Trapping of embolic particles in a vessel phantom by cavitation-enhanced acoustic streaming

Timber DNA release using focused ultrasound extraction (FUSE) for genetic species identification

The use of genetic data for timber species and population assignment is a powerful tool for combating the illegal timber trade, but the challenges of extracting DNA from timber have prevented the routine use of genetics as a supply chain management tool. To overcome these challenges, we explored the feasibility of focused ultrasound extraction (FUSE) for rapid DNA release from timber. Using high-pressure ultrasound pulses, FUSE generates a cavitation bubble cloud that disintegrates samples into acellular debris, resulting in the mechanical release of DNA. In this work, FUSE was applied to white oak (Quercus alba) timber shavings to test the feasibility of using FUSE for timber DNA extraction for the first time. Results showed that FUSE processing disintegrated the tissue samples and released significant quantities of DNA. After five minutes of tissue processing DNA quantities of 0.21 ± 0.02 ng/mg, 0.99 ± 0.32 ng/mg, and 0.14 ± 0.01 ng/mg, were released from medium, coarse, and combination shaving groups, respectively. Amplification and sequencing of regions within the matK and rbcL chloroplast genes confirmed that the quality of DNA prepared with FUSE was suitable for PCR and short-read sequencing applications. Overall, these results show that FUSE can serve as a DNA sample preparation method capable of releasing high-quality DNA from timber in a fraction of the time required by conventional extraction methods. Based on the improved efficiency of DNA release with FUSE, ongoing work aims to develop this technology into portable systems that can be used to rapidly prepare timber samples for genetic species identification.

A. Mhjoob A, Raja A. Histotripsy: an innovative approach for minimally invasive tumour and disease treatment

Histotripsy is a noninvasive medical technique that uses high-intensity focused ultrasound (HIFU) to treat liver tumours. The two main histotripsy methods are boiling histotripsy and cavitation cloud histotripsy. Boiling histotripsy uses prolonged ultrasound pulses to create small boiling bubbles in the tissue, which leads to the breakdown of the tissue into smaller subcellular fragments. Cavitation cloud histotripsy uses the ultrasonic cavitation effect to disintegrate target tissue into precisely defined liquefied lesions. Both methods show similar treatment effectiveness; however, boiling histotripsy ensures treatment stability by producing a stable boiling bubble with each pulse. The therapeutic effect is ascribed to mechanical damage at the subcellular level rather than thermal damage. This article discusses the mechanisms, treatment parameters, and potential of histotripsy as a minimally invasive procedure that provides precise and controlled subcellular damage.

Sonothrombolysis with an acoustic net-assisted boiling histotripsy: A proof-of-concept study

Whilst sonothrombolysis is a promising and noninvasive ultrasound technique for treating blood clots, bleeding caused by thrombolytic agents used for dissolving clots and potential obstruction of blood flow by detached clots (i.e., embolus) are the major limitations of the current approach. In the present study, a new sonothrombolysis method is proposed for treating embolus without the use of thrombolytic drugs. Our proposed method involves (a) generating a spatially localised acoustic radiation force in a blood vessel against the blood flow to trap moving blood clots (i.e., generation of an acoustic net), (b) producing acoustic cavitation to mechanically destroy the trapped embolus, and (c) acoustically monitoring the trapping and mechanical fractionation processes. Three different ultrasound transducers with different purposes were employed in the proposed method: (1) 1-MHz dual focused ultrasound (dFUS) transducers for capturing moving blood clots, (2) a 2-MHz High Intensity Focused Ultrasound (HIFU) source for fractionating blood clots and (3) a passive acoustic emission detector with broad bandwidth (10 kHz to 20 MHz) for receiving and analysing acoustic waves scattered from a trapped embolus and acoustic cavitation. To demonstrate the feasibility of the proposed method, in vitro experiments with an optically transparent blood vessel-mimicking phantom filled with a blood mimicking fluid and a blood clot (1.2 to 5 mm in diameter) were performed with varying the dFUS and HIFU exposure conditions under various flow conditions (from 1.77 to 6.19 cm/s). A high-speed camera was used to observe the production of acoustic fields, acoustic cavitation formation and blood clot fragmentation within a blood vessel by the proposed method. Numerical simulations of acoustic and temperature fields generated under a given exposure condition were also conducted to further interpret experimental results on the proposed sonothrombolysis. Our results clearly showed that fringe pattern-like acoustic pressure fields (fringe width of 1 mm) produced in a blood vessel by the dFUS captured an embolus (1.2 to 5 mm in diameter) at the flow velocity up to 6.19 cm/s. This was likely to be due to the greater magnitude of the dFUS-induced acoustic radiation force exerted on an embolus in the opposite direction to the flow in a blood vessel than that of the drag force produced by the flow. The acoustically trapped embolus was then mechanically destructed into small pieces of debris (18 to 60 μm sized residual fragments) by the HIFU-induced strong cavitation without damaging the blood vessel walls. We also observed that acoustic emissions emitted from a blood clot captured by the dFUS and cavitation produced by the HIFU were clearly distinguished in the frequency domain. Taken together, these results can suggest that our proposed sonothrombolysis method could be used as a promising tool for treating thrombosis and embolism through capturing and destroying blood clots effectively.

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.

