as a sham-treated control group. Blood specimens were drawn immediately prior to treatment and serially over 24 hours afterward. Plasma microRNA was quantified with reverse-transcription quantitative polymerase chain reaction, and statistical significance was determined with one-way analysis of variance (Kruskal-Wallis and Friedman tests), followed by the Dunn multiple-comparisons test. Results After tissue liquefaction and cavitation treatments (but not mild heating), plasma quantities of candidate biomarkers increased significantly (P value range, <.0001 to .04) relative to sham-treated controls. A threefold to 32-fold increase occurred within 15 minutes after initiation of pulsed focused ultrasound tumor treatment, and these increases persisted for 3 hours. Histologic examination confirmed complete liquefaction of the targeted tumor area with boiling histotripsy, in addition to areas of petechial hemorrhage and tissue disruption by means of cavitation-based treatment. Conclusion Mechanical tumor tissue disruption with pulsed focused ultrasound-induced bubble activity significantly increases the plasma abundance of tumor-derived microRNA rapidly after treatment. © RSNA, 2016 Online supplemental material is available for this article.

Ultrasound-guided tissue fractionation by high intensity focused ultrasound in an in vivo porcine liver model.

The clinical use of high intensity focused ultrasound (HIFU) therapy for noninvasive tissue ablation has been recently gaining momentum. In HIFU, ultrasound energy from an extracorporeal source is focused within the body to ablate tissue at the focus while leaving the surrounding organs and tissues unaffected. Most HIFU therapies are designed to use heating effects resulting from the absorption of ultrasound by tissue to create a thermally coagulated treatment volume. Although this approach is often successful, it has its limitations, such as the heat sink effect caused by the presence of a large blood vessel near the treatment area or heating of the ribs in the transcostal applications. HIFU-induced bubbles provide an alternative means to destroy the target tissue by mechanical disruption or, at its extreme, local fractionation of tissue within the focal region. Here, we demonstrate the feasibility of a recently developed approach to HIFU-induced ultrasound-guided tissue fractionation in an in vivo pig model. In this approach, termed boiling histotripsy, a millimeter-sized boiling bubble is generated by ultrasound and further interacts with the ultrasound field to fractionate porcine liver tissue into subcellular debris without inducing further thermal effects. Tissue selectivity, demonstrated by boiling histotripsy, allows for the treatment of tissue immediately adjacent to major blood vessels and other connective tissue structures. Furthermore, boiling histotripsy would benefit the clinical applications, in which it is important to accelerate resorption or passage of the ablated tissue volume, diminish pressure on the surrounding organs that causes discomfort, or insert openings between tissues.

HIFU for Palliative Treatment of Pancreatic Cancer.

Pancreatic cancer is one of the deadliest malignancies, with only a 6 % 5-year survival rate and over 50 % of patients being diagnosed at the advanced stage. Current therapies are ineffective, and the treatment of patients with advanced disease is palliative. In the past decade, HIFU ablation has emerged as a modality for palliative treatment of pancreatic tumors. Multiple preclinical and non-randomized clinical trials have been performed to evaluate the safety and efficacy of this procedure. Substantial tumor-related pain reduction was achieved in most cases after HIFU treatment and few significant side effects were observed. In addition, some studies indicate that combination of HIFU ablation with chemotherapy may provide a survival benefit. This chapter summarizes the pre-clinical and clinical experience obtained to date in HIFU treatment of pancreatic tumors and discusses the challenges, limitations and new approaches in this modality.

Endoscopic high-intensity focused US: technical aspects and studies in an in vivo porcine model (with video).

BACKGROUND: High-intensity focused US (HIFU) is becoming more widely used for noninvasive and minimally invasive ablation of benign and malignant tumors. Recent studies suggest that HIFU can also enhance targeted drug delivery and stimulate an antitumor immune response in many tumors. However, targeting pancreatic and liver tumors by using an extracorporeal source is challenging due to the lack of an adequate acoustic window. The development of an EUS-guided HIFU transducer has many potential benefits including improved targeting, decreased energy requirements, and decreased potential for injury to intervening structures. OBJECTIVE: To design, develop, and test an EUS-guided HIFU transducer for endoscopic applications. DESIGN: A preclinical, pilot characterization and feasibility study. SETTING: Academic research center. PATIENTS: Studies were performed in an in vivo porcine model. INTERVENTION: Thermal ablation of in vivo porcine pancreas and liver was performed with EUS-guided focused US through the gastric tract. RESULTS: The transducer successfully created lesions in gel phantoms and ex vivo bovine livers. In vivo studies demonstrated that targeting and creating lesions in the porcine pancreas and liver are feasible. LIMITATIONS: This was a preclinical, single-center feasibility study with a limited number of subjects. CONCLUSION: An EUS-guided HIFU transducer was successfully designed and developed with dimensions that are appropriate for endoscopic use. The feasibility of performing EUS-guided HIFU ablation in vivo was demonstrated in an in vivo porcine model. Further development of this technology will allow endoscopists to perform precise therapeutic ablation of periluminal lesions without breaching the wall of the gastric tract.

