In silico assessment of histotripsy-induced changes in catheter-directed thrombolytic delivery.

Introduction: For venous thrombosis patients, catheter-directed thrombolytic therapy is the standard-of-care to recanalize the occluded vessel. Limitations with thrombolytic drugs make the development of adjuvant treatments an active area of research. One potential adjuvant is histotripsy, a focused ultrasound therapy that lyses red blood cells within thrombus via the spontaneous generation of bubbles. Histotripsy has also been shown to improve the efficacy of thrombolytic drugs, though the precise mechanism of enhancement has not been elucidated. In this study, in silico calculations were performed to determine the contribution of histotripsy-induced changes in thrombus diffusivity to alter catheter-directed therapy. Methods: An established and validated Monte Carlo calculation was used to predict the extent of histotripsy bubble activity. The distribution of thrombolytic drug was computed with a finite-difference time domain (FDTD) solution of the perfusion-diffusion equation. The FDTD calculation included changes in thrombus diffusivity based on outcomes of the Monte Carlo calculation. Fibrin degradation was determined using the known reaction rate of thrombolytic drug. Results: In the absence of histotripsy, thrombolytic delivery was restricted in close proximity to the catheter. Thrombolytic perfused throughout the focal region for calculations that included the effects of histotripsy, resulting in an increased degree of fibrinolysis. Discussion: These results were consistent with the outcomes of in vitro studies, suggesting histotripsy-induced changes in the thrombus diffusivity are a primary mechanism for enhancement of thrombolytic drugs.

(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.

Design and Characterization of an Ultrasound Transducer for Combined Histotripsy-Thrombolytic Therapy.

Chronic thrombi of the deep veins of the leg are resistant to dissolution or removal by current interventions and can act as thrombogenic sources. Histotripsy, a focused ultrasound therapy, uses the mechanical activity of bubble clouds to liquefy target tissues. In vitro experiments have shown that histotripsy enhances thrombolytic agent recombinant tissue plasminogen activator in a highly retracted clot model resistant to lytic therapy alone. Although these results are promising, further refinement of the acoustic source is necessary for in vivo studies and clinical translation. The source parameters for use in vivo were defined, and a transducer was fabricated for transcutaneous exposure of porcine and human iliofemoral deep-vein thrombosis (DVT) as the target. Based on the design criteria, a 1.5-MHz elliptical source with a 6-cm focal length and a focal gain of 60 was selected. The source was characterized by fiber-optic hydrophone and holography. High-speed photography showed that the cavitation cloud could be confined to dimensions smaller than the specified vessel lumen. The source was also demonstrated in vitro to create confined lesions within clots. The results support that this design offers an appropriate clinical prototype for combined histotripsy-thrombolytic therapy.

Assessment of histological characteristics, imaging markers, and rt-PA susceptibility of ex vivo venous thrombi.

Venous thromboembolism is a significant source of morbidity and mortality worldwide. Catheter-directed thrombolytics is the primary treatment used to relieve critical obstructions, though its efficacy varies based on the thrombus composition. Non-responsive portions of the specimen often remain in situ, which prohibits mechanistic investigation of lytic resistance or the development of diagnostic indicators for treatment outcomes. In this study, thrombus samples extracted from venous thromboembolism patients were analyzed ex vivo to determine their histological properties, susceptibility to lytic therapy, and imaging characteristics. A wide range of thrombus morphologies were observed, with a dependence on age and etymology of the specimen. Fibrinolytic inhibitors including PAI-1, alpha 2-antiplasmin, and TAFI were present in samples, which may contribute to the response venous thrombi to catheter-directed thrombolytics. Finally, a weak but significant correlation was observed between the response of the sample to lytic drug and its magnetic microstructure assessed with a quantitative MRI sequence. These findings highlight the myriad of changes in venous thrombi that may promote lytic resistance, and imaging metrics that correlate with treatment outcomes.

An In vitro System to Gauge the Thrombolytic Efficacy of Histotripsy and a Lytic Drug.