(More than) doubling down: Effective fibrinolysis at a reduced rt-PA dose for catheter-directed thrombolysis combined with histotripsy

Deep vein thrombosis is a major source of morbidity and mortality worldwide. For acute proximal deep vein thrombosis, catheter-directed thrombolytic therapy is an accepted method for vessel recanalization. Thrombolytic therapy is not without risk, including the potential for hemorrhagic bleeding that increases with lytic dose. Histotripsy is a focused ultrasound therapy that generates bubble clouds spontaneously in tissue at depth. The mechanical activity of histotripsy increases the efficacy of thrombolytic therapy at doses consistent with current pharmacomechanical treatments for venous thrombosis. The objective of this study was to determine the influence of lytic dose on histotripsy-enhanced fibrinolysis. Human whole blood clots formed in vitro were exposed to histotripsy and a thrombolytic agent (recombinant tissue plasminogen activator, rt-PA) in a venous flow model perfused with plasma. Lytic was administered into the clot via an infusion catheter at concentrations ranging from 0 (control) to 4.54 μg/mL (a common clinical dose for catheter-directed thrombolysis). Following treatment, perfusate samples were assayed for markers of fibrinolysis, hemolysis, and intact red blood cells and platelets. Fibrinolysis was equivalent between the common clinical dose of rt-PA (4.54 μg/mL) and rt-PA at a reduction to one-twentieth of the common clinical dose (0.23 μg/mL) when combined with histotripsy. Minimal changes were observed in hemolysis for treatment arms with or without histotripsy, potentially due to clot damage from insertion of the infusion catheter. Likewise, histotripsy did not increase the concentration of red blood cells or platelets in the perfusate following treatment compared to rt-PA alone. At the highest lytic dose, a refined histotripsy exposure scheme was implemented to cover larger areas of the clot. The updated exposure scheme improved clot mass loss and fibrinolysis relative to administration of lytic alone. Overall, the data collected in this study indicate the rt-PA dose can be reduced by more than a factor of ten and still promote fibrinolysis when combined with histotripsy.

Clot Degradation Under the Action of Histotripsy Bubble Activity and a Lytic Drug

Deep vein thrombosis is a major source of morbidity worldwide. For critical obstructions, catheter-directed thrombolytics are the frontline therapy to achieve vessel recanalization. Techniques that aid lytic therapy are under development to improve treatment efficacy and reduce procedure-related complications. Histotripsy is one such adjuvant under development that relies on focused ultrasound for in situ nucleation of bubble clouds. Prior studies have demonstrated synergistic effects for clot dissolution when histotripsy is combined with lytic therapy. The success of this combination approach is hypothesized to promote thrombolytic efficacy via two mechanisms: erythrocyte fractionation (hemolysis) and increased lytic activity (fibrinolysis). In this study, the contributions of hemolysis and fibrinolysis to clot degradation under histotripsy and a lytic were quantified with measurements of hemoglobin and D-dimer, respectively. A linear regression analysis was used to determine the relationship between hemoglobin, D-dimer, and the overall treatment efficacy (clot mass loss). A similar analysis was conducted to gauge the role of bubble activity, which was assessed with passive cavitation imaging, on hemolysis and fibrinolysis. Tabulation of these data demonstrated hemolysis and fibrinolysis contributed equally to clot mass loss. Furthermore, bubble cloud activity promoted the generation of hemoglobin and D-dimer in equal proportion. These studies indicate a multifactorial process for clot degradation under the action of histotripsy and a lytic therapy.

In Vitro Thrombolytic Efficacy of Single- and Five-Cycle Histotripsy Pulses and rt-PA

Although primarily known as an ablative modality, histotripsy can increase the efficacy of lytic therapy in a retracted venous clot model. Bubble cloud oscillations are the primary mechanism of action for histotripsy, and the type of bubble activity is dependent on the pulse duration. A retracted human venous clot model was perfused with and without the thrombolytic recombinant tissue plasminogen activator (rt-PA). The clot was exposed to histotripsy pulses of single- or five-cycle duration and peak negative pressures of 0-30 MPa. Bubble activity within the clot was monitored via passive cavitation imaging. The combination of histotripsy and rt-PA was more efficacious than rt-PA alone for single- and five-cycle pulses with peak negative pressures of 25 and 20 MPa, respectively. For both excitation schemes, the detected acoustic emissions correlated with the degree of thrombolytic efficacy. These results indicate that rt-PA and single- or multicycle histotripsy pulses enhance thrombolytic therapy.

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.

Histotripsy Liquefaction of Large Hematomas

Intra- and extra-muscular hematomas result from repetitive injury as well as sharp and blunt limb trauma. The clinical consequences can be serious, including debilitating pain and functional deficit. There are currently no short-term treatment options for large hematomas, only lengthy conservative treatment. The goal of this work was to evaluate the feasibility of a high intensity focused ultrasound (HIFU)-based technique, termed histotripsy, for rapid (within a clinically relevant timeframe of 15-20 min) liquefaction of large volume (up to 20 mL) extra-vascular hematomas for subsequent fine-needle aspiration. Experiments were performed using in vitro extravascular hematoma phantoms-fresh bovine blood poured into 50 mL molds and allowed to clot. The resulting phantoms were treated by boiling histotripsy (BH), cavitation histotripsy (CH) or a combination in a degassed water tank under ultrasound guidance. Two different transducers operating at 1 MHz and 1.5 MHz with f-number = 1 were used. The liquefied lysate was aspirated and analyzed by histology and sized in a Coulter Counter. The peak instantaneous power to achieve BH was lower than (at 1.5 MHz) or equal to (at 1 MHz) that which was required to initiate CH. Under the same exposure duration, BH-induced cavities were one and a half to two times larger than the CH-induced cavities, but the CH-induced cavities were more regularly shaped, facilitating easier aspiration. The lysates contained a small amount of debris larger than 70 μm, and 99% of particulates were smaller than 10 μm. A combination treatment of BH (for initial debulking) and CH (for liquefaction of small residual fragments) yielded 20 mL of lysate within 17.5 minutes of treatment and was found to be most optimal for liquefaction of large extravascular hematomas.

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.