Biological and physical mechanisms of HIFU-induced hyperecho in ultrasound images

Guidance and monitoring of high intensity focused ultrasound (HIFU) therapy, using ultrasound imaging, has primarily utilized formation of a hyperechoic region at the HIFU focus. We investigated biologic and physical mechanisms of a hyperecho, as well as safety of this phenomenon, using thermal, acoustic and light microscopy observations. Single, short-duration HIFU pulses (30-60 ms) were able to produce a hyperechoic region at the HIFU focus, 2 cm deep in a rabbit thigh muscle. When hyperechoic regions appeared, inertial cavitation was detected in vivo using a custom-made passive cavitation detection system. Light micrographs showed a large number of cavities (approximately 100/mm3), 1-10 microm in diameter, in a cytoplasm of cells located at the HIFU focus. Blood congestion was observed around a focal region, indicating an injury of microvasculature. Cellular necrosis was observed at 2 d after the treatment, while healing, scar tissue formation and regeneration were observed at 7 d and 14 d. The results indicate that a possibility of adverse tissue effects has to be taken into consideration when the hyperecho formation, induced by very-short HIFU pulses, is used for pretreatment targeting.     

Real-time visualization of high-intensity focused ultrasound treatment using ultrasound imaging

High-intensity focused ultrasound (HIFU) and conventional B-mode ultrasound (US) imaging were synchronized to develop a system for real-time visualization of HIFU treatment. The system was tested in vivo in pig liver. The HIFU application resulted in the appearance of a hyperechoic spot at the focus that faded gradually after cessation of HIFU exposure. The duration of HIFU exposure needed for a hyperechoic spot to appear, was inversely related to the HIFU intensity. The threshold intensity required to produce a hyperechoic spot in liver in < 1 s was 970 W/cm(2), in situ. At this HIFU dose, no immediate cellular damage was observed, providing a potential for pretreatment targeting. The real-time visualization method was used in hemostasis of actively bleeding internal pelvic vessels, allowing targeting and monitoring of successful treatment. Real-time US imaging may provide a useful tool for image-guided HIFU therapy.     

Temporal and Spatial Detection of HIFU-Induced Inertial and Hot-Vapor Cavitation with a Diagnostic Ultrasound System

The onset and presence of inertial cavitation and near-boiling temperatures in high-intensity focused ultrasound (HIFU) therapy have been identified as important indicators of energy deposition for therapy guidance. Passive cavitation detection is commonly used to detect bubble emissions, where a fixed-focus single-element acoustic transducer is typically used as a passive cavitation detector (PCD). This technique is suboptimal for clinical applications, because most PCD transducers are tightly focused and afford limited spatial coverage of the HIFU focal region. A Terason 2000 Ultrasound System was used as a PCD array to expand the spatial detection region for cavitation by operating in passive mode, obtaining the radiofrequency signals corresponding to each scan line and filtering the contribution from scattering of the HIFU signal harmonics. This approach allows for spatially resolved detection of both inertial and stable cavitation throughout the focal region. Measurements with the PCD array during sonication with a 1.1-MHz HIFU source in tissue phantoms were compared with single-element PCD and thermocouple sensing. Stable cavitation signals at the harmonics and superharmonics increased in a threshold fashion for temperatures >90°C, an effect attributed to high vapor pressure in the cavities. Incorporation of these detection techniques in a diagnostic ultrasound platform could result in a powerful tool for improving HIFU guidance and treatment. (E-mail: cfarny@bwh.harvard.edu)     

Activation, aggregation and adhesion of platelets exposed to high-intensity focused ultrasound.

Using platelet-rich plasma, we investigated the effect of 1.1-MHz continuous wave high-intensity focused ultrasound (HIFU) on platelet activation, aggregation and adhesion to a collagen-coated surface. Platelets were exposed for durations of 10-500 s at spatial average intensities of up to 4860 W/cm(2). To avoid heating effects, the average temperature in the HIFU tank was maintained at 33.8 +/- 4.0 degrees C during platelet experiments. Flow cytometry, laser aggregometry, environmental scanning electron microscopy and passive cavitation detection were used to observe and to quantify platelet activation, aggregation, adhesion to a collagen-coated surface and associated cavitation. It was determined that HIFU can activate platelets, stimulate them to aggregate and promote their adherence to a collagen-coated surface. In principle, HIFU can stimulate primary, or platelet-related, hemostasis. Cavitation was monitored by a passive cavitation detector during aggregation trials and was quantified to provide a relative measure of the amount of cavitation that occurred in each aggregation trial. Regression analysis shows a weak correlation (r(2) = 0.11) between aggregation and ultrasound intensity, but a substantial correlation (r(2) = 0.76) between aggregation and cavitation occurrence.     

