Overview of Therapeutic Ultrasound Applications and Safety Considerations: 2024 Update

A 2012 review of therapeutic ultrasound was published to educate researchers and physicians on potential applications and concerns for unintended bioeffects (doi: 10.7863/jum.2012.31.4.623). This review serves as an update to the parent article, highlighting advances in therapeutic ultrasound over the past 12 years. In addition to general mechanisms for bioeffects produced by therapeutic ultrasound, current applications, and the pre-clinical and clinical stages are outlined. An overview is provided for image guidance methods to monitor and assess treatment progress. Finally, other topics relevant for the translation of therapeutic ultrasound are discussed, including computational modeling, tissue-mimicking phantoms, and quality assurance protocols.

Ultrasound Molecular Imaging Enhances High-Intensity Focused Ultrasound Ablation on Liver Cancer With B7-H3-Targeted Microbubbles

BACKGROUND

High-intensity focused ultrasound (HIFU) is a promising minimally invasive treatment for liver cancer; however, its efficacy is often limited by the attenuation of ultrasonic energy. This study investigates the effectiveness of B7-H3-targeted microbubbles (T-MBs) in enhancing HIFU ablation of liver cancer and explores their potential for clinical translation.

METHODS

T-MBs and isotype control microbubbles (I-MBs) were synthesized through the conjugation of biotinylated anti-B7-H3 antibody and isotype control antibody to the microbubble surface, respectively. Contrast-enhanced ultrasound imaging was performed to compare the accumulation of T-MBs and I-MBs in liver cancer at various time points. The efficacy of T-MBs in enhancing HIFU treatment was evaluated by measuring the immediate tumor ablation rate and long-term tumor growth suppression. Additionally, the induced antitumor immune response was assessed through cytokine quantification in serum and tumor tissue, along with immunofluorescence staining conducted on days 1, 3, and 7 post-treatment.

RESULTS

T-MBs demonstrated superior liver cancer-specific accumulation, characterized by higher concentrations and prolonged retention compared to I-MBs. The combination of T-MBs with HIFU resulted in significantly enhanced tumor ablation rates and superior tumor growth suppression. Post-treatment analysis revealed a gradual uptick in cytokine levels within the tumor microenvironment, along with progressive infiltration of antitumor immune cells.

CONCLUSION

T-MBs effectively enhance the therapeutic efficacy of HIFU for liver cancer treatment while simultaneously promoting an antitumor immune response. These findings provide a strong experimental foundation for the clinical translation of ultrasound molecular imaging combined with HIFU as a novel approach for tumor therapy.

Safety of Sonazoid in Assisting High-Intensity Focused Ultrasound Ablation Therapy for Advanced Liver Malignant Lesions: A Single-Arm Clinical Study

OBJECTIVE

The aim of the study described here was to evaluate the safety of Sonazoid-assisted high-intensity focused ultrasound (HIFU) in the treatment of advanced malignant liver lesions.

METHODS

A single-arm study was designed to enroll participants who were diagnosed with advanced primary liver cancer or liver metastases and proposed to receive Sonazoid assistance during HIFU treatment. Serological examination was conducted within 1 wk, and side effects in each patient were monitored for 1 mo. To evaluate therapeutic efficacy, the contrast-enhanced magnetic resonance imaging was performed 1 mo after treatment, and short-term follow-up was conducted a year later.

RESULTS

A total of 17 participants (12 male, 5 female) with an average age of 58 y (range: 46-73 y) were enrolled, including 11 patients with hepatocellular carcinoma, 2 patients with hepatic metastasis and 4 patients with cholangiocarcinoma. The total volume of tumor mass was 111.82 (11.01-272.30) cm3. The average total ablation time for a patient was 2021 ± 1030 s, and the energy efficiency factor was 5979.7 (3108.0, 45634.5) J/cm3. Immediately after HIFU treatment, 1 patient (5.9%) achieved complete response (CR), 4 patients (23.5%) had a moderate response, 8 patients (47.1%) had partial reperfusion and 4 patients (23.5%) had stable disease (SD). The average ablation rate for all the tumors was 51.5 ± 26.7%. The level of glutamic-pyruvic transaminase (ALT) was mildly increased in 71.6% (12/17) of patients after HIFU therapy. Mean ALT values before and after treatment were 22 (14, 35) U/L and 36 (25, 41) U/L, respectively (Z = 1.947, p = 0.051). Mild or obvious edema in skin and subcutaneous soft tissues were observed in 76.5% of patients, but no serious side effects were found. Twelve months after treatment, the follow-up results revealed that 1 patient (5.8%) achieved a CR, 8 patients (47.1%) had SD and 8 patients (47.1%) had progressive disease. The estimated median time to progression was 11 mo after treatment, with a 95% confidence interval of 6, 11 for all involved patients.

CONCLUSION

Use of Sonazoid is safe and feasible for improving HIFU ablation efficiency during the treatment of advanced malignant liver lesions. The therapeutic efficacy of Sonazoid-assisted HIFU needs to be explored in additional controlled clinical investigations.

