Cell-cycle-specific Cellular Responses to Sonoporation.

Microbubble-mediated sonoporation has shown its great potential in facilitating intracellular uptake of gene/drugs and other therapeutic agents that are otherwise difficult to enter cells. However, the biophysical mechanisms underlying microbubble-cell interactions remain unclear. Particularly, it is still a major challenge to get a comprehensive understanding of the impact of cell cycle phase on the cellular responses simultaneously occurring in cell membrane and cytoskeleton induced by microbubble sonoporation. Methods: Here, efficient synchronizations were performed to arrest human cervical epithelial carcinoma (HeLa) cells in individual cycle phases. The, topography and stiffness of synchronized cells were examined using atomic force microscopy. The variations in cell membrane permeabilization and cytoskeleton arrangement induced by sonoporation were analyzed simultaneously by a real-time fluorescence imaging system. Results: The results showed that G1-phase cells typically had the largest height and elastic modulus, while S-phase cells were generally the flattest and softest ones. Consequently, the S-Phase was found to be the preferred cycle for instantaneous sonoporation treatment, due to the greatest enhancement of membrane permeability and the fastest cytoskeleton disassembly at the early stage after sonoporation. Conclusion: The current findings may benefit ongoing efforts aiming to pursue rational utilization of microbubble-mediated sonoporation in cell cycle-targeted gene/drug delivery for cancer therapy.

Resource Paper: Sonoluminescence.

Sonoluminescence is the production of electromagnetic radiation, much of it in the form of visible light, that is emitted from a gas-filled cavity that has grown and collapsed under the influence of a varying pressure field. This resource paper provides a guide to the literature of sonoluminescence, from its early history to the present.

Deep Bleeder Acoustic Coagulation (DBAC)-part II: in vivo testing of a research prototype system.

BACKGROUND: Deep Bleeder Acoustic Coagulation (DBAC) is an ultrasound image-guided high-intensity focused ultrasound (HIFU) method proposed to automatically detect and localize (D&L) and treat deep, bleeding, combat wounds in the limbs of soldiers. A prototype DBAC system consisting of an applicator and control unit was developed for testing on animals. To enhance control, and thus safety, of the ultimate human DBAC autonomous product system, a thermal coagulation strategy that minimized cavitation, boiling, and non-linear behaviors was used. MATERIAL AND METHODS: The in vivo DBAC applicator design had four therapy tiles (Tx) and two 3D (volume) imaging probes (Ix) and was configured to be compatible with a porcine limb bleeder model developed in this research. The DBAC applicator was evaluated under quantitative test conditions (e.g., bleeder depths, flow rates, treatment time limits, and dose exposure time limits) in an in vivo study (final exam) comprising 12 bleeder treatments in three swine. To quantify blood flow rates, the "bleeder" targets were intact arterial branches, i.e., the superficial femoral artery (SFA) and a deep femoral artery (DFA). D&L identified, characterized, and targeted bleeders. The therapy sequence selected Tx arrays and determined the acoustic power and Tx beam steering, focus, and scan patterns. The user interface commands consisted of two buttons: "Start D&L" and "Start Therapy." Targeting accuracy was assessed by necropsy and histologic exams and efficacy (vessel coagulative occlusion) by angiography and histology. RESULTS: The D&L process (Part I article, J Ther Ultrasound, 2015 (this issue)) executed fully in all cases in under 5 min and targeting evaluation showed 11 of 12 thermal lesions centered on the correct vessel subsection, with minimal damage to adjacent structures. The automated therapy sequence also executed properly, with select manual steps. Because the dose exposure time limit (t dose ≤ 30 s) was associated with nonefficacious treatment, 60-s dosing and dual-dosing was also pursued. Thrombogenic evidence (blood clotting) and collagen denaturation (vessel shrinkage) were found in necropsy and histologically in all targeted SFAs. Acute SFA reductions in blood flow (20-30 %) were achieved in one subject, and one partial and one complete vessel occlusion were confirmed angiographically. The complete occlusion case was achieved with a dual dose (90 s total exposure) with focal intensity ≈500 W/cm(2) (spatial average, temporal average). CONCLUSIONS: While not meeting all in vivo objectives, the overall performance of the DBAC applicator was positive. In particular, D&L automation workflow was verified during each of the tests, with processing times well under specified (10 min) limits, and all bleeder branches were detected and localized. Further, gross necropsy and tissue examination confirmed that the HIFU thermal lesions were coincident with the target vessel locations in over 90 % of the multi-array dosing treatments. The SFA/DFA bleeder models selected, and the protocols used, were the most suitable practical model options for the given DBAC anatomical and bleeder requirements. The animal models were imperfect in some challenging aspects, including requiring tissue-mimicking material (TMM) standoffs to achieve deep target depths, thereby introducing device-tissue motion, with resultant imaging artifacts. The model "bleeders" involved intact vessels, which are subject to less efficient heating and coagulation cascade behaviors than true puncture injuries.

Deep bleeder acoustic coagulation (DBAC)-Part I: development and in vitro testing of a research prototype cuff system.

BACKGROUND: Bleeding from limb injuries is a leading cause of death on the battlefield, with deep wounds being least accessible. High-intensity focused ultrasound (HIFU) has been shown capable of coagulation of bleeding (cautery). This paper describes the development and refereed in vitro evaluation of an ultrasound (US) research prototype deep bleeder acoustic coagulation (DBAC) cuff system for evaluating the potential of DBAC in the battlefield. The device had to meet quantitative performance metrics on automated operation, therapeutic heating, bleeder detection, targeting accuracy, operational time limits, and cuff weight over a range of limb sizes and bleeder depths. These metrics drove innovative approaches in image segmentation, bleeder detection, therapy transducers, beam targeting, and dose monitoring. A companion (Part II) paper discusses the in vivo performance testing of an animal-specific DBAC system. MATERIALS AND METHODS: The cuff system employed 3D US imaging probes ("Ix") for detection and localization (D&L) and targeting, with the bleeders being identified by automated spectral Doppler analysis of flow waveforms. Unique high-element-count therapeutic arrays ("Tx") were developed, with the final cuff prototype having 21 Tx's and 6 Ix's. Spatial registration of Ix's and Tx's was done with a combination of image-registration, acoustic time-of-flight measurement, and tracking of the cuff shape via a fiber optic sensor. Acoustic radiation force impulse (ARFI) imaging or thermal strain imaging (TSI) at low-power doses were used to track the HIFU foci in closed-loop targeting. Recurrent neural network (RNN) acoustic thermometry guided closed-loop dosing. The cuff was tested on three phantom "limb" sizes: diameters = 25, 15, and 7.5 cm, with bleeder depths from 3.75 to 12.5 cm. "Integrated Phantoms" (IntP) were used for assessing D&L, closed-loop targeting, and closed-loop dosing. IntPs had surrogate arteries and bleeders, with blood-mimicking fluids moved by a pulsatile pump, and thermocouples (TCs) on the bleeders. Acoustic dosing was developed and tested using "HIFU Phantoms" having precisely located TCs, with end-of-dose target ∆T = 33-58 °C, and skin temperature ∆T ≤ 20 °C, being required. RESULTS: Most DBAC cuff performance requirements were met, including cuff weight, power delivery, targeting accuracy, skin temperature limit, and autonomous operation. The automated D&L completed in 9 of 15 tests (65 %), detecting the smallest (0.6 mm) bleeders, but it had difficulty with the lowest flow (3 cm/sec) bleeders, and in localizing bleeders in the smallest (7.5 cm) phantoms. D&L did not complete within the 9-min limit (results ranged 10-21 min). Closed-loop targeting converged in 20 of 31 tests (71 %), and closed-loop dosing power shut-off at preset ∆Ts was operational. SUMMARY AND CONCLUSION: The main performance objectives of the prototype DBAC cuff were met, however the designs required a number of challenging new technology developments. The novel Tx arrays exhibited high power with significant beam steering and focusing flexibility, while their integrated electronics enabled the required compact, lightweight configurability and simplified driving controls and cable/connector architecture. The compounded 3D imaging, combined with sophisticated software algorithms, enabled automated D&L and initial targeting and closed-loop targeting feedback via TSI. The development of RNN acoustic thermometry made possible feedback-controlled dosing. The lightweight architecture required significant design and fabrication effort to meet mechanical functionalities. Although not all target specifications were met, future engineering solutions addressing these performance deficiencies are proposed. Lastly, the program required very complex limb test phantoms and, while very challenging to develop, they performed well.

