Contrast-enhanced ultrasound assessment of tissue response to high-intensity focused ultrasound

Combination of HIFU with sulfur hexafluoride microbubbles in the treatment of solitary uterine fibroids: a systematic review and meta-analysis

OBJECTIVES

To assess the efficacy and safety of sulfur hexafluoride microbubbles on ultrasound-guided high-intensity focused ultrasound (HIFU) ablation of uterine fibroids.

METHODS

Studies that compared HIFU-microbubble combination with HIFU-only in patients with uterine fibroids were searched from inception to April 2022. The standardized mean difference (SMD) or relative risk (RR) with 95% confidence interval (CI) for different outcome parameters was calculated.

RESULTS

Seven studies were included, with a total of 901 patients (519 in the combination group and 382 in the HIFU-only group). The energy consumption for treating 1 cm3 of the lesion in the combination group was less than that in the HIFU-only group [SMD =  - 2.19, 95%CI (- 3.81, - 0.57), p = 0.008]. The use of microbubbles was associated with shortening the duration of the treatment and sonication [SMD =  - 2.60, 95%CI (- 4.09, - 1.10), p = 0.0007; SMD =  - 2.11, 95%CI (- 3.30, - 0.92), p = 0.0005]. The rates of significant greyscale changes during HIFU were greater in the combination group, as well as the increase of non-perfused volume ratio [RR = 1.26, 95%CI (1.04, 1.54), p = 0.02; SMD = 0.32, 95%CI (0.03, 0.61), p = 0.03]. The average sonication durations to reach significant greyscale changes and for ablating 1 cm3 of the fibroid lesion were shorter in the combination group [SMD =  - 1.24, 95%CI (- 2.02, - 0.45), p = 0.002; SMD =  - 0.22, 95%CI (- 0.42, - 0.02), p = 0.03]. The two groups had similar post-HIFU adverse effects, while the combination group had fewer intraprocedural adverse events like abdominal pain, sacrum pain, and leg pain.

CONCLUSIONS

Sulfur hexafluoride microbubbles can be safely used to enhance and accelerate the ablation effects of HIFU in the treatment of uterine fibroids.

CLINICAL RELEVANCE STATEMENT

The combination of HIFU with sulfur hexafluoride microbubbles offers a promising non-invasive treatment option for patients with uterine fibroids.

KEY POINTS

• Sulfur hexafluoride microbubbles combined with ultrasound-guided high-intensity focused ultrasound (USgHIFU) has potential advantages in the treatment of uterine fibroids. • Sulfur hexafluoride microbubbles not only enhance the effects of USgHIFU treatment for uterine fibroids but also shorten its duration. • Sulfur hexafluoride microbubbles do not increase the incidence of USgHIFU-related adverse events in the treatment of uterine fibroids.

Artificial intelligence-assisted ultrasound-guided focused ultrasound therapy: a feasibility study

OBJECTIVES

Focused ultrasound (FUS) therapy has emerged as a promising noninvasive solution for tumor ablation. Accurate monitoring and guidance of ultrasound energy is crucial for effective FUS treatment. Although ultrasound (US) imaging is a well-suited modality for FUS monitoring, US-guided FUS (USgFUS) faces challenges in achieving precise monitoring, leading to unpredictable ablation shapes and a lack of quantitative monitoring. The demand for precise FUS monitoring heightens when complete tumor ablation involves controlling multiple sonication procedures.

METHODS

To address these challenges, we propose an artificial intelligence (AI)-assisted USgFUS framework, incorporating an AI segmentation model with B-mode ultrasound imaging. This method labels the ablated regions distinguished by the hyperechogenicity effect, potentially bolstering FUS guidance. We evaluated our proposed method using the Swin-Unet AI architecture, conducting experiments with a USgFUS setup on chicken breast tissue.

RESULTS

Our results showed a 93% accuracy in identifying ablated areas marked by the hyperechogenicity effect in B-mode imaging.

CONCLUSION

Our findings suggest that AI-assisted ultrasound monitoring can significantly improve the precision and control of FUS treatments, suggesting a crucial advancement toward the development of more effective FUS treatment strategies.

A Review of Imaging Methods to Assess Ultrasound-Mediated Ablation

Ultrasound ablation techniques are minimally invasive alternatives to surgical resection and have rapidly increased in use. The response of tissue to HIFU ablation differs based on the relative contributions of thermal and mechanical effects, which can be varied to achieve optimal ablation parameters for a given tissue type and location. In tumor ablation, similar to surgical resection, it is desirable to include a safety margin of ablated tissue around the entirety of the tumor. A factor in optimizing ablative techniques is minimizing the recurrence rate, which can be due to incomplete ablation of the target tissue. Further, combining focal ablation with immunotherapy is likely to be key for effective treatment of metastatic cancer, and therefore characterizing the impact of ablation on the tumor microenvironment will be important. Thus, visualization and quantification of the extent of ablation is an integral component of ablative procedures. The aim of this review article is to describe the radiological findings after ultrasound ablation across multiple imaging modalities. This review presents readers with a general overview of the current and emerging imaging methods to assess the efficacy of ultrasound ablative treatments.

Methods of monitoring thermal ablation of soft tissue tumors - A comprehensive review

Thermal ablation is a form of hyperthermia in which oncologic control can be achieved by briefly inducing elevated temperatures, typically in the range 50-80°C, within a target tissue. Ablation modalities include high intensity focused ultrasound, radiofrequency ablation, microwave ablation, and laser interstitial thermal therapy which are all capable of generating confined zones of tissue destruction, resulting in fewer complications than conventional cancer therapies. Oncologic control is contingent upon achieving predefined coagulation zones; therefore, intraoperative assessment of treatment progress is highly desirable. Consequently, there is a growing interest in the development of ablation monitoring modalities. The first section of this review presents the mechanism of action and common applications of the primary ablation modalities. The following section outlines the state-of-the-art in thermal dosimetry which includes interstitial thermal probes and radiologic imaging. Both the physical mechanism of measurement and clinical or pre-clinical performance are discussed for each ablation modality. Thermal dosimetry must be coupled with a thermal damage model as outlined in Section 4. These models estimate cell death based on temperature-time history and are inherently tissue specific. In the absence of a reliable thermal model, the utility of thermal monitoring is greatly reduced. The final section of this review paper covers technologies that have been developed to directly assess tissue conditions. These approaches include visualization of non-perfused tissue with contrast-enhanced imaging, assessment of tissue mechanical properties using ultrasound and magnetic resonance elastography, and finally interrogation of tissue optical properties with interstitial probes. In summary, monitoring thermal ablation is critical for consistent clinical success and many promising technologies are under development but an optimal solution has yet to achieve widespread adoption.