Combination of thermal and cavitation effects to generate deep lesions with an endocavitary applicator using a plane transducer: ex vivo studies

In the high-intensity focused ultrasound (US), or HIFU, field, it is well-known that the cavitation effect can be used to induce lesions of larger volume. The principle is based on the increase in the equivalent attenuation coefficient of the tissue in the presence of the bubbles created by cavitation. The elementary lesions produced by combination of cavitation and thermal effects, using focused transducers, were spherical and developed upstream of the focal point. This paper presents a method that combines cavitation with a thermal effect to obtain deeper lesions using a plane transducer, rather than a focused one. The cavitation effect was produced by delivering intensities of 60 W/cm2 at the face of the transducer for 0.5 s. The applicator was then rotated through 90 degrees at a constant speed of between 0.5 and 1.5 degrees /s. During this rotation, ex vivo tissues were exposed continuously to an acoustic intensity of 14 W/cm2 to combine the cavitation effect with a thermal effect. The necroses were, on average, twice as deep when the cavitation effect was used as those obtained with a thermal effect alone. Observed macroscopically, the lesions have a very well-delimited geometry. Temperature measurements made at different angles of treatment have shown that they were coagulation necroses.     

Gel phantom for use in high-intensity focused ultrasound dosimetry

An optically transparent phantom was developed for use in high-intensity focused ultrasound (US), or HIFU, dosimetry studies. The phantom is composed of polyacrylamide hydrogel, embedded with bovine serum albumin (BSA) that becomes optically opaque when denatured. Acoustic and optical properties of the phantom were characterized as a function of BSA concentration and temperature. The speed of sound (1544 m/s) and acoustic impedance (1.6 MRayls) were similar to the values in soft tissue. The attenuation coefficient was approximately 8 times lower than that of soft tissues (0.02 Np/cm/MHz for 9% BSA). The nonlinear (B/A) coefficient was similar to the value in water. HIFU lesions were readily seen during formation in the phantom. In US B-mode images, the HIFU lesions were observed as hyperechoic regions only if the cavitation activity was present. The phantom can be used for fast characterization and calibration of US-image guided HIFU devices before animal or clinical studies.     

The relation between cavitation and platelet aggregation during exposure to high-intensity focused ultrasound

Our previous study showed that high-intensity focused ultrasound (HIFU) is capable of producing "primary acoustic hemostasis" in the form of ultrasound (US)-induced platelet activation, aggregation and adhesion to a collagen-coated surface. In the current study, 1.1 MHz continuous-wave HIFU was used to investigate the role of cavitation as a mechanism for platelet aggregation in samples of platelet-rich plasma. A 5 MHz passive cavitation detector was used to monitor cavitation activity and laser aggregometry was used to measure platelet aggregation. Using spatial average intensities from 0 to 3350 W/cm2, the effects of HIFU-induced cavitation on platelet aggregation were investigated by enhancing cavitation activity through use of US contrast agents and by limiting cavitation activity through use of an overpressure system. Our results show that increased cavitation activity lowers the intensity threshold to produce platelet aggregation and decreased cavitation activity in the overpressure system raises the intensity threshold for platelet aggregation.     

Numerical and Experimental Study of Mechanisms Involved in Boiling Histotripsy

The aim of boiling histotripsy is to mechanically fractionate tissue as an alternative to thermal ablation for therapeutic applications. In general, the shape of a lesion produced by boiling histotripsy is tadpole like, consisting of a head and a tail. Although many studies have demonstrated the efficacy of boiling histotripsy for fractionating solid tumors, the exact mechanisms underpinning this phenomenon are not yet well understood, particularly the interaction of a boiling vapor bubble with incoming incident shockwaves. To investigate the mechanisms involved in boiling histotripsy, a high-speed camera with a passive cavitation detection system was used to observe the dynamics of bubbles produced in optically transparent tissue-mimicking gel phantoms exposed to the field of a 2.0-MHz high-intensity focused ultrasound (HIFU) transducer. We observed that boiling bubbles were generated in a localized heated region and cavitation clouds were subsequently induced ahead of the expanding bubble. This process was repeated with HIFU pulses and eventually resulted in a tadpole-shaped lesion. A simplified numerical model describing the scattering of the incident ultrasound wave by a vapor bubble was developed to help interpret the experimental observations. Together with the numerical results, these observations suggest that the overall size of a lesion induced by boiling histotripsy is dependent on the sizes of (i) the heated region at the HIFU focus and (ii) the backscattered acoustic field by the original vapor bubble.     