Therapeutic Ultrasound Promotes Reperfusion and Angiogenesis in a Rat Model of Peripheral Arterial Disease.

BACKGROUND: Shock wave therapy (SWT) is an acoustic technology clinically used for the non-invasive treatment of ischemic heart disease (IHD). Therapeutic ultrasound (TUS) has more recently been developed for the same indication, although its effects on reperfusion and angiogenesis have yet to be directly compared to those of SWT. METHODS AND RESULTS: TUS and SWT acoustic parameters were matched, and their ability to promote angiogenesis and reperfusion in a rat hindlimb ischemia model was compared. After left femoral artery excision, 3-weekly TUS, SWT or sham treatments (n=10 rats each) of the left hindlimb were performed for 2 weeks. Laser Doppler perfusion imaging demonstrated improved perfusion with TUS (66±4% L:R hindlimb perfusion, mean±SEM, P=0.02), but not with SWT (59±4%, P=0.13) compared with sham (50±4%). Immunohistochemistry of CD31 demonstrated increased microvascular density with TUS (222.6 vessels/high-power field, P=0.001) and SWT (216.9, P=0.01) compared to sham-treated rats (196.0). Tissue vascular endothelial growth factor mRNA levels were elevated in the left hindlimb of TUS-, but not SWT- or sham-treated rats. CONCLUSIONS: Direct comparison demonstrates that TUS is more effective than SWT at promoting reperfusion, whereas both therapies promote angiogenesis in ischemic gastrocnemius muscle. These results suggest that TUS may be more effective than SWT for the treatment of IHD and peripheral arterial disease.

Histotripsy methods in mechanical disintegration of tissue: towards clinical applications.

In high intensity focused ultrasound (HIFU) therapy, an ultrasound beam is focused within the body to locally affect the targeted site without damaging intervening tissues. The most common HIFU regime is thermal ablation. Recently there has been increasing interest in generating purely mechanical lesions in tissue (histotripsy). This paper provides an overview of several studies on the development of histotripsy methods toward clinical applications. Two histotripsy approaches and examples of their applications are presented. In one approach, sequences of high-amplitude, short (microsecond-long), focused ultrasound pulses periodically produce dense, energetic bubble clouds that mechanically disintegrate tissue. In an alternative approach, longer (millisecond-long) pulses with shock fronts generate boiling bubbles and the interaction of shock fronts with the resulting vapour cavity causes tissue disintegration. Recent preclinical studies on histotripsy are reviewed for treating benign prostatic hyperplasia (BPH), liver and kidney tumours, kidney stone fragmentation, enhancing anti-tumour immune response, and tissue decellularisation for regenerative medicine applications. Potential clinical advantages of the histotripsy methods are discussed. Histotripsy methods can be used to mechanically ablate a wide variety of tissues, whilst selectivity sparing structures such as large vessels. Both ultrasound and MR imaging can be used for targeting and monitoring the treatment in real time. Although the two approaches utilise different mechanisms for tissue disintegration, both have many of the same advantages and offer a promising alternative method of non-invasive surgery.

Dynamic Mode Decomposition for Transient Cavitation Bubbles Imaging in Pulsed High Intensity Focused Ultrasound Therapy.