Oxford's clinical experience in the development of high intensity focused ultrasound therapy

High Intensity Focused Ultrasound (HIFU) capably bridges the disciplines of surgery, oncology and biomedical engineering science. It provides the precision associated with a surgical tool whilst remaining a truly non-invasive technique. Oxford has been a centre for both clinical and preclinical research in HIFU over the last twenty years. Research into this technology in the UK has a longer history, with much of the early research being carried out by Professor Gail ter Haar and her team at the Institute of Cancer Research at Sutton in Surrey. A broad range of potential applications have been explored extending from tissue ablation to novel drug delivery. This review presents Oxford's clinical studies and applications for the development of this non-invasive therapy. This includes treatment of solid abdominal tumours comprising those of the liver, kidney, uterus, pancreas, pelvis and prostate. It also briefly introduces preclinical and translational works that are currently being undertaken at the Institute of Biomedical Engineering, University of Oxford. The safety, wide tolerability and effectiveness of this technology is comprehensively demonstrated across these studies. These results can facilitate the incorporation of HIFU as a key clinical management strategy.

Clinical significance of performing Sonazoid-based contrast-enhanced ultrasonography before ablation of uterine fibroids by high-intensity focused ultrasound: A preliminary cohort study

High-intensity focused ultrasound (HIFU) is effective for the ablation of uterine fibroids. However, no research has indicated whether HIFU ablation of uterine fibroids might be improved by application of contrast-enhanced ultrasonography (CEUS) with Sonazoid as a contrast agent. This study aimed to assess the clinical significance of Sonazoid-based CEUS 30 minute before HIFU ablation of uterine fibroids.This retrospective cohort study included Asian patients with solitary uterine fibroids who were treated with HIFU at Seoul HICARE Clinic (South Korea; n = 34) and the Second Affiliated Hospital of Chongqing Medical University (China; n = 30) between August 1, 2017, and October 31, 2017. The patients in Seoul underwent Sonazoid-based CEUS 30 minute before HIFU. All the patients received contrast-enhanced magnetic resonance imaging to diagnose uterine fibroids. The ablation results were evaluated 1 day after HIFU by contrast-enhanced magnetic resonance imaging or Sonazoid-based CEUS.All the patients were successfully treated with HIFU. The CEUS+HIFU group had lower values for sonication power, treatment time, sonication time, total energy applied, and energy efficiency factor compared with HIFU alone group (P < .001). There were no major adverse events after ablation therapy in either group. The incidence of post-procedure sacrococcygeal pain was lower in the CEUS+HIFU group than that in the HIFU alone group (P = .045), while the incidences of all other intraoperative and postoperative adverse events were similar between the 2 groups.Our findings suggest that Sonazoid-based CEUS before HIFU may enhance the ablation of uterine fibroids.

Durability study of a gellan gum-based tissue-mimicking phantom for ultrasonic thermal therapy

Stability and duration of ultrasonic phantoms are still subjects of research. This work presents a tissue-mimicking material (TMM) to evaluate high-intensity therapeutic ultrasound (HITU) devices, composed of gellan gum (matrix), microparticles (scatterers), and chemicals. The ultrasonic velocity and attenuation coefficient were characterized as a function of temperature (range 20 °C-85 °C). The nonlinear parameter B/A was determined by the finite amplitude insertion substitution (FAIS) method, and the shear modulus was determined by a transient elastography technique. The thermal conductivity and specific heat were determined by the line source method. The attenuation was stable for 60 days, and in an almost linear frequency dependence (0.51f^0.96dB cm^-1), at 20 °C (1-10 MHz). All other evaluated physical parameters are also close to typical soft tissue values. Longitudinal ultrasonic velocities were between 1.49 and 1.75 mm μs^-1, the B/A parameter was 7.8 at 30 °C, and Young's modulus was 23.4 kPa. The thermal conductivity and specific heat values were 0.7 W(m K)^-1 and 4.7 kJ(kg K)^-1, respectively. Consistent temperature increases and thermal doses occurred under identical HITU exposures. Low cost, longevity, thermal stability, and thermal repeatability make TMM an excellent material for ultrasonic thermal applications. The TMM developed has the potential to assess the efficacy of hyperthermia devices and could be used to adjust the ultrasonic emission of HITU devices.

Image-guided ultrasound phased arrays are a disruptive technology for non-invasive therapy

Focused ultrasound offers a non-invasive way of depositing acoustic energy deep into the body, which can be harnessed for a broad spectrum of therapeutic purposes, including tissue ablation, the targeting of therapeutic agents, and stem cell delivery. Phased array transducers enable electronic control over the beam geometry and direction, and can be tailored to provide optimal energy deposition patterns for a given therapeutic application. Their use in combination with modern medical imaging for therapy guidance allows precise targeting, online monitoring, and post-treatment evaluation of the ultrasound-mediated bioeffects. In the past there have been some technical obstacles hindering the construction of large aperture, high-power, densely-populated phased arrays and, as a result, they have not been fully exploited for therapy delivery to date. However, recent research has made the construction of such arrays feasible, and it is expected that their continued development will both greatly improve the safety and efficacy of existing ultrasound therapies as well as enable treatments that are not currently possible with existing technology. This review will summarize the basic principles, current statures, and future potential of image-guided ultrasound phased arrays for therapy.

Intraprocedure contrast enhanced ultrasound: the value in assessing the effect of ultrasound-guided high intensity focused ultrasound ablation for uterine fibroids

PURPOSE

To investigate the value of microbubble contrast-enhanced ultrasound (CEUS) in evaluating the treatment response of uterine fibroids to HIFU ablation.