Dynamic Changes of Integrated Backscatter,Attenuation Coefficient and Bubble Activities During High-Intensity Focused Ultrasound (HIFU) Treatment

This paper simultaneously investigated the transient characteristics of integrated backscatter (IBS), attenuation coefficient and bubble activities as time traces before, during and after HIFU treatment, with different HIFU parameters (acoustic power and duty cycle) in both transparent tissue-mimicking phantoms and freshly excised bovine livers. These dynamic changes of acoustic parameters and bubble activities were correlated with the visualization of lesion development selected from photos, conventional B-mode ultrasound images and differential IBS images over the whole procedure of HIFU treatment. Two-dimensional radiofrequency (RF) data were acquired by a modified diagnostic ultrasound scanner to estimate the changes of mean IBS and attenuation coefficient averaged in the lesion region, and to construct the differential IBS images and B-mode ultrasound images simultaneously. Bubble activities over the whole procedure of HIFU treatment were investigated by the passive cavitation detection (PCD) method and the changes in subharmonic and broadband noise were correlated with the transient characteristics of IBS and attenuation coefficient. When HIFU was switched on, IBS and attenuation coefficient increased with the appearance of bubble clouds in the B-mode and differential IBS image. At the same time, the level of subharmonic and broadband noise rose abruptly. Then, there was an initial decrease in the attenuation coefficient, followed by an increase when at lower HIFU power. As the lesion appeared, IBS and attenuation coefficient both increased rapidly to a value twice that of normal. Then the changes in IBS and attenuation coefficient showed more complex patterns, but still showed a slower trend of increases with lesion development. Violent bubble activities were visible in the gel and were evident as strongly echogenic regions in the differential IBS images and B-mode images simultaneously. This was detected by a dramatic high level of subharmonic and broadband noise at the same time. These bubble activities caused fluctuations in IBS and attenuation coefficient during HIFU treatment. After HIFU, IBS and attenuation coefficient decreased gradually accompanied by the fadeout of bright hyperechoic spot in the B-mode and differential IBS image, but were still higher than normal when they were stable. The increases of IBS and attenuation coefficient were greater when using higher acoustic power or a higher duty cycle of the therapeutic emission. These experiments indicated that the bubble activities had the dominant effects on the transient characteristics of IBS and attenuation. This should be taken into consideration when using the dynamic acoustic-property changes for the potentially real-time monitoring imaging of HIFU treatment. (E-mail: mxwan@mail.xjtu.edu.cn)     

Erythrocytes, as well as microbubble contrast agents, are important factors in improving thermal and therapeutic effects of high-intensity focused ultrasound

Erythrocytes, as well as microbubble contrast agents, are important factors in improving thermal effect of high-intensity focused ultrasound (US), or HIFU, and increasing the coagulation volume produced by HIFU irradiation. In vitro experiments used human plasma with various concentrations of human erythrocytes in combination with or without Levovist. In vivo experiments used eight Japan white rabbits with three degrees of anemia. Using a 2.17-MHz transducer, HIFU was applied for 60 s, and the temperature rise and the volume of coagulation necrosis was evaluated. There was a significant correlation between the HIFU-induced temperature rise and hematocrit, with a correlation coefficient of 0.998 (p=0.0001). Although the temperature rise was smaller at low hematocrit, it was significantly increased by adding Levovist in the suspension (p<0.01). The mean volume of coagulation necrosis was significantly greater in the rabbits with higher hematocrits (p<0.01), and that in the moderate anemia group was significantly increased by using Levovist (p<0.01).     