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.

Ultrasonic atomization of liquids in drop-chain acoustic fountains.

When focused ultrasound waves of moderate intensity in liquid encounter an air interface, a chain of drops emerges from the liquid surface to form what is known as a drop-chain fountain. Atomization, or the emission of micro-droplets, occurs when the acoustic intensity exceeds a liquid-dependent threshold. While the cavitation-wave hypothesis, which states that atomization arises from a combination of capillary-wave instabilities and cavitation bubble oscillations, is currently the most accepted theory of atomization, more data on the roles of cavitation, capillary waves, and even heat deposition or boiling would be valuable. In this paper, we experimentally test whether bubbles are a significant mechanism of atomization in drop-chain fountains. High-speed photography was used to observe the formation and atomization of drop-chain fountains composed of water and other liquids. For a range of ultrasonic frequencies and liquid sound speeds, it was found that the drop diameters approximately equalled the ultrasonic wavelengths. When water was exchanged for other liquids, it was observed that the atomization threshold increased with shear viscosity. Upon heating water, it was found that the time to commence atomization decreased with increasing temperature. Finally, water was atomized in an overpressure chamber where it was found that atomization was significantly diminished when the static pressure was increased. These results indicate that bubbles, generated by either acoustic cavitation or boiling, contribute significantly to atomization in the drop-chain fountain.

Investigation into the mechanisms of tissue atomization by high-intensity focused ultrasound.

Ultrasonic atomization, or the emission of a fog of droplets, was recently proposed to explain tissue fractionation in boiling histotripsy. However, even though liquid atomization has been studied extensively, the mechanisms underlying tissue atomization remain unclear. In the work described here, high-speed photography and overpressure were used to evaluate the role of bubbles in tissue atomization. As static pressure increased, the degree of fractionation decreased, and the ex vivo tissue became thermally denatured. The effect of surface wetness on atomization was also evaluated in vivo and in tissue-mimicking gels, where surface wetness was found to enhance atomization by forming surface instabilities that augment cavitation. In addition, experimental results indicated that wetting collagenous tissues, such as the liver capsule, allowed atomization to breach such barriers. These results highlight the importance of bubbles and surface instabilities in atomization and could be used to enhance boiling histotripsy for transition to clinical use.

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.

Cardiovascular applications of therapeutic ultrasound.

Ultrasound (US) has gained widespread use in diagnostic cardiovascular applications. At amplitudes and frequencies typical of diagnostic use, its biomechanical effects on tissue are largely negligible. However, these parameters can be altered to harness US's thermal and non-thermal effects for therapeutic indications. High-intensity focused ultrasound (HIFU) and extracorporeal shock wave therapy (ECWT) are two therapeutic US modalities which have been investigated for treating cardiac arrhythmias and ischemic heart disease, respectively. Here, we review the biomechanical effects of HIFU and ECWT, their potential therapeutic mechanisms, and pre-clinical and clinical studies demonstrating their efficacy and safety limitations. Furthermore, we discuss other potential clinical applications of therapeutic US and areas in which future research is needed.

