Retracted blood clots have been previously recognized to be more resistant to drug-based thrombolysis methods, even with ultrasound and microbubble enhancements. Microtripsy, a new histotripsy approach, has been investigated as a non-invasive, drug-free, and image-guided method that uses ultrasound to break up clots with improved treatment accuracy and a lower risk of vessel damage when compared to the traditional histotripsy thrombolysis approach. Unlike drug-mediated thrombolysis, which is dependent on the permeation of the thrombolytic agents into the clot, microtripsy controls acoustic cavitation to fractionate clots. We hypothesize that microtripsy thrombolysis is effective on retracted clots and that the treatment efficacy can be enhanced using strategies incorporating electronic focal steering. To test our hypothesis, retracted clots were prepared in vitro and the mechanical properties were quantitatively characterized. Microtripsy thrombolysis was applied on the retracted clots in an in vitro flow model using three different strategies: single-focus, electronically-steered multi-focus, and a dual-pass multi-focus strategy. Results show that microtripsy was used to successfully generate a flow channel through the retracted clot and the flow was restored. The multi-focus and the dual-pass treatments incorporating the electronic focal steering significantly increased the recanalized flow channel size compared to the single-focus treatments. The dual-pass treatments achieved a restored flow rate up to 324 mL/min without cavitation contacting the vessel wall. The clot debris particles generated from microtripsy thrombolysis remained within the safe range. The results in this study show the potential of microtripsy thrombolysis for retracted clot recanalization with the enhancement of electronic focal steering.

Histotripsy is a noninvasive therapeutic technique that uses ultrasound generated from outside the body to create controlled cavitation in targeted tissue, and fractionates it into acellular debris. We have developed a new histotripsy approach, termed microtripsy, to improve targeting accuracy and to avoid collateral tissue damage. This in vivo study evaluates the safety and efficacy of microtripsy for noninvasive thrombolysis in a porcine deep vein thrombosis (DVT) model. Acute thrombi were formed in left femoral veins of pigs (~35 kg) by occluding the vessel using two balloon catheters and infusing with thrombin. Guided by real-time ultrasound imaging, microtripsy thrombolysis treatment was conducted in 14 pigs. 10 pigs were euthanized on the same day (acute) and 4 at 2 weeks (subacute). To evaluate vessel damage, 30-min free-flow treatment in right femoral vein (no thrombus) was also conducted in 8 acute pigs. Blood flow was successfully restored or significantly increased after treatment in 13 out of the 14 pigs. The flow channels reopened by microtripsy had a diameter up to 64% of the vessel diameter (~6 mm). The average treatment time was 16 minute per cm-long thrombus. Only mild intravascular hemolysis was induced during microtripsy thrombolysis. No damage was observed on vessel walls after two weeks of recovery, venous valves were preserved, and there was no sign of pulmonary embolism. The results of this study indicate that microtripsy has the potential to be a safe and effective treatment for DVT in a porcine model.

Following the collapse of a cavitation bubble cloud, residual microbubbles can persist for up to seconds and function as weak cavitation nuclei for subsequent pulses in a phenomenon known as cavitation memory effect. In histotripsy, the cavitation memory effect can cause bubble clouds to repeatedly form at the same discrete set of sites. This effect limits the efficacy of histotripsy-based tissue fractionation. Our previous studies have shown that low-amplitude bubble coalescing (BC) ultrasound sequences interleaved between high-amplitude histotripsy pulses can coalescence the residual bubbles into one large bubble quickly. This reduces the cavitation memory effect and may increase treatment efficacy. Histotripsy has been investigated for thrombolysis by breaking up clots to debris smaller than red blood cells. However, this treatment has low efficacy for aged or retracted clot. In this study, we investigate the use of histotripsy with BC to improve the treatment efficacy for retracted clots. An integrated histotripsy and bubble coalescing (HBC) transducer system with specialized electronic driving system was built in-house. One high amplitude (32 MPa), 1-cycle histotripsy pulse followed by 36 low amplitude (2.4 MPa), 1-cycle BC pulses formed one HBC sequence. Results show that HBC sequences successfully generated a flow channel through the retracted clots under scan speeds of 0.2 – 0.5 mm/s. The created channel size was 128–480% larger using the HBC sequence compared to using histotripsy alone. The clot debris particles generated during HBC treatments were within the safe range. These results demonstrate the concept that BC improves treatment efficacy of histotripsy thrombolysis for retracted clots.

Histotripsy is an ultrasonic tissue ablation method based on acoustic cavitation. It has been shown that cavitation dynamics change depending on the mechanical properties of the host medium. During histotripsy treatment, the target-tissue is gradually fractionated and eventually liquefied to acellular homogenate. In this study, the change in the collapse time (t ) of the cavitation bubble cloud over the course of histotripsy treatment is investigated as an indicator for progression of the tissue fractionation process throughout treatment. A 500 kHz histotripsy transducer is used to generate single-location lesions within tissue-mimicking agar phantoms of varying stiffness levels as well as ex vivo bovine liver samples. Cavitation collapse signals are acquired with broadband hydrophones, and cavitation is imaged optically using a high-speed camera in transparent tissue-mimicking phantoms. The high-speed-camera-acquired measurements of t validate the acoustic hydrophone measurements. Increases in t are observed both with decreasing phantom stiffness and throughout histotripsy treatment with increasing number of pulses applied. Theincreasing trend of t throughout the histotripsy treatment correlates well with the progression of lesion formation generated in tissue-mimicking phantoms (R =  0.87). Finally, the increasing trend of t over the histotripsy treatment is validated in ex vivo bovine liver.

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