Image-guided HIFU neurolysis of peripheral nerves to treat spasticity and pain

Spasticity, a major complication of central nervous system disorders, signified by uncontrollable muscle contractions, is very difficult to treat effectively. We report on the use of ultrasound (US) image-guided high-intensity focused US (HIFU) to target and suppress the function of the sciatic nerve complex of rabbits in vivo, as a possible treatment of spasticity. The image-guided HIFU device included a 3.2-MHz spherically curved transducer and an intraoperative imaging probe. A focal acoustic intensity of 1480 to 1850 W/cm(2), applied using a scanning method, was effective in achieving complete conduction block in 100% of 22 nerve complexes with HIFU treatment times of 36 +/- 14 s (mean +/- SD). Gross examination showed blanching of the nerve at the HIFU treatment site and lesion volumes of 2.8 +/- 1.4 cm(3) encompassing the nerve complex. Histologic examination indicated axonal demyelination and necrosis of Schwann cells as probable mechanisms of nerve block. With accurate localization and targeting of peripheral nerves using US imaging, HIFU could become a promising tool for the suppression of spasticity.     

Using the acoustic interference pattern to locate the focus of a high-intensity focused ultrasound (HIFU) transducer

One of the main problems encountered when using conventional B-mode ultrasound (US) for targeting and monitoring purposes during ablation therapies employing high-intensity focused US (HIFU) is the appearance of strong interference in the obtained diagnostic US images. In this study, instead of avoiding the interference noise, we demonstrate how we used it to locate the focus of the HIFU transducer in both in vitro tissue-mimicking phantoms and an ex vivo tissue block. We found that when the B-mode image plane coincided with the HIFU focal plane, the interference noise was maximally converged and enhanced compared with the off-focus situations. Stronger interference noise was recorded when the angle (alpha) between the US image plane and the HIFU axis was less than or equal to 90 degrees. By intentionally creating a target (group of bubbles) at the 3.5-MHz HIFU focus (7.1 mm in length and 0.7 mm in diameter), the position of the maximal noise convergence coincided well with the target. The difference between the predicted focus and the actual one (bubbles) on x and z axes (axes perpendicular to the HIFU central axis, Fig. 1) were both about 0.9 mm. For y axis (HIFU central axis), the precision was within 1.0 mm. For tissue block ablation, the interference noise concentrated at the position of maximal heating of the HIFU-induced lesions. The proposed method can also be used to predict the position of the HIFU focus by using a low intensity output scheme before permanent changes in the target tissue were made.     

Use of overpressure to assess the role of bubbles in focused ultrasound lesion shape in vitro

Overpressure--elevated hydrostatic pressure--was used to assess the role of gas or vapor bubbles in distorting the shape and position of a high-intensity focused ultrasound (HIFU) lesion in tissue. The shift from a cigar-shaped lesion to a tadpole-shaped lesion can mean that the wrong area is treated. Overpressure minimizes bubbles and bubble activity by dissolving gas bubbles, restricting bubble oscillation and raising the boiling temperature. Therefore, comparison with and without overpressure is a tool to assess the role of bubbles. Dissolution rates, bubble dynamics and boiling temperatures were determined as functions of pressure. Experiments were made first in a low-overpressure chamber (0.7 MPa maximum) that permitted imaging by B-mode ultrasound (US). Pieces of excised beef liver (8 cm thick) were treated in the chamber with 3.5 MHz for 1 to 7 s (50% duty cycle). In situ intensities (I(SP)) were 600 to 3000 W/cm(2). B-mode US imaging detected a hyperechoic region at the HIFU treatment site. The dissipation of this hyperechoic region following HIFU cessation corresponded well with calculated bubble dissolution rates; thus, suggesting that bubbles were present. Lesion shape was then tested in a high-pressure chamber. Intensities were 1300 and 1750 W/cm(2) ( +/- 20%) at 1 MHz for 30 s. Hydrostatic pressures were 0.1 or 5.6 MPa. At 1300 W/cm(2), lesions were cigar-shaped, and no difference was observed between lesions formed with or without overpressure. At 1750 W/cm(2), lesions formed with no overpressure were tadpole-shaped, but lesions formed with high overpressure (5.6 MPa) remained cigar-shaped. Data support the hypothesis that bubbles contribute to the lesion distortion.     

Laser-Induced Fluorescence Thermometry of Heating in Water from Short Bursts of High Intensity Focused Ultrasound

Free field experimental measurements of the temperature rise of water in the focal region of a 2 MHz high intensity focused ultrasound (HIFU) transducer were performed. The transducer was operated in pulse-mode with millisecond bursts, at acoustic intensities of 5 to 18.5 kW/cm² at the focus, resulting in non-linear wave propagation and shock wave formation. Pulsed, planar, laser-induced fluorescence (LIF) was used as a fast rise-time, non-intrusive, temperature measurement method of the water present in the focal region. LIF thermometry is based on calibrating the temperature-dependent fluorescence intensity signal emitted by a passive dye dissolved in water when excited by a pulse of laser light. The laser beam was formed into a thin light sheet to illuminate a planar area in the HIFU focal region. The laser light sheet was oriented transverse to the acoustic axis. Cross-sectional, instantaneous temperature field measurements within the HIFU focal volume showed that the water temperature increased steadily with increasing HIFU drive voltage. Heating rates of 4000–7000°C/s were measured within the first millisecond of the HIFU burst. Increasing the length of the burst initially resulted in an increase in the water temperature within the HIFU focal spot (up to ∼3 ms), after which it steadied or slightly dropped. Acoustic streaming was measured and shown to be consistent with the reduction in heating with increased burst length due to convective cooling. LIF thermometry may thus be a viable non-invasive method for the characterization of HIFU transducers at high power intensities.     

HIFU联合脂质纳米氟碳液滴消融兔肝的实验研究

目的 探讨脂质纳米氟碳液滴增效HIFU消融兔肝脏的空化活动及术后病理学变化。方法 首先制备全氟戊烷脂质纳米氟碳液滴(L-PFP);然后将24只正常新西兰兔随机分为对照组(单纯HIFU组)和L-PFP组;在 B 超引导下进行HIFU定点消融兔肝脏(超声能量：900 J);通过被动空化检测系统(PCD)监控空化活动;分别将消融即刻、1d、3d、7d的兔肝脏标本取出进行H E染色,观察消融灶转归过程中的病理学变化。结果 经耳缘静脉注射L-PFP后HIFU 辐照兔肝脏所产生的空化泡群更明显,其灰度变化值为对照组的1.93倍,累积瞬态空化剂量为对照组的6.3倍,空化活动表现强烈;大体病理及H E结果显示L-PFP组造成的组织损伤严重,细胞变性更为彻底,炎性反应更为强烈;单纯HIFU组消融灶7d修复为正常组织,转归所需时间显著短于L-PFP组。结论 脂质纳米氟碳液滴通过增强空化效应有效提高HIFU消融效果,延长消融灶转归所需时间。     

HIFU-induced cavitation and heating in ex vivo porcine subcutaneous fat

The present study is motivated by the fact that there are no published studies quantifying cavitation activity and heating induced by ultrasound in adipose tissue and that there are currently no reliable techniques for monitoring successful deposition of ultrasound energy in fat in real time. High-intensity focused ultrasound (HIFU) exposures were performed in excised porcine fat at four different frequencies (0.5, 1.1, 1.6 and 3.4 MHz) over a range of pressure amplitudes and exposure durations. The transmission losses arising from reflection at the skin interface and attenuation through skin and fat were quantified at all frequencies using an embedded needle hydrophone. A 15 MHz passive cavitation detector (PCD) coaxial to the HIFU transducer was used to capture acoustic emissions emanating from the focus during HIFU exposures, while the focal temperature rise was measured using minimally invasive needle thermocouples. Repeatable temperature rises in excess of 10°C could be readily instigated across all four frequencies for acoustic intensities (Ispta) in excess of 50 W/cm² within the first 2 s of exposure. Even though cavitation could not be initiated at 1.1, 1.6 and 3.4 MHz over the in situ peak rarefactional (p_-) pressure range 0-3 MPa explored in the present study, inertial cavitation activity was always initiated at 0.5 MHz for pressures greater than 1.6 MPa (p_-) and was found to enhance focal heat deposition. A good correlation was identified between the energy of broadband emissions detected by the PCD and the focal temperature rise at 0.5 MHz, particularly for short 2 s exposures, which could be exploited as a tool for noninvasive monitoring of successful treatment delivery. (E-mail: zoe.kyriakou@eng.ox.ac.uk)     

生物学焦域的概念及在高强度聚焦超声切除组织中的作用

本实验研究和比较了高强度聚焦超声(HIFU)所致体内深部组织凝固性坏死灶的形状和大小.结果显示参照同样的治疗参数,HIFU所致凝固性坏死灶的大小和形状在体内和体外有较大的差异.体内体积与治疗参数、靶组织的结构、功能和运动有密切关系.与物理学焦域比较,凝固性坏死灶体积亦可能命名为HIFU的生物学焦域.     