MATERIALS AND METHODS

Sixty-eight patients with a solitary uterine fibroid from the First Affiliated Hospital of Chongqing Medical University were included and analyzed. All patients underwent pre- and post-treatment magnetic resonance imaging (MRI) with a standardized protocol, as well as pre-evaluation, intraprocedure, and immediate post-treatment CEUS. CEUS and MRI were compared by different radiologists.

RESULTS

In comparison with MRI, CEUS showed that the size of fibroids, volume of fibroids, size of non-perfused regions, non-perfused volume (NPV) or fractional ablation (NPV ratio) was similar to that of MRI. In terms of CEUS examination results, the median volume of fibroids was 75.2 (interquartile range, 34.2-127.3) cm(3), the median non-perfused volume was 54.9 (interquartile range, 28.0-98.1) cm(3), the mean fractional ablation was 83.7±13.6 (range, 30.0-100.0)%. In terms of MRI examination results, the median volume of fibroids was 74.1 (interquartile range, 33.4-116.2) cm(3). On the basis of contrast-enhanced T1-weighted images immediately after HIFU treatment, the median non-perfused volume was 58.5 (interquartile range, 27.7-100.0) cm(3), the average fractional ablation was 84.2±14.2 (range, 40.0-100.0)%.

CONCLUSIONS

CEUS clearly showed the size of fibroids and the non-perfused areas of the fibroid. Results from CEUS correlated well with results obtained from MRI.

Ultrasound-guided therapeutic focused ultrasound: current status and future directions

This paper reviews ultrasound imaging methods for the guidance of therapeutic focused ultrasound (USgFUS), with emphasis on real-time preclinical methods. Guidance is interpreted in the broadest sense to include pretreatment planning, siting of the FUS focus, real-time monitoring of FUS-tissue interactions, and real-time control of exposure and damage assessment. The paper begins with an overview and brief historical background of the early methods used for monitoring FUS-tissue interactions. Current imaging methods are described, and discussed in terms of sensitivity and specificity of the localisation of the FUS effects in both therapeutic and sub-therapeutic modes. Thermal and non-thermal effects are considered. These include cavitation-enhanced heating, tissue water boiling and cavitation. Where appropriate, USgFUS methods are compared with similar methods implemented using other guidance modalities, e.g. magnetic resonance imaging. Conclusions are drawn regarding the clinical potential of the various guidance methods, and the feasibility and current status of real-time implementation.

Quantitative ultrasound imaging for monitoring in situ high-intensity focused ultrasound exposure

Quantitative ultrasound (QUS) imaging is hypothesized to map temperature elevations induced in tissue with high spatial and temporal resolution. To test this hypothesis, QUS techniques were examined to monitor high-intensity focused ultrasound (HIFU) exposure of tissue. In situ experiments were conducted on mammary adenocarcinoma tumors grown in rats and lesions were formed using a HIFU system. A thermocouple was inserted into the tumor to provide estimates of temperature at one location. Backscattered time-domain waveforms from the tissue during exposure were recorded using a clinical ultrasonic imaging system. Backscatter coefficients were estimated using a reference phantom technique. Two parameters were estimated from the backscatter coefficient (effective scatterer diameter (ESD) and effective acoustic concentration (EAC). The changes in the average parameters in the regions corresponding to the HIFU focus over time were correlated to the temperature readings from the thermocouple. The changes in the EAC parameter were consistently correlated to temperature during both heating and cooling of the tumors. The changes in the ESD did not have a consistent trend with temperature. The mean ESD and EAC before exposure were 120 ± 16 μm and 32 ± 3 dB/cm3, respectively, and changed to 144 ± 9 μm and 51 ± 7 dB/cm3, respectively, just before the last HIFU pulse was delivered to the tissue. After the tissue cooled down to 37 °C, the mean ESD and EAC were 126 ± 8 μm and 35 ± 4 dB/cm3, respectively. Peak temperature in the range of 50-60 °C was recorded by a thermocouple placed just behind the tumor. These results suggest that QUS techniques have the potential to be used for non-invasive monitoring of HIFU exposure.

Potential enhancement of intravenous nano-hydroxyapatite in high-intensity focused ultrasound ablation for treating hepatocellular carcinoma in a rabbit model

The aim of the present study was to evaluate the safety and efficiency of an intravenously delivered nano-hydroxyapatite (Nano-HA) solution into a rabbit model (Oryctolagus cuniculus) to determine the potential enhancement of high-intensity focused ultrasound (HIFU) for the ablation of hepatocellular carcinoma (HCC) in liver tissue. The present study clearly indicated that the intravenous delivery of large quantities of Nano-HA into the body of the rabbit model over relatively short periods of time may be absorbed by the hepatic reticuloendothelial system. Subsequent HIFU treatment for HCC, as well as intravenous Nano-HA, produced a rapid increase in temperature and an enlargement of the coagulated necrotic area during ablation in the in vivo and ex vivo environments. In addition, it was found that the therapeutic doses of Nano-HA produced mild and transient abnormalities in the normal renal function and hepatic enzymes during the first 24 h following administration. The results of the current study indicated that the combination of Nano-HA and HIFU may provide a safe and effective alternative to conventional surgical procedures.

Enhancing ablation effects of a microbubble-enhancing contrast agent ("SonoVue") in the treatment of uterine fibroids with high-intensity focused ultrasound: a randomized controlled trial

PURPOSE

To evaluate the role of the ultrasound contrast agent SonoVue in enhancing the ablation effects of ultrasound-guided high-intensity focused ultrasound (HIFU) on uterine fibroids.

METHODS

Eighty patients with solitary uterine fibroids at a single center were randomly assigned to a control or SonoVue group. Of these, 40 were treated using HIFU alone; 40 who were pretreated with SonoVue received a bolus before the HIFU procedure. All patients underwent magnetic resonance imaging (MRI) scan before and after HIFU treatment.