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.

Numerical Study of Bubble Cloud and Thermal Lesion Evolution During Acoustic Droplet Vaporization Enhanced HIFU Treatment

Acoustic droplet vaporization (ADV) has been proven to enhance high intensity focused ultrasound (HIFU) thermal ablation of tumor. It has also been demonstrated that triggering droplets before HIFU exposure could be a potential way to control both the size and the shape of the thermal lesion. In this paper, a numerical model is proposed to predict the thermal lesion created in ADV enhanced HIFU treatment. Bubble oscillation was coupled into a viscoelastic medium in the model to more closely represent real applications in tissues. Several physical processes caused by continuous wave ultrasound and elevated temperature during the HIFU exposure were considered, including rectified diffusion, gas solubility variation with temperature in the medium, and boiling. Four droplet concentrations spanning two orders of magnitude were calculated. The bubble cloud formed from triggering of the droplets by the pulse wave ultrasound, along with the evolution of the shape and location of the bubble cloud and thermal lesion during the following continuous wave exposure was obtained. The increase of bubble void fraction caused by continuous wave exposure was found to be consistent with the experimental observation. With the increase of droplet concentration, the predicted bubble cloud shapes vary from tadpole to triangular and double triangular, while the thermal lesions move toward the transducer. The results show that the assumptions used in this model increased the accuracy of the results. This model may be used for parametrical study of ADV enhanced HIFU treatment and be further used for treatment planning and optimization in the future.

Ultrasound-Guided High Intensity Focused Ultrasound Ablation for Symptomatic Uterine Fibroids: Preliminary Clinical Experience

OBJECTIVE

 To evaluate the middle-term efficacy and complications of ultrasound-guided high intensity focused ultrasound (USgHIFU) for the treatment of symptomatic uterine fibroids in an NHS population.

METHODS

 A prospective observational single-center study at a single university hospital in Oxford, UK. Patients with symptomatic uterine fibroids who declined standard surgical/radiological intervention and were referred to the HIFU unit were considered for USgHIFU treatment. Clinical evaluation, adverse event monitoring, uterine fibroid symptoms and health-related quality of life questionnaire (UFS-QOL) and contrast-enhanced pelvic magnetic resonance imaging (MRI) were performed before and at regular intervals after treatment to assess patient outcome.

RESULTS

 12 of 22 referred patients underwent one session of USgHIFU ablation of 14 fibroids overall and received a two-year follow-up. No serious adverse events were reported, but a second-degree skin burn was observed in one patient who had a surgical scar from a previous caesarean section. Mean symptom severity scores (SSS-QOL) improved significantly from 56.5 ± 29.1 (SD) at baseline to 33.4 ± 23.3 (p < 0.01) at three months, 45.0 ± 35.4 (p < 0.05) at one year and 40.6 ± 32.7 (p < 0.01) at two years post-treatment. The mean non-perfused volume ratio was 67.7 ± 39.0 % (SD) in the treated fibroids (n = 14) within three months of treatment. The mean volume reduction rates of the treated fibroids were 23.3 ± 25.5 % (SD) at 3 months post-treatment (p < 0.01, n = 14), 49.3 ± 23.7 % at 12 months (p < 0.05, n = 8), and 51.9 ± 11.1 % at 24 months (p < 0.005, n = 8).

CONCLUSION

 This study demonstrates the clinical efficacy of USgHIFU ablation of uterine fibroids and the low risk of complications. We believe that this noninvasive approach may offer an alternative therapy for women with symptomatic uterine fibroids. While HIFU is fast becoming the standard of care for fibroid ablation in other countries, to our knowledge, this study is the first to present clinical experience of US-guided HIFU ablation of symptomatic uterine fibroids in an NHS population.

PLAIN LANGUAGE SUMMARY

High intensity focused ultrasound (HIFU) can be used for the noninvasive ablation of symptomatic uterine fibroids, and MR-guided treatment has already gained FDA approval. Ultrasound-guided HIFU has the advantage of offering practicalities in anesthesia and considerable cost-savings over MR-guided treatments. In this prospective study we have demonstrated the middle-term efficacy and favorable safety profile of ultrasound-guided HIFU for the treatment of symptomatic uterine fibroids for the first time in an NHS population.

The effects on thermal lesion shape and size from bubble clouds produced by acoustic droplet vaporization

BACKGROUND

Bubbles formed by acoustic droplet vaporization (ADV) have proven to be an effective method for significant enlargement of the thermal lesions produced by high intensity focused ultrasound (HIFU). We investigated the influences of bubble cloud shape and droplet concentration on HIFU thermal lesions, as these relate to the ADV technique.

METHODS

Unlike previous studies where the droplets were simultaneously vaporized with the HIFU exposure for thermal lesion formation, droplets were vaporized by pulse wave (PW) ultrasound prior to continuous wave (CW) ultrasound heating in this experimental study. Under different experimental conditions, we recorded and quantified by the image processing methods the morphology and size of the bubble clouds created and the corresponding thermal lesions formed.

RESