Purpose 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. Material and Methods 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 vapor cavity causes tissue disintegration. Results Recent pre-clinical studies on histotripsy are reviewed for treating benign prostatic hyperplasia (BPH), liver and kidney tumors, kidney stone fragmentation, enhancing antitumor immune response, and tissue decellularization for regenerative medicine applications. Potential clinical advantages of the histotripsy methods are discussed. Conclusions 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 utilize different mechanisms for tissue disintegration, both have many of the same advantages and offer a promising alternative method of noninvasive surgery.

Recent studies have shown that shock wave heating and millisecond boiling in high intensity focused ultrasound (HIFU) fields can result in mechanical fractionation or emulsification of tissue - named boiling histotripsy. Visual observations of the change in color and contents indicated that the degree of thermal damage in the emulsified lesions can be controlled by varying the parameters of the exposure. The goal of this work was to examine thermal and mechanical effects in boiling histotripsy lesions using histological and biochemical analysis. The lesions were induced in ex vivo bovine heart and liver using a 2-MHz single-element transducer operating at duty factors of 0.005–0.01, pulse durations of 5–500 ms, and in situ shock amplitude of 73 MPa. Mechanical and thermal damage to tissue was evaluated histologically using conventional staining techniques (H&E and NADH-diphorase). Thermal effects were quantified by measuring denaturation of salt soluble proteins in the treated region. According to histology, the lesions that visually appeared as a liquid, contained no cellular structures larger than a cell nucleus and had a very sharp border of 1–2 cells. Both histology and protein analysis showed that lesions obtained with short pulses (< 10 ms) did not contain any thermal damage. Increasing the pulse duration resulted in an increase in thermal damage. However, both protein analysis and NADH-diaphorase staining showed less denaturation than visually observed as whitening of tissue. The number of HIFU pulses delivered per exposure did not change the lesion shape or the degree of thermal denaturation, whereas the size of the lesion showed a saturating behaviour thus suggesting optimal exposure duration. This study confirmed that boiling histotripsy offers an effective, predictable way to non-invasively fractionate tissue into subcellular fragments with or without inducing thermal damage.

Atomization and fountain formation is a well-known phenomenon that occurs when a focused ultrasound wave in liquid encounters an air interface. High intensity focused ultrasound (HIFU) has been shown to fractionate tissue into submicron-size fragments in a process termed boiling histotripsy, wherein the focused ultrasound wave superheats the tissue at the focus, producing a millimetre-size boiling or vapour bubble in several milliseconds. Yet the question of how this millimetre-size boiling bubble creates submicron-size tissue fragments remains. The hypothesis of this work is that tissue can behave as a liquid such that it forms a fountain and atomization within the vapour bubble produced in boiling histotripsy. We describe an experiment, in which a 2-MHz HIFU transducer (maximum in situ intensity of 24,000 W/cm2) was aligned with an air-tissue interface meant to simulate the boiling bubble. Atomization and fountain formation were observed with high-speed photography and resulted in tissue erosion. Histological examination of the atomized tissue showed whole and fragmented cells and nuclei. Air-liquid interfaces were also filmed. Our conclusion was that HIFU can fountain and atomize tissue. Although this process does not entirely mimic what was observed in liquids, it does explain many aspects of tissue fractionation in boiling histotripsy.

Significance High intensity focused ultrasound (HIFU) therapy is a promising, clinically adopted method of noninvasive tissue ablation used to treat both benign and malignant conditions. This work presents, to our knowledge, the first in vivo validation of a previously developed HIFU-based method that allows for noninvasive fractionation of targeted tissue into subcellular debris—boiling histotripsy—in a large animal model. While fractionating the targeted soft tissue, boiling histotripsy is shown to spare the adjacent connective tissue structures such as blood vessels. The process can be readily targeted and monitored by B-mode ultrasound. The resulting tissue debris are liquid, which provides a potential clinical benefit over thermal ablation in the treatment of tumors that exert uncomfortable pressure on surrounding tissues.

Pulsed high-intensity focused ultrasound (pHIFU) has been demonstrated to enhance vascular permeability, disrupt tumor barriers and enhance drug penetration into tumor tissue through acoustic cavitation. Monitoring of cavitation activity during pHIFU treatments and knowing the ultrasound pressure levels sufficient to reliably induce cavitation in a given tissue are therefore very important. Here, three metrics of cavitation activity induced by pHIFU and evaluated by confocal passive cavitation detection were introduced: cavitation probability, cavitation persistence and the level of the broadband acoustic emissions. These metrics were used to characterize cavitation activity in several ex vivo tissue types (bovine tongue and liver and porcine adipose tissue and kidney) and gel phantoms (polyacrylamide and agarose) at varying peak-rarefactional focal pressures (1–12 MPa) during the following pHIFU protocol: frequency 1.1 MHz, pulse duration 1 ms, pulse repetition frequency 1 Hz. To evaluate the relevance of the measurements in ex vivo tissue, cavitation metrics were also investigated and compared in the ex vivo and in vivo murine pancreatic tumors that develop spontaneously in transgenic KPC mice and closely recapitulate human disease in their morphology. The cavitation threshold, defined at 50 % cavitation probability, was found to vary broadly among the investigated tissues (within 2.5–10 MPa), depending mostly on the water-lipid ratio that characterizes the tissue composition. Cavitation persistence and the intensity of broadband emissions depended both on tissue structure and lipid concentration. Both the cavitation threshold and broadband noise emission level were similar between ex vivo and in vivo pancreatic tumor tissue. The largest difference between in vivo and ex vivo settings was found in the pattern of cavitation occurrence throughout pHIFU exposure: it was sporadic in vivo, but ex vivo it decreased rapidly and stopped over the first few pulses. Cavitation activity depended on the interplay between the destruction and circulation of cavitation nuclei, which are not only used up by HIFU treatment but also replenished or carried away by circulation in vivo. These findings are important for treatment planning and optimization in pHIFU-induced drug delivery, in particular for pancreatic tumors.

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Background High-intensity focused ultrasound (HIFU) is becoming more widely used for non-invasive and minimally invasive ablation of benign and malignant tumors. Recent studies suggest that HIFU can also enhance targeted drug delivery and stimulate an anti-tumor immune response in many tumors. However, targeting of pancreatic and liver tumors using an extracorporeal source is challenging due to the lack of an adequate acoustic window. The development of an endoscopic ultrasound (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 pre-clinical, 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 ultrasound 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 is a pre-clinical, 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. Feasibility of performing EUS-guided HIFU ablation in vivo has been demonstrated in an in vivo porcine model. Further development of this technology will allow endoscopists to perform precise therapeutic ablation of peri-lumenal lesions without breaching the wall of the gastric tract.

Background Diabetic foot ulceration has a complex and multi-factorial etiology and can involve changes in the pathophysiology of the plantar soft tissue. In the current study, histomorphological analyses of diabetic and non-diabetic plantar tissue were performed. It was hypothesized that the diabetic tissue would have thicker skin (epidermis and dermis), less interdigitation between the dermis and epidermis, thicker elastic septa and decreased adipose cell size. Materials and Methods Two locations of the foot (the heel and the first metatarsal) were examined, both of which have been reported to be locations with a high incidence of ulceration. Stereological methods and quantitative morphological techniques were used to evaluate the skin thickness, interdigitation index, elastic septae thickness and adipocyte cell size. Results The diabetic donors had a greater body mass index (BMI) than the non-diabetic donors. The diabetic tissue had significantly thicker elastic septae and dermis. However, no significant difference was observed in the interdigitation index or adipocyte size. Conclusion These findings demonstrate that morphological changes can be evaluated histologically to give a better understanding of the pathological changes in the plantar soft tissue with diabetes. These evaluations can then be associated with biomechanical changes that occur in diabetes to provide new insight into how microstructural changes can alter macroscopic properties. Clinical Relevance An understanding of the histomorphological changes in the soft tissue in relationship to the location on the foot could help to explain the biomechanical changes that occur in diabetes and the subsequent increase in susceptibility to breakdown.