RESULTS

The post-HIFU MRI showed the nonperfused volume (NPV) in all of the treated uterine fibroids; the mean fractional ablation (NPV ratio) was 90.4 ± 8.3 % (range 66.4-100 %) in the SonoVue group and 82.8 ± 13.3 % (range 53.4-100 %) in the control group. The frequency of massive gray-scale changes that occurred during HIFU was greater in the group that received SonoVue than the group that did not. The average sonication time to reach massive gray-scale changes was significantly shorter in the group receiving SonoVue than the group without SonoVue. The acoustic energy for treating 1 mm(3) of uterine fibroid was less in the SonoVue group than the control group. No any major complication occurred in this study.

CONCLUSION

Based on the results of this randomized controlled trial, SonoVue could be safely used to enhance the effects of HIFU treatment for uterine fibroids.

Current status and prospects for microbubbles in ultrasound theranostics

Encapsulated microbubbles have been developed over the past two decades to provide improvements both in imaging as well as new therapeutic applications. Microbubble contrast agents are used currently for clinical imaging where increased sensitivity to blood flow is required, such as echocardiography. These compressible spheres oscillate in an acoustic field, producing nonlinear responses which can be uniquely distinguished from surrounding tissue, resulting in substantial enhancements in imaging signal-to-noise ratio. Furthermore, with sufficient acoustic energy the oscillation of microbubbles can mediate localized biological effects in tissue including the enhancement of membrane permeability or increased thermal energy deposition. Structurally, microbubbles are comprised of two principal components--an encapsulating shell and an inner gas core. This configuration enables microbubbles to be loaded with drugs or genes for additional therapeutic effect. Application of sufficient ultrasound energy can release this payload, resulting in site-specific delivery. Extensive preclinical studies illustrate that combining microbubbles and ultrasound can result in enhanced drug delivery or gene expression at spatially selective sites. Thus, microbbubles can be used for imaging, for therapy, or for both simultaneously. In this sense, microbubbles combined with acoustics may be one of the most universal theranostic tools.

HIFU for palliative treatment of pancreatic cancer

High intensity focused ultrasound (HIFU) is a novel non-invasive modality for ablation of various solid tumors including uterine fibroids, prostate cancer, hepatic, renal, breast and pancreatic tumors. HIFU therapy utilizes mechanical energy in the form of a powerful ultrasound wave that is focused inside the body to induce thermal and/or mechanical effects in tissue. Multiple preclinical and non-randomized clinical trials have been performed to evaluate the safety and efficacy of HIFU for palliative treatment of pancreatic tumors. Substantial tumor-related pain reduction was achieved in most cases after HIFU treatment, and no significant side-effects were observed. This review provides a description of different physical mechanisms underlying HIFU therapy, summarizes the clinical experience obtained to date in HIFU treatment of pancreatic tumors, and discusses the challenges, limitations and new approaches in this modality.

Development and characterization of a tissue-mimicking material for high-intensity focused ultrasound

A tissue-mimicking material (TMM) for the acoustic and thermal characterization of high-intensity focused ultrasound (HIFU) devices has been developed. The material is a high-temperature hydrogel matrix (gellan gum) combined with different sizes of aluminum oxide particles and other chemicals. The ultrasonic properties (attenuation coefficient, speed of sound, acoustical impedance, and the thermal conductivity and diffusivity) were characterized as a function of temperature from 20 to 70°C. The backscatter coefficient and nonlinearity parameter B/A were measured at room temperature. Importantly, the attenuation coefficient has essentially linear frequency dependence, as is the case for most mammalian tissues at 37°C. The mean value is 0.64f(0.95) dB·cm(-1) at 20°C, based on measurements from 2 to 8 MHz. Most of the other relevant physical parameters are also close to the reported values, although backscatter signals are low compared with typical human soft tissues. Repeatable and consistent temperature elevations of 40°C were produced under 20-s HIFU exposures in the TMM. This TMM is appropriate for developing standardized dosimetry techniques, validating numerical models, and determining the safety and efficacy of HIFU devices.

Effect of microbubble contrast agent during high intensity focused ultrasound ablation on rabbit liver in vivo

OBJECTIVE

To evaluate the effect of a microbubble contrast agent (SonoVue) during HIFU ablation of a rabbit liver.

MATERIALS AND METHODS

HIFU ablations (intensity of 400W/cm(2) for 4s, six times, with a 5s interval between exposures) were performed upon 16 in vivo rabbit livers before and after intravenous injection of a microbubble contrast agent (0.8ml). A Wilcoxon signed rank test was used to compare mean ablation volume and time required to tissue ablation on real-time US. Shape of ablation and pattern of coagulative necrosis were analyzed by Fisher's exact test.

RESULTS

The volume of coagulative necrosis was significantly larger in the combination microbubble and HIFU group than in the HIFU alone group (P<0.05). Also, time to reach ablation was shorter in the combination microbubble and HIFU group than in the HIFU alone group (P<0.05). When analyzing the shape of tissue ablation, a pyramidal shape was more prevalently in the HIFU alone group compared to the combination microbubble and HIFU group (P<0.05). Following an analysis of the pattern of coagulative necrosis, non-cavitary necrosis was found in ten and cavitary necrosis in six of the samples in the combination microbubble and HIFU group. Conversely, non-cavitary necrosis occurred in all 16 samples in the HIFU alone group (P<0.05).

CONCLUSION

HIFU of in vivo rabbit livers with a microbubble contrast agent produced larger zones of ablation and more cavitary tissue necrosis than without the use of a microbubble contrast agent. Microbubble contrast agents may be useful in tissue ablation by enhancing the treatment effect of HIFU.

A rabbit bone tumor model for high-intensity focused ultrasound therapy

The incidence of malignant bone tumor increases every year. Because the application of high-intensity focused ultrasound (HIFU) for the treatment of bone tumors is still at a nascent stage, it is essential to study the effectiveness of this technique in ideal animal models in order to obtain a beneficial reference for imaging studies. In this study, we established a VX2 malignant bone tumor model and evaluated this model by contrast-enhanced sonography and magnetic resonance imaging (MRI). The results show that all tumors were enhanced after injection of SonoVue. A contrast-enhanced MRI scan revealed obvious enhancement within the tumors. Histological examination revealed the presence of a large number of tumor cells. The model can serve as an ideal experimental model for the study of HIFU therapy in the treatment of malignant bone tumors and as a reference for imaging studies during follow-ups.

Egg white as a blood coagulation surrogate

Egg white, a protein-containing solution, is characterized as a blood coagulation surrogate for the acoustical and thermal evaluation of therapeutic ultrasound, especially high intensity focused ultrasound (HIFU) devices. Physical properties, including coagulation temperature, frequency dependent attenuation, sound speed, viscosity, and thermal properties, were measured as a function of temperature (20-95 degrees C). Thermal coagulation and attenuation (5-12 and 1 MHz) of cow blood, pig blood, and human blood also were assessed and compared with egg white. For a 30 s thermal exposure, both egg white and blood samples (3 mm thickness) started to denature at 65 degrees C and coagulate into an elastic gel at 85 degrees C. The attenuation of egg white was found to be similar to that of the blood samples, having values of 0.23f(1.09), 1.58f(0.61), and 2.7f(0.5) dB/cm at 20, 75, and 95 degrees C, respectively. This significant attenuation increase with temperature was determined to be caused mainly by bubble cavity formation. The other temperature-dependent parameters are also similar to the reported values for blood. These properties make egg white a potentially useful bench testing tool for the safety and efficacy evaluation of therapeutic ultrasound devices.

High-intensity focused ultrasound induced apoptosis with caspase 3, 8, and 9/6 activation in rat hepatoma

PURPOSE

The purpose of the present study is to investigate anticancer efficacy and apoptosis confirmed by caspase under several exposure conditions of high-intensity focused ultrasound (HIFU).

MATERIALS AND METHODS

Twenty-five rats with KDH-8 hepatoma were treated by HIFU at several acoustic energies to evaluate treatment efficacy. Apoptosis was examined by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) and Hoechst 33258 staining, and caspase 3, 8, and 9/6 activity was respectively assayed.

RESULTS

The KDH-8 subcutaneous tumors were reduced by HIFU, and these rats survived longer than the nontreatment rats (P < 0.01). The minimal threshold of HIFU energy was 30 W × 1.0 s for tumor control and long-term survival. The tumors exposed to HIFU exhibited marked apoptotic features under conditions of less than 10 W × 1.0 s. In cultured KDH-8 cells, apoptosis was caused at less than 30 W × 1.0 s (P < 0.01), and more was induced as the energy went down. Caspase 3, 8, and 9/6 were more activated at low energy under 10 W × 1.0 s (P < 0.01), and caspase 8, which is death receptor dependent, was significantly more activated than caspase 9/6, which is mitochondria dependent (P < 0.01).

CONCLUSION

HIFU-induced apoptosis in vivo and in vitro is one of the mechanisms for tumor control and is mediated by caspase 3, 8, and 9/6. The significantly greater activation of caspase 8 than of caspase 9/6 suggests that the apoptosis pathway induced by HIFU might be more mitochondria dependent than death receptor dependent. However, further examination will be needed.

PET Imaging of Angiogenesis

Angiogenesis is a highly-controlled process that is dependent on the intricate balance of both promoting and inhibiting factors, involved in various physiological and pathological processes. A comprehensive understanding of the molecular mechanisms that regulate angiogenesis has resulted in the design of new and more effective therapeutic strategies. Due to insufficient sensitivity to detect therapeutic effects by using standard clinical endpoints or by looking for physiological improvement, a multitude of imaging techniques have been developed to assess tissue vasculature on the structural, functional and molecular level. Imaging is expected to provide a novel approach to noninvasively monitor angiogenesis, to optimize the dose of new antiangiogenic agents and to assess the efficacy of therapies directed at modulation of the angiogenic process. All these methods have been successfully used preclinically and will hopefully aid in antiangiogenic drug development in animal studies. In this review article, the application of PET in angiogenesis imaging at both functional and molecular level will be discussed. For PET imaging of angiogenesis related molecular markers, we emphasize integrin alpha(v)beta(3), VEGF/VEGFR, and MMPs.

High-intensity focused ultrasound in small renal masses

High-intensity focused ultrasound (HIFU) competes with radiofrequency and cryotherapy for the treatment of small renal masses as a third option among ablative approaches. As an emerging technique, its possible percutaneous or laparoscopic application, low discomfort to the patient and the absence of complications make this technology attractive for the management of small renal masses. This manuscript will focus on the principles, basic research and clinical applications of HIFU in small renal masses, reviewing the present literature. Therapeutic results are controversial and from an clinical view, HIFU must be considered a technique under investigation at present time. Further research is needed to settle its real indications in the management of small renal masses; maybe technical improvements will certainly facilitate its use in the management of small renal masses in the near future.

Development and characterization of a blood mimicking fluid for high intensity focused ultrasound

A blood mimicking fluid (BMF) has been developed for the acoustic and thermal characterizations of high intensity focused ultrasound (HIFU) ablation devices. The BMF is based on a degassed and de-ionized water solution dispersed with low density polyethylene microspheres, nylon particles, gellan gum, and glycerol. A broad range of physical parameters, including attenuation coefficient, speed of sound, viscosity, thermal conductivity, and diffusivity, were characterized as a function of temperature (20-70 degrees C). The nonlinear parameter B/A and backscatter coefficient were also measured at room temperature. Importantly, the attenuation coefficient is linearly proportional to the frequency (2-8 MHz) with a slope of about 0.2 dB cm(-1) MHz(-1) in the 20-70 degrees C range as in the case of human blood. Furthermore, sound speed and bloodlike backscattering indicate the usefulness of the BMF for ultrasound flow imaging and ultrasound-guided HIFU applications. Most of the other temperature-dependent physical parameters are also close to the reported values in human blood. These properties make it a unique HIFU research tool for developing standardized exposimetry techniques, validating numerical models, and determining the safety and efficacy of HIFU ablation devices.

Hyperecho as the indicator of tissue necrosis during microbubble-assisted high intensity focused ultrasound: sensitivity, specificity and predictive value

The purpose of this study was to determine the sensitivity, specificity and predictive values of hyperecho in grayscale ultrasonic images as the indicator of tissue necrosis in microbubble-assisted high-intensity focused ultrasound (HIFU) exposure in vivo. Livers and kidneys of the rabbit were exposed to HIFU (control group) or microbubble-assisted HIFU (experimental group); a continuous line of ablation, viz. linear scan, was performed to destruct tissues. Tissue responses were evaluated macroscopically and microscopically 24 h after HIFU. The cases of positive (hyperecho occurred and tissue necrotized), false positive (tissue was unaffected although hyperecho appeared), negative (echo was not changed and tissue was intact) and false negative (tissue was destructed despite the lack of hyperecho) were counted, and then the sensitivity, specificity and positive and negative predictive values of hyperecho were calculated. The sensitivity, specificity, positive predictive value and negative predictive value were 49.25% vs. 79.63% (p < 0.001), 45.45% vs. 30.00%, 84.62% vs. 86.00% and 12.82% vs. 21.43% for liver and 76.06% vs. 81.25%, 26.53% vs. 41.67%, 60.00% vs. 82.28% (p = 0.002) and 43.33% vs. 40.00% for kidney, in control and experimental groups, respectively. Rates varied between tissue types in control group. These findings indicated that the use of microbubble during HIFU improved the sensitivity in liver and the positive predictive value in kidney. The specificity and negative predictive value were poor. Hyperecho could only be used as the indicator of tissue necrosis in some tissue types.

Advances in anatomic, functional, and molecular imaging of angiogenesis

Angiogenesis is a fundamental process in various physiologic and pathologic processes. The ability to visualize and quantify angiogenesis will allow early diagnosis and monitoring for clinical determination of angiogenesis states before, during, and after adjuvant antiangiogenic and therapeutic angiogenesis treatments.

High-intensity focused ultrasound: current potential and oncologic applications

OBJECTIVE

The objective of this article is to introduce the reader to the principles and applications of high-intensity focused ultrasound (HIFU).

CONCLUSION

Although a great deal about HIFU physics is understood, its clinical applications are currently limited, and multiple trials are underway worldwide to determine its efficacy.

Short and long term efficacy of high intensity focused ultrasound therapy for advanced hepatocellular carcinoma

BACKGROUND

The aim of this study was to investigate the short and long term efficacy of high intensity focused ultrasound therapy (HIFU) in patients with advanced hepatocellular carcinoma (HCC).

METHODS

Patients with surgically unresectable HCC received either HIFU plus supportive treatment (HIFU group, n = 151) or supportive treatment only (control group, n = 30), according to their willingness. Short term efficacy, including improvement in tumor imaging parameters, decrease in serum alpha-fetoprotein (AFP) levels, symptom relief (i.e. Karnofsky Performance Status and numerical rating scales) and response rates, and long term efficacy, including an increase in survival rates and improvement of quality of life (QOL), was monitored.

RESULTS

Tumor imaging parameters, serum AFP levels and symptom scores improved significantly in the HIFU group compared with the control group (all P < 0.05). In the HIFU group, a complete and a partial response were achieved in 28.5% (n = 43) and 60.3% (n = 91) of cases, respectively, while the rates were 0% and 16.7% (n = 5), respectively, in the control group. The overall response rate (88.8%) was significantly greater in the HIFU group (16.7%) than in the control group (P < 0.01). In addition, the 1- and 2-year survival rates were 50.0% and 30.9%, respectively, in the HIFU group, which were significantly greater than those (3.4% and 0%, respectively) in the control group (both P < 0.01). The QOL score was 83.1 +/- 8.0 at 3 months after HIFU, which was significantly greater than the pre-HIFU score (67.7 +/- 5.9) and the score at 3 months after treatment (69.0 +/- 8.5) in the control group (both P < 0.05). No severe complications occurred during and after HIFU.

CONCLUSION

HIFU is an effective and safe ablation therapy with satisfactory short and long term efficacy for patients with advanced HCC.

Host antitumour immune responses to HIFU ablation

The ideal cancer therapy not only induces the death of all localized tumour cells without damage to surrounding normal tissue, but also activates a systemic antitumour immunity. High-intensity focused ultrasound (HIFU) has the potential to be such a treatment, as it can non-invasively ablate a targeted tumour below the skin surface, and may subsequently augment host antitumour immunity. In addition to thermal and cavitation effects, which act directly and locally on the tumour, there is increasing evidence linking systemic anti-tumour immune response to HIFU ablation. This may provide micro-metastatic control and long-term tumour resistance for cancer patients. The goal of this article is to review the emerging pre-clinical and clinical results suggesting that HIFU ablation may enhance host anti-tumour immunity, and to discuss its potential mechanisms. It is concluded that the systemic immune response induced by thermal ablation may play an important role in local recurrence and metastasis control after HIFU treatment.

Enhancing effects of SonoVue, a microbubble sonographic contrast agent, on high-intensity focused ultrasound ablation in rabbit livers in vivo

OBJECTIVE

The purpose of this study was to explore the enhancing biological effects of SonoVue (Bracco SpA, Milan, Italy), a sulfur hexafluoride sonographic contrast agent, on high-intensity focused ultrasound (HIFU) ablation in vivo.

METHODS

Forty-five rabbits were randomly divided into 3 groups and underwent laparotomy. Animals in group 1 were given injections of 0.2 mL of SonoVue intravenously; animals in group 2 were given physiologic saline; and those in group 3 were not given injections as control. The exposure time was set at 2 seconds with the acoustic power at 600 W. After HIFU ablations, volumes of coagulated regions were measured. Liver tissues were examined under light microscopy with hematoxylin-eosin staining and under transmission electron microscopy.

RESULTS

Coagulated volumes in group 1 (mean +/- SD, 2.41 +/- 0.44 cm(3)) were larger than those in group 2 (0.80 +/- 0.13 cm(3)) and group 3 (0.83 +/- 0.16 cm(3)) (P < .05). Separated from the surrounding area with a clear demarcation line, the targeted area in each group showed a few necrotic cells on light microscopy with the hematoxylin-eosin stain. More bubbles were observed under light microscopy in exposed areas in group 1 than in the other 2 groups (P < .05). Electron microscopy showed more severe cell ultrastructure disorder, including more interrupted cell nuclear membranes, in targeted areas in group 1 than in the other 2 groups. Conversely, in all the groups, untreated areas were not affected.