Pulsed high-intensity focused ultrasound (pHIFU) can induce sparse de novo inertial cavitation without the introduction of exogenous contrast agents, promoting mild mechanical disruption in targeted tissue. Because the bubbles are small and rapidly dissolve after each HIFU pulse, mapping transient bubbles and obtaining real-time quantitative metrics correlated to tissue damage are challenging. Prior work introduced Bubble Doppler, an ultrafast power Doppler imaging method as a sensitive means to map cavitation bubbles. The main limitation of that method was its reliance on conventional wall filters used in Doppler imaging and optimized for imaging blood flow rather than transient scatterers. This study explores Bubble Doppler enhancement using dynamic mode decomposition (DMD) of a matrix created from a Doppler ensemble for mapping and extracting the characteristics of transient cavitation bubbles. DMD was first tested in silico with a numerical dataset mimicking the spatiotemporal characteristics of backscattered signal from tissue and bubbles. The performance of DMD filter was compared to other widely used Doppler wall filters - singular value decomposition (SVD) and infinite impulse response (IIR) highpass filter. DMD was then applied to an ex vivo tissue dataset where each HIFU pulse was immediately followed by a plane wave Doppler ensemble. In silico DMD outperformed SVD and IIR high pass filter and ex vivo provided physically interpretable images of the modes associated with bubbles and their corresponding temporal decay rates. These DMD modes can be trackable over the duration of pHIFU treatment using k-means clustering method, resulting in quantitative indicators of treatment progression.

Dynamic Mode Decomposition for Transient Cavitation Bubbles Imaging in Pulsed High-Intensity Focused Ultrasound Therapy.

Pulsed high-intensity focused ultrasound (pHIFU) can induce sparse de novo inertial cavitation without the introduction of exogenous contrast agents, promoting mild mechanical disruption in targeted tissue. Because the bubbles are small and rapidly dissolve after each HIFU pulse, mapping transient bubbles and obtaining real-time quantitative metrics correlated with tissue damage are challenging. Prior work introduced Bubble Doppler, an ultrafast power Doppler imaging method as a sensitive means to map cavitation bubbles. The main limitation of that method was its reliance on conventional wall filters used in Doppler imaging and its optimization for imaging blood flow rather than transient scatterers. This study explores Bubble Doppler enhancement using dynamic mode decomposition (DMD) of a matrix created from a Doppler ensemble for mapping and extracting the characteristics of transient cavitation bubbles. DMD was first tested in silico with a numerical dataset mimicking the spatiotemporal characteristics of backscattered signal from tissue and bubbles. The performance of DMD filter was compared to other widely used Doppler wall filter-singular value decomposition (SVD) and infinite impulse response (IIR) high-pass filter. DMD was then applied to an ex vivo tissue dataset where each HIFU pulse was immediately followed by a plane wave Doppler ensemble. In silico DMD outperformed SVD and IIR high-pass filter and ex vivo provided physically interpretable images of the modes associated with bubbles and their corresponding temporal decay rates. These DMD modes can be trackable over the duration of pHIFU treatment using k-means clustering method, resulting in quantitative indicators of treatment progression.

Treatment Planning and Aberration Correction Algorithm for HIFU Ablation of Renal Tumors.

High-intensity focused ultrasound (HIFU) applications for thermal or mechanical ablation of renal tumors often encounter challenges due to significant beam aberration and refraction caused by oblique beam incidence, inhomogeneous tissue layers, and presence of gas and bones within the beam. These losses can be significantly mitigated through sonication geometry planning, patient positioning, and aberration correction using multielement phased arrays. Here, a sonication planning algorithm is introduced, which uses the simulations to select the optimal transducer position and evaluate the effect of aberrations and acoustic field quality at the target region after aberration correction. Optimization of transducer positioning is implemented using a graphical user interface (GUI) to visualize a segmented 3-D computed tomography (CT)-based acoustic model of the body and to select sonication geometry through a combination of manual and automated approaches. An HIFU array (1.5 MHz, 256 elements) and three renal cell carcinoma (RCC) cases with different tumor locations and patient body habitus were considered. After array positioning, the correction of aberrations was performed using a combination of backpropagation from the focus with an ordinary least squares (OLS) optimization of phases at the array elements. The forward propagation was simulated using a combination of the Rayleigh integral and k-space pseudospectral method (k-Wave toolbox). After correction, simulated HIFU fields showed tight focusing and up to threefold higher maximum pressure within the target region. The addition of OLS optimization to the aberration correction method yielded up to 30% higher maximum pressure compared to the conventional backpropagation and up to 250% higher maximum pressure compared to the ray-tracing method, particularly in strongly distorted cases.

Histology-based quantification of boiling histotripsy outcomes via ResNet-18 network: Towards mechanical dose metrics.