Deep vein thrombosis (DVT) is a global health concern. The primary approach to achieve vessel recanalization for critical obstructions is catheter-directed thrombolytics (CDT). To mitigate caustic side effects and the long treatment time associated with CDT, adjuvant and alternative approaches are under development. One such approach is histotripsy, a focused ultrasound therapy to ablate tissue via bubble cloud nucleation. Pre-clinical studies have demonstrated strong synergy between histotripsy and thrombolytics for clot degradation. This report outlines a benchtop method to assess the efficacy of histotripsy-aided thrombolytic therapy, or lysotripsy. Clots manufactured from fresh human venous blood were introduced into a flow channel whose dimensions and acousto-mechanical properties mimic an iliofemoral vein. The channel was perfused with plasma and the lytic recombinant tissue-type plasminogen activator. Bubble clouds were generated in the clot with a focused ultrasound source designed for the treatment of femoral venous clots. Motorized positioners were used to translate the source focus along the clot length. At each insonation location, acoustic emissions from the bubble cloud were passively recorded, and beamformed to generate passive cavitation images. Metrics to gauge treatment efficacy included clot mass loss (overall treatment efficacy), and the concentrations of D-dimer (fibrinolysis) and hemoglobin (hemolysis) in the perfusate. There are limitations to this in vitro design, including lack of means to assess in vivo side effects or dynamic changes in flow rate as the clot lyses. Overall, the setup provides an effective method to assess the efficacy of histotripsy-based strategies to treat DVT.

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.

Assessment of Collaborative Robot (Cobot)-Assisted Histotripsy for Venous Clot Ablation.

OBJECTIVE: The application of bubble-based ablation with the focus ultrasound therapy histotripsy is gaining traction for the treatment of venous thrombosis, among other pathologies. For extensive clot burden, the histotripsy source must be translated to ensure uniform bubble activity throughout the vascular obstruction. The purpose of this study was to evaluate the targeting accuracy of a histotripsy system comprised of a focused source, ultrasound image guidance, and a collaborative robot (cobot) positioner. The system was designed with a primary emphasis for treating deep vein thrombosis. METHODS: Studies to test treatment planning and targeting bubble activity with the histotripsy-cobot system were conducted in an in vitro clot model. A tissue-mimicking phantom was also targeted with the system, and the predicted and actual areas of liquefaction were compared to gauge the spatial accuracy of ablation. RESULTS: The system provided submillimeter accuracy for both tracking along an intended path (within 0.6 mm of a model vessel) and targeting bubble activity within the venous clot model (0.7 mm from the center of the clot). Good correlation was observed between the planned and actual liquefaction locations in the tissue phantom, with an average Dice similarity coefficient of 77.8%, and average Hausdorff distance of 1.6 mm. CONCLUSION: Cobots provide an effective means to apply histotripsy pulses over a treatment volume, with the ablation precision contingent on the quality of image guidance. SIGNIFICANCE: Overall, these results demonstrate cobots can be used to guide histotripsy ablation for targets that extend beyond the natural focus of the transducer.

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.

In vitro assessment of stiffness-dependent histotripsy bubble cloud activity in gel phantoms and blood clots.

As a bubble-based ablative therapy, the efficacy of histotripsy has been demonstrated in healthy or acutely diseased models. Chronic conditions associated with stiff tissues may require additional bubble activity prior to histotripsy liquefaction. In this study, histotripsy pulses were generated in agarose phantoms of Young's moduli ranging from 12.3 to 142 kPa, and in vitro clot models with mild and strong platelet-activated retraction. Bubble cloud emissions were tracked with passive cavitation imaging, and the threshold acoustic power associated with phantom liquefaction was extracted with receiver operator characteristic analysis. The power of histotripsy-generated emissions and the degree of liquefaction were tabulated for both clot models. For the agarose phantoms, the acoustic power associated with liquefaction increased with Young's modulus. When grouped based on agarose concentration, only two arms displayed a significant difference in the liquefaction threshold acoustic power (22.1 kPa versus 142 kPa Young's modulus). The bubble cloud dynamics tracked with passive cavitation imaging indicated no strong changes in the bubble dynamics based on the phantom stiffness. For identical histotripsy exposure, the power of acoustic emissions and degree of clot lysis did not vary based on the clot model. Overall, these results indicate that a fixed threshold acoustic power mapped with passive cavitation imaging can be utilized for predicting histotripsy liquefaction over a wide range of tissue stiffness.

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

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