载IR780/PFH纳米粒双模态成像及增效高强度聚焦超声消融的体外实验研究

摘 要 目的 制备一种近红外荧光多功能纳米粒,评估其体外超声、光声成像能力以及协同高强度聚焦超声（HIFU）消融牛肝的效果,通过空化检测探讨其增效HIFU的机制。方法 采用超声薄膜法制备载 IR780和全氟己烷（PFH）的脂质体纳米粒(IR780-CLs),使用透射电镜观察纳米粒的结构特征,多角度粒度分析及Zeta电位仪检测纳米粒的粒径和电位,紫外分光光度计检测IR780包封率,观察其体外超声、光声成像效果,通过离体牛肝实验评估IR780-CLs体外增效HIFU的作用,并通过被动空化检测,取宽带噪声信号幅度的均方根值（RMS）随时间的变化曲线反应实时空化信号。结果 制备的IR780-CLs纳米粒呈大小均匀的球形,平均电位(34.5 ± 3.2) mV,粒径(246.5 ± 12.4) nm,IR780包封率为88%,IR780-CLs纳米粒具有良好的体外超声、光声成像效果,纳米粒光声信号强度与浓度间存在良好的线性关系,体外牛肝实验表明其具有显著增效HIFU的作用,被动空化检测进一步验证了IR780-CLs组空化信号RMS值高于PBS组。结论 本实验成功制备多功能纳米粒IR780-CLs,具有良好的超声、光声成像能力,并能显著提高HIFU消融牛肝的效率,空化效应为增效HIFU的主要机制。     

Correlation between inertial cavitation dose and endothelial cell damage in vivo

Previous in vivo studies have demonstrated that vascular endothelial damage can result when vessels containing gas-based microbubble ultrasound contrast agent (UCA) are exposed to MHz-frequency pulsed ultrasound (US) of sufficient pressure amplitudes, presumably as a result of inertial cavitation (IC). The hypothesis guiding this research was that IC is the primary mechanism by which the vascular endothelium (VE) is damaged when a vessel is exposed to pulsed 1-MHz frequency US in the presence of circulating UCA. The expectation was that a correlation should exist between the magnitude and duration of IC activity and the degree of VE damage. Rabbit auricular vessels were exposed in vivo to 1.17-MHz focused US of variable peak rarefaction pressure amplitude (1, 3, 6.5 or 9 MPa), using low duty factors (0.04% or 0.4%), pulse lengths of 500 or 5000 cycles, with varying treatment durations and with or without infusion of a shelled microbubble contrast agent. A broadband passive cavitation detection system was used to measure IC activity in vivo within the targeted segment of the blood vessel. The magnitude of the detected IC activity was quantified using a previously reported measure of IC dose. Endothelial damage was assessed via scanning electron microscopy image analysis. The results supported the hypothesis and demonstrate that the magnitude of the measured IC dose correlates with the degree of VE damage when UCA is present. These results have implications for therapeutic US-induced vascular occlusion.     

Synergistic Ablation of Liver Tissue and Liver Cancer Cells with High-Intensity Focused Ultrasound and Ethanol

We investigated the combined effect of ethanol and high-intensity focused ultrasound (HIFU), first, on heating and cavitation bubble activity in tissue-mimicking phantoms and porcine liver tissues and, second, on the viability of HepG2 liver cancer cells. Phantoms or porcine tissues were injected with ethanol and then subjected to HIFU at acoustic power ranging from 1.2 to 20.5 W (HIFU levels 1–7). Cavitation events and the temperature around the focal zone were measured with a passive cavitation detector and embedded type K thermocouples, respectively. HepG2 cells were subjected to 4% ethanol solution in growth medium (v/v) just before the cells were exposed to HIFU at 2.7, 8.7 or 12.0 W for 30 s. Cell viability was measured 2, 24 and 72 h post-treatment. The results indicate that ethanol and HIFU have a synergistic effect on liver cancer ablation as manifested by greater temperature rise and lesion volume in liver tissues and reduced viability of liver cancer cells. This effect is likely caused by reduction of the cavitation threshold in the presence of ethanol and the increased rate of ethanol diffusion through the cell membrane caused by HIFU-induced streaming, sonoporation and heating.