Evidence for trapped surface bubbles as the cause for the twinkling artifact in ultrasound imaging.

The mechanism of the twinkling artifact (TA) that occurs during Doppler ultrasound imaging of kidney stones was investigated. The TA expresses itself in Doppler images as time-varying color. To define the TA quantitatively, beam-forming and Doppler processing were performed on raw per channel radio-frequency data collected when imaging human kidney stones in vitro. Suppression of twinkling by an ensemble of computer-generated replicas of a single radio frequency signal demonstrated that the TA arises from variability among the acoustic signals and not from electronic signal capture or processing. This variability was found to be random, and its suppression by elevated static pressure and return when the pressure was released suggest that the presence of bubbles on the stone surface is the mechanism that gives rise to the TA.

Characterizing an agar/gelatin phantom for image guided dosing and feedback control of high-intensity focused ultrasound.

The temperature dependence of an agar/gelatin phantom was evaluated. The purpose was to predict the material property response to high-intensity focused ultrasound (HIFU) for developing ultrasound guided dosing and targeting feedback. Changes in attenuation, sound speed, shear modulus and thermal properties with temperature were examined from 20°C to 70°C for 3 weeks post-manufacture. The attenuation decreased with temperature by a power factor of 0.15. Thermal conductivity, diffusivity and specific heat all increased linearly with temperature for a total change of approximately 16%, 10% and 6%, respectively. Sound speed had a parabolic dependence on temperature similar to that of water. Initially, the shear modulus irreversibly declined with even a slight increase in temperature. Over time, the gel maintained its room temperature shear modulus with moderate heating. A stable phantom was achieved within 2 weeks post-manufacture that possessed quasi-reversible material properties up to nearly 55°C.

Ultrasonic Atomization: A Mechanism of Tissue Fractionation.

High intensity focused ultrasound (HIFU) can be used to atomize liquid by creating a fountain on the surface exposed to air. The mechanism of atomization can be most accurately described by the cavitation-wave hypothesis wherein a combination of capillary waves excited on the liquid surface with cavitation beneath the surface produces a fine spray. Here, we show experimentally that a free tissue surface can also be atomized resulting in erosion of tissue from the surface. A 2-MHz spherically focused transducer operating at linearly predicted in situ intensities up to 14,000 W/cm2 was focused at ex vivo bovine liver and in vivo porcine liver tissue surfaces without the capsule. The end result for both in vivo and ex vivo tissues was erosion from the surface. In bovine liver at the maximum intensity, the erosion volume reached 25.7±10.9 mm3 using 300 10-ms pulses repeated at 1 Hz. Jet velocities for all tissues tested here were on the order of 10 m/s. Besides providing a mechanism for how HIFU can mechanically disrupt tissue, atomization may also explain how tissue is fractionated in boiling histotripsy.

1pPAb5. Acoustic radiation force to reposition kidney stones.

Our group has introduced transcutaneous ultrasound to move kidney stones in order to expel small stones or relocate an obstructing stone to a nonobstructing location. Human stones and metalized beads (2-8 mm) were implanted ureteroscopically in kidneys of eight domestic swine. Ultrasonic propulsion was performed using a diagnostic imaging transducer and a Verasonics ultrasound platform. Stone propulsion was visualized using fluoroscopy, ultrasound, and the ureteroscope. Successful stone movement was defined as relocating a stone to the renal pelvis, ureteropelvic junction (UPJ) or proximal ureter. Three blinded experts evaluated for histologic injury in control and treatment arms. All stones were moved. 65% (17/26) of stones/beads were moved the entire distance to the renal pelvis (3), UPJ (2), or ureter (12). Average successful procedure per stone required 14±8 min and 23±16 pushes. Each push averaged 0.9 s in duration. Mean interval between pushes was 41±13 sec. No gross or histologic kidney damage was identified in six kidneys from exposure to 20 1-s pushes spaced by 33 s. Ultrasonic propulsion is effective with most stones being relocated to the renal pelvis, UPJ, or ureter. The procedure appears safe without evidence of injury.