This work was focused on the newly developed ultrasonic approach for non-invasive surgery - boiling histotripsy (BH) - recently proposed for mechanical ablation of tissues using pulsed high intensity focused ultrasound (HIFU). The BH lesion is known to depend in size and shape on exposure parameters and mechanical properties, structure and composition of tissue being treated. The aim of this work was to advance the concept of BH dose by investigating quantitative relationships between the parameters of the lesion, pulsing protocols, and targeted tissue properties. A HIFU focus of a 1.5 MHz 256-element array driven by power-enhanced Verasonics system was electronically steered along the grid within 12 × 4 × 12 mm volume to produce volumetric lesions in porcine liver (soft, with abundant collagenous structures) and bovine myocardium (stiff, homogenous cellular) ex vivo tissues with various pulsing protocols (1-10 ms pulses, 1-15 pulses per point). Quantification of the lesion size and completeness was performed through serial histological sectioning, and a computer vision approach using a combination of manual and automated detection of fully fractionated and residual tissue based on neural network ResNet-18 was developed. Histological sample fixation led to underestimation of BH ablation rate compared to the ultrasound-based estimations, and provided similar qualitative feedback as did gross inspection. This suggests that gross observation may be sufficient for qualitatively evaluating the BH treatment completeness. BH efficiency in liver tissue was shown to be insensitive to the changes in pulsing protocol within the tested parameter range, whereas in bovine myocardium the efficiency increased with either increasing pulse length or number of pulses per point or both. The results imply that one universal mechanical dose metric applicable to an arbitrary tissue type is unlikely to be established. The dose metric as a product of the BH pulse duration and the number of pulses per sonication point (BHD1) was shown to be more relevant for initial planning of fractionation of collagenous tissues. The dose metric as a number of pulses per point (BHD2) is more suitable for the treatment planning of softer targets primarily containing cellular tissue, allowing for significant acceleration of treatment using shorter pulses.

Elastic Properties of Aging Human Hematoma Model In Vitro and Its Susceptibility to Histotripsy Liquefaction.

OBJECTIVE: Tissue susceptibility to histotripsy disintegration has been reported to depend on its elastic properties. This work was aimed at investigation of histotripsy efficiency for liquefaction of human hematomas, depending on their stiffness and degree of retraction over time (0-10 d). METHODS: As an in vitro hematoma model, anticoagulated human blood samples (200 mL) were recalcified at different temperatures. In one set of samples, the shear modulus was measured by shear wave elastography during blood clotting at 10℃, 22℃ and 37℃, and then daily during further aging. The ultrastructure of the samples was analyzed daily with scanning electron microscopy (SEM). Another set of blood samples (50-200 mL) were recalcified at 37℃ for density and retraction measurements over aging and exposed to histotripsy at varying time points. Boiling histotripsy (2.5 ms pulses) and hybrid histotripsy (0.2 ms pulses) exposures (2 MHz, 1% dc, P+/P-/As = 182/-27/207 MPa in situ) were used to produce either individual cigar-shaped or volumetric (0.8-3 mL) lesions in samples incubated for 3 h, 5 d and 10 d. The obtained lesions were sized, then the lysate aspirated under B-mode guidance was analyzed ultrastructurally and diluted in distilled water for sizing of residual fragments. RESULTS: It was found that clotting time decreased from 113 to 25 min with the increase in blood temperature from 10℃ to 37℃. The shear modulus increased to 0.53 ± 0.17 kPa during clotting and remained constant within 8 d of incubation at 2℃. Sample volumes decreased by 57% because of retraction within 10 d. SEM revealed significant echinocytosis but unchanged ultrastructure of the fibrin meshwork. Liquefaction rate and lesion dimensions produced with the same histotripsy protocols correlated with the increase in the degree of retraction and were lower in retracted samples versus freshly clotted samples. More than 80% of residual fibrin fragments after histotripsy treatment were shorter than 150 µm; the maximum length was 208 µm, allowing for unobstructed aspiration of the lysate with most clinically used needles. CONCLUSION: The results indicate that hematoma susceptibility to histotripsy liquefaction is not entirely determined by its stiffness, and correlates with the retraction degree.

Enhancement of Boiling Histotripsy by Steering the Focus Axially During the Pulse Delivery.