CONCLUSIONS

SonoVue can substantially enhance the ablation effects of HIFU, suggesting that microbubble contrast agents may be useful for improving HIFU efficiency.

Blood flow occlusion via ultrasound image-guided high-intensity focused ultrasound and its effect on tissue perfusion

This study investigated the induction of tissue necrosis by arterial blood flow occlusion using ultrasound image-guided high-intensity focused ultrasound (HIFU). We constructed a prototype HIFU transducer in combination with an imaging probe that provided color Doppler imaging and ultrasound contrast imaging. The HIFU beam was aimed into a branch of the renal artery in vivo. The renal artery branches of eight rabbits were occluded by HIFU at an intensity of 4 kW/cm(2) (from 2 to 10 times of each sonication for 5 s). When the HIFU exposure was successful, complete cessation of blood flow was observed by color Doppler imaging with success rate of 100% (8/8). Furthermore, lack of perfusion was observed in the renal cortex with a contrast-enhanced image. Postmortem histologic evaluation showed a wedge-shaped area of infarction in six of seven cases, corresponding to the lack of the contrast medium in the ultrasound image. These results demonstrated that ultrasound image-guided HIFU can be used to induce arterial occlusion, thus producing infarction and necrosis of the perfused tissue.

Efficient transfection of tumors facilitated by long-term therapeutic ultrasound in combination with contrast agent: from in vitro to in vivo setting

Therapeutic ultrasound (TUS) is a promising non-viral clinical approach for the delivery of genes. This study demonstrates the efficient delivery and localization of DNA in subcutaneous tumors facilitated by TUS application and examines the contribution of ultrasound contrast-agent (USCA) addition on transfection. The study addresses the importance of in vivo optimization when using long-term TUS and USCA based on data achieved in vitro. In vitro results showed that transfection of TrampC2 prostate cancer (Pca) cells using genes encoding for luciferase and green fluorescent protein was enhanced when DNA and Optison were added together and TUS was applied for 20 or 30 min. In vivo results showed that the highest transfection was achieved when Optison and DNA were co-injected intratumorally, and TUS was applied for 20 min. Using Optison significantly increased protein distribution in the tumor. However, in vivo expression level was decreased by two and four fold at 7 and 14 days, respectively, post-TUS. The study establishes the potential of intratumoral delivery of DNA-Optison, followed by TUS as an effective, non-toxic, gene delivery method that could provide a safe, clinical alternative to current viral gene delivery approaches where short-term gene expression is needed.

Monitoring imaging of lesions induced by high intensity focused ultrasound based on differential ultrasonic attenuation and integrated backscatter estimation

We investigated the feasibility of two monitoring imaging methods to visualize and evaluate the high intensity focused ultrasound (HIFU) induced lesions in vitro during and after their formation, which were based on differential ultrasonic parameter estimation. Firstly, ultrasonic attenuation slope of tissue sample was estimated based on the spectral analysis of ultrasound RF backscattered signals. The differential attenuation slope maps were acquired, which were interpreted as the differences between the pretreatment image and those obtained in different stages during HIFU therapy. Secondly, ultrasonic integrated backscatter (IBS), defined as the frequency average of the backscatter transfer function over the useful bandwidth, was proposed quantitatively to evaluate the extent of lesions with the same RF signals as the first method. Differential IBS maps were also acquired to visualize temporal evolution of lesion formation. It was found in pig liver in vitro that more precise definition of the treated area was obtained from the differential IBS images than from differential attenuation slope images. Dramatic increase in both attenuation and IBS value was observed during the therapy, which may be related to dramatic enhancement of cavitation due to boiling and accompanying tissue damage. Two methods to obtain one differential image were compared and the cumulative differential image was found to be able to eliminate noises and artifacts to some extent, which was the cumulation of a series of differential images acquired from the differences between the temporally adjacent RF data frames. Moreover, we presented a bidirectional color code for identification of the artifacts due to tissue movements caused by HIFU radiation force. We conclude that cumulative differential IBS images have the potential to monitor the formation of HIFU-induced lesions.

Therapeutic applications of ultrasound

Therapeutic applications of ultrasound predate its use in imaging. A range of biological effects can be induced by ultrasound, depending on the exposure levels used. At low levels, beneficial, reversible cellular effects may be produced, whereas at high intensities instantaneous cell death is sought. Therapy ultrasound can therefore be broadly divided into "low power" and "high power" applications. The "low power" group includes physiotherapy, fracture repair, sonophoresis, sonoporation and gene therapy, whereas the most common use of "high power" ultrasound in medicine is probably now high intensity focused ultrasound. Therapeutic effect through the intensity spectrum is obtained by both thermal and non-thermal interaction mechanisms. At low intensities, acoustic streaming is likely to be significant, but at higher levels, heating and acoustic cavitation will predominate. While useful therapeutic effects are now being demonstrated clinically, the mechanisms by which they occur are often not well understood.

Enhancement of ultrasound contrast agent in high-intensity focused ultrasound ablation

High-intensity focused ultrasound (HIFU) is becoming an increasingly attractive modality for ablation. Enhancement of HIFU is an important issue that has been discussed and investigated worldwide. Ultrasound contrast agents are considered to constitute an efficient medium for changing acoustic characteristics and improving energy deposition in the focal region. The role of microbubbles in inducing enhanced heating, cavitation, and other related events in HIFU ablation has been investigated, with the goal of improving coagulation necrosis volume or decreasing acoustic power and exposure duration. Consequently, with the use of ultrasound contrast agents, applications of HIFU are expected to become more efficient, safe, and accurate and to produce fewer adverse effects. This paper reviews studies that have been conducted to investigate the enhancement of ultrasound contrast agents in HIFU ablation through experiments that were carried out in vitro and in vivo; an analysis of results of this enhancement mechanism is provided.

Heat fixation of cancer cells ablated with high-intensity-focused ultrasound in patients with breast cancer

BACKGROUND

High-intensity-focused ultrasound (HIFU) is a noninvasive thermal ablation technique. This study reports the use of histological techniques for the pathological assessment of HIFU effects in patients with breast cancer.