2aBA6. Bubbles trapped on the surface of kidney stones as a cause of the twinkling artifact in ultrasound imaging.

A twinkling artifact (TA) associated with urinary calculi has been described as rapidly changing colors on Doppler ultrasound. The purpose of this study was to investigate the mechanism of the TA. Doppler processing was performed on raw per channel radio-frequency data collected when imaging human kidney stones in degassed water. Suppression of twinkling by an ensemble of computer generated replicas of a single received signal demonstrated that the TA arises from variability among the acoustic signals and not from electronic signal processing. This variability was found to be random in nature, and its suppression by elevated static pressure, and its return when the pressure was released, suggests that the presence of surface bubbles on the stone is the mechanism of the TA. Submicron size bubbles are often trapped in crevices on solid objects, but the presence of these bubbles in vivo is unexpected. To further check this mechanism under conditions identical to in vivo, stone-producing porcine kidneys were harvested en bloc with a ligated ureter and then placed into a pressure chamber and imaged at elevated atmospheric pressure. The result was similar to in vitro. Work supported by NIH DK43881, DK092197, RFBR 11-02-01189, 12-02-00114, and NSBRI through NASA NCC 9-58.

Ultrasonic atomization of tissue and its role in tissue fractionation by high intensity focused ultrasound.

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 a tissue into submicron-sized fragments in a process termed boiling histotripsy, wherein the focused ultrasound wave superheats the tissue at the focus, producing a millimetre-sized boiling or vapour bubble in several milliseconds. Yet the question of how this millimetre-sized boiling bubble creates submicron-sized tissue fragments remains. The hypothesis of this work is that the tissue can behave as a liquid such that it atomizes and forms a fountain 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 cm(-2)) was aligned with an air-tissue interface meant to simulate the boiling bubble. Atomization and fountain formation was 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.

Controllable in vivo hyperthermia effect induced by pulsed high intensity focused ultrasound with low duty cycles.

High intensity focused ultrasound (HIFU)-induced hyperthermia is a promising tool for cancer therapy. Three-dimensional nonlinear acoustic-bioheat transfer-blood flow-coupling model simulations and in vivo thermocouple measurements were performed to study hyperthermia effects in rabbit auricular vein exposed to pulsed HIFU (pHIFU) at varied duty cycles (DCs). pHIFU-induced temperature elevations are shown to increase with increasing DC. A critical DC of 6.9% is estimated for temperature at distal vessel wall exceeding 44 °C, although different tissue depths and inclusions could affect the DC threshold. The results demonstrate clinic potentials of achieving controllable hyperthermia by adjusting pHIFU DCs, while minimizing perivascular thermal injury.

Novel high-intensity focused ultrasound clamp--potential adjunct for laparoscopic partial nephrectomy.

BACKGROUND AND PURPOSE: Partial nephrectomy (PN) can be technically challenging, especially if performed in a minimally invasive manner. Although ultrasound technology has been shown to have therapeutic capabilities, including tissue ablation and hemostasis, it has not gained clinical use in the PN setting. The purpose of this study is to evaluate the ability of a high-intensity ultrasound clamp to create an ablation plane in the kidney providing hemostasis that could potentially aid in laparoscopic PN. METHODS: A new instrument was created using a laparoscopic Padron endoscopic exposing retractor. Ultrasound elements were engineered on both sides of the retractor to administer high-intensity ultrasound energy between the two sides of the clamp. This high-intensity focused ultrasound (HIFU) clamp was placed 2 to 2.5 cm from the upper and lower poles of 10 porcine kidneys to evaluate its effectiveness at different levels and duration of energy delivery. PN transection was performed through the distal portion of the clamped margin. Kidneys postintervention and after PN were evaluated and blood loss estimated by weighing gauze placed at the defect. Histologic analysis was performed with hematoxylin and eosin and nicotinamide adenine dinucleotide staining to evaluate for tissue viability and thermal spread. RESULTS: Gross parenchymal changes were seen with obvious demarcation between treated and untreated tissue. Increased ultrasound exposure time (10 vs 5 and 2 min), even at lower power settings, was more effective in causing destruction and necrosis of tissue. Transmural ablation was achieved in three of four renal units after 10 minutes of exposure with significantly less blood loss (<2 g vs 30-100 g). Nonviable tissue was confirmed histologically. There was minimal thermal spread outside the clamped margin (1.2-3.2 mm). CONCLUSION: In this preliminary porcine evaluation, a novel HIFU clamp induced hemostasis and created an ablation plane in the kidney. This technology could serve as a useful adjunct to laparoscopic PN in the future and potentially obviate the need for renal hilar clamping.

Tissue Atomization by High Intensity Focused Ultrasound.

Liquid atomization and fountain formation by focused ultrasound was first published by Wood and Loomis [1]. Since then, the cavitation-wave hypothesis emerged to explain atomization in a fountain, which states atomization arises from a combination of surface capillary waves and the collapse of cavitation bubbles. More recently, high intensity focused ultrasound (HIFU) has been shown to fractionate tissue through either pulsed-cavitation or millisecond boiling histotripsy therapies; however it is unclear how millimeter-size boiling bubbles or cavitation bubble clouds fractionate tissue into submicron-size fragments. The objective of this work is to test the hypothesis experimentally that atomization and fountain formation occurs similarly in liquids and tissues and results in tissue erosion. A 2-MHz HIFU transducer operating at peak in situ pressures of 50 MPa and -11 MPa (linear intensity = 14,000 W/cm2) was focused at the interface between a liquid or tissue and air. A high-speed camera was used to monitor atomization and fountain formation in water, ethanol, glycerol, bovine liver, and porcine blood clots. The in situ linear intensity threshold for consistent atomization in one 10-ms pulse increased in the order: ethanol (180 W/cm2) < blood clot (250 W/cm2) < water (350 W/cm2) < liver (6200 W/cm2); glycerol did not atomize. Average jet velocities for the initial spray at the maximum acoustic intensity were similar for all materials and on the order of 20 m/s. The tissue erosion rate of liver approached saturation at around 300 10-ms pulses repeated at 1 Hz, which had an average erosion volume of 25.7±10.9 mm3. While tissue atomization and fountain formation does not completely mimic what is observed in liquids, atomization provides a plausible explanation of how tissue is fractionated in millisecond boiling and possibly even cavitation cloud histotrispy therapies.

Controlled tissue emulsification produced by high intensity focused ultrasound shock waves and millisecond boiling.

In high intensity focused ultrasound (HIFU) applications, tissue may be thermally necrosed by heating, emulsified by cavitation, or, as was recently discovered, emulsified using repetitive millisecond boiling caused by shock wave heating. Here, this last approach was further investigated. Experiments were performed in transparent gels and ex vivo bovine heart tissue using 1, 2, and 3 MHz focused transducers and different pulsing schemes in which the pressure, duty factor, and pulse duration were varied. A previously developed derating procedure to determine in situ shock amplitudes and the time-to-boil was refined. Treatments were monitored using B-mode ultrasound. Both inertial cavitation and boiling were observed during exposures, but emulsification occurred only when shocks and boiling were present. Emulsified lesions without thermal denaturation were produced with shock amplitudes sufficient to induce boiling in less than 20 ms, duty factors of less than 0.02, and pulse lengths shorter than 30 ms. Higher duty factors or longer pulses produced varying degrees of thermal denaturation combined with mechanical emulsification. Larger lesions were obtained using lower ultrasound frequencies. The results show that shock wave heating and millisecond boiling is an effective and reliable way to emulsify tissue while monitoring the treatment with ultrasound.