Boiling histotripsy (BH) is a pulsed high-intensity focused ultrasound (HIFU) method relying on the generation of high-amplitude shocks at the focus, localized enhanced shock-wave heating, and bubble activity driven by shocks to induce tissue liquefaction. BH uses sequences of 1-20 ms long pulses with shock fronts of over 60 MPa amplitude, initiates boiling at the focus of the HIFU transducer within each pulse, and the remainder shocks of the pulse then interact with the boiling vapor cavities. One effect of this interaction is the creation of a prefocal bubble cloud due to reflection of shocks from the initially generated mm-sized cavities: the shocks are inverted when reflected from a pressure-release cavity wall resulting in sufficient negative pressure to reach intrinsic cavitation threshold in front of the cavity. Secondary clouds then form due to shock-wave scattering from the first one. Formation of such prefocal bubble clouds has been known as one of the mechanisms of tissue liquefaction in BH. Here, a methodology is proposed to enlarge the axial dimension of this bubble cloud by steering the HIFU focus toward the transducer after the initiation of boiling until the end of each BH pulse and thus to accelerate treatment. A BH system comprising a 1.5 MHz 256-element phased array connected to a Verasonics V1 system was used. High-speed photography of BH sonications in transparent gels was performed to observe the extension of the bubble cloud resulting from shock reflections and scattering. Volumetric BH lesions were then generated in ex vivo tissue using the proposed approach. Results showed up to almost threefold increase of the tissue ablation rate with axial focus steering during the BH pulse delivery compared to standard BH.

Chronic effects of pulsed high intensity focused ultrasound aided delivery of gemcitabine in a mouse model of pancreatic cancer.

Pulsed high intensity focused ultrasound (pHIFU) is a non-invasive method that allows to permeabilize pancreatic tumors through inertial cavitation and thereby increase the concentration of systemically administered drug. In this study the tolerability of weekly pHIFU-aided administrations of gemcitabine (gem) and their influence on tumor progression and immune microenvironment were investigated in genetically engineered KrasLSL.G12D/þ; p53R172H/þ; PdxCretg/þ (KPC) mouse model of spontaneously occurring pancreatic tumors. KPC mice were enrolled in the study when the tumor size reached 4-6 mm and treated once a week with either ultrasound-guided pHIFU (1.5 MHz transducer, 1 ms pulses, 1% duty cycle, peak negative pressure 16.5 MPa) followed by administration of gem (n = 9), gem only (n = 5) or no treatment (n = 8). Tumor progression was followed by ultrasound imaging until the study endpoint (tumor size reaching 1 cm), whereupon the excised tumors were analyzed by histology, immunohistochemistry (IHC) and gene expression profiling (Nanostring PanCancer Immune Profiling panel). The pHIFU + gem treatments were well tolerated; the pHIFU-treated region of the tumor turned hypoechoic immediately following treatment in all mice, and this effect persisted throughout the observation period (2-5 weeks) and corresponded to areas of cell death, according to histology and IHC. Enhanced labeling by Granzyme-B was observed within and adjacent to the pHIFU treated area, but not in the non-treated tumor tissue; no difference in CD8 + staining was observed between the treatment groups. Gene expression analysis showed that the pHIFU + gem combination treatment lead to significant downregulation of 162 genes related to immunosuppression, tumorigenesis, and chemoresistance vs gem only treatment.

Comparative Characterization of Nonlinear Ultrasound Fields Generated by Sonalleve V1 and V2 MR-HIFU Systems.

A Sonalleve magnetic resonance-guided high-intensity focused ultrasound (MR-HIFU) clinical system (Profound Medical, Mississauga, ON, Canada) has been shown to generate nonlinear ultrasound fields with shocks up to 100 MPa at the focus as required for HIFU applications such as boiling histotripsy of hepatic and renal tumors. The Sonalleve system has two versions V1 and V2 of the therapeutic array, with differences in focusing angle, focus depth, arrangement of elements, and the size of a central opening that is twice larger in the V2 system compared to the V1. The goal of this study was to compare the performance of the V1 and V2 transducers for generating high-amplitude shock-wave fields and to reveal the impact of different array geometries on shock amplitudes at the focus. Nonlinear modeling of the field in water using boundary conditions reconstructed from holography measurements shows that at the same power output, the V2 array generates 10-15-MPa lower shock amplitudes at the focus. Consequently, substantially higher power levels are required for the V2 system to reach the same shock-wave exposure conditions in histotripsy-type treatments. Although this difference is mainly caused by the smaller focusing angle of the V2 array, the larger central opening of the V2 array has a nontrivial impact. By excluding coherently interacting weakly focused waves coming from the central part of the source, the presence of the central opening results in a somewhat higher effective focusing angle and thus higher shock amplitudes at the focus. Axisymmetric equivalent source models were constructed for both arrays, and the importance of including the central opening was demonstrated. These models can be used in the "HIFU beam" software for simulating nonlinear fields of the Sonalleve V1 and V2 systems in water and flat-layered biological tissues.