METHODS

Twenty-three patients with biopsy-proven breast cancer underwent HIFU treatment for primary breast lesion. Mastectomy was performed on all patients after HIFU. By using histological examinations, the surgical specimens were assessed to explore HIFU effects on breast cancer.

RESULTS

Coagulation necrosis of targeted tumors was confirmed by microscopy in 23 patients. Tumor cells presented typical characteristics of coagulation necrosis in the peripheral region of the ablated tumor in all patients. However, in 11 of 23 patients, hematoxylin and eosin staining showed normal cellular structure in the central ablated tumor. By using electronic microscopy and nicotinamide adenine dinucleotide-diaphorase stain, those who had normal-appearing cancer cells were not viable.

CONCLUSIONS

HIFU can cause the heat fixation of ablated tumor through thermal effect.

Extracorporeal high intensity focused ultrasound in the treatment of patients with solid malignancy

The ideal treatment of localized cancer should cause the complete death of tumor cells without damage to surrounding normal tissue. High intensity focused ultrasound (HIFU) is such a potential treatment, which can induce complete coagulation necrosis of a targeted tumor, at depth, through the intact skin. The concept of using HIFU as a non-invasive therapy has attracted attention in medicine for 60 years. Recently, it has received increasing interest as a promising modality for the treatment of localized solid malignancies. The goal of this article is to introduce recent clinical developments in the use of extracorporeal HIFU ablation for solid tumors, including those of liver, breast, bone, kidney, pancreas, soft tissue, and uterus. It describes the physical principles and ablative mechanisms, three-dimensional therapeutic regimes, and medical imaging used in HIFU. Currently, large numbers of patients with solid malignancy are already treated using HIFU, and short-term clinical results are very encouraging. However, large-scale randomized clinical trials are necessary to evaluate long-term efficacy of HIFU treatment for solid malignancies. It is concluded that this non-invasive ablation can be considered as a conventional therapy for widespread clinical use only when the results from prospective, randomized clinical trials worldwide are available.

Combination of HIFU therapy with contrast-enhanced sonography for quantitative assessment of therapeutic efficiency on tumor grafted mice

The objective was to evaluate treatment efficiency of a new high-intensity focused ultrasound (HIFU) prototype combining a therapeutic transducer with a sonographic probe. The optimal HIFU sequence was defined on ex vivo samples before in vivo evaluation of tumor ablation was performed by perfusion quantification after contrast agent injection. The original feature of this prototype is a 9-MHz sonographic probe in a HIFU device and connected to an Aplio (Toshiba) sonograph. Acoustical power and treatment time were determined on ex vivo livers to generate 1-cm-long lesions. Lesion reproducibility was assessed for the power and treatment time selected. The gap between lesions and HIFU displacement shot procedures were optimized to ablate a 1-cm3 volume. The optimized protocol was applied to five murine tumors in vivo. Tumor ablation was quantified according to (1) contrast uptake (CU) after HIFU using perfusion software (Toshiba) in "vascular recognition imaging" mode and Sonovue (Bracco) contrast agent, and (2) the percentage of necrosis quantified on histologic slides. Ex vivo results: optimized settings, at 442 W/cm2 applied during three cycles (3 s on/5 s off) generated 10 identical elementary lesions measuring 9.78 (+/-0.66) * 2.11 (+/-0.33) mm2. A 4-mm gap between adjacent lesions and a 2-min pause between shot lines were found optimal. In vivo results: 60 % (+/-22) mean reduction in CU after HIFU and tumor necrosis histologically estimated at 58 % (+/-5.7) were quantified for the five animals. The therapeutic potential of this HIFU prototype was demonstrated in vivo through objective quantification of tumor ablation based on CU.

[Ultrasound contrast agents for liver diagnostics]

Ultrasound contrast agents have achieved a high level of acceptance in diagnostics of liver tumors. Contrast-enhanced ultrasound can, on the one hand, be used for detection of liver tumors, e.g., during the search for metastases in tumor staging, and, on the other hand, for tumor characterization. The dispersion behavior of the ultrasound contrast agent plays a decisive role in the characterization and the enhancement patterns correspond to a large extent to those already known from contrast-enhanced computed tomography. Contrast-enhanced ultrasound can also be employed for monitoring ablative tumor therapies, visualization of vessels difficult to depict, and measurement of the so-called liver transit time.

High-intensity focused ultrasound in the treatment of solid tumours

Traditionally, surgery has been the only cure for many solid tumours. Technological advances have catalysed a shift from open surgery towards less invasive techniques. Laparoscopic surgery and minimally invasive techniques continue to evolve, but for decades high-intensity focused ultrasound has promised to deliver the ultimate objective - truly non-invasive tumour ablation. Only now, however, with recent improvements in imaging, has this objective finally emerged as a real clinical possibility.

Monitoring of liver metastases after stereotactic radiotherapy using low-MI contrast-enhanced ultrasound--initial results

The purpose of this study was to monitor liver metastases after radiotherapy using contrast-enhanced ultrasound (CEUS). In 15 patients, follow-up examinations after stereotactic, single-dose radiotherapy were performed using CEUS (low mechanical index (MI), 2.4-ml SonoVue) and computed tomography (CT). Besides tumor size, the enhancement of the liver and the metastases was assessed at the arterial, portal venous, and delayed phases. The sizes of the tumor and of a perifocal liver reaction after radiotherapy measured with CEUS significantly correlated with those measured at CT (r=0.93, p<0.001). CEUS found a significant reduction of the arterial vascularization in treated tumors (p<0.05). In the arterial phase, the perifocal liver tissue was hypervascularized compared to the treated tumor (p<0.001); in the late phase, it was less enhanced than the liver (p<0.001) and more than the tumor (p<0.01). The perifocal liver reaction was also seen in CT, but with a variable enhancement at the arterial (50% hyperdense compared to normal liver tissue), venous, or delayed phase (each with 70% hyperdense reactions). CEUS allows for the assessment of tumor and liver perfusion, in addition to morphological tumor examination, which was comparable with CT. Thus, changes of tumor perfusion, which may indicate tumor response, as well as the perifocal liver reaction after radiotherapy, which must be differentiated from perifocal tumor growth, can be sensitively visualized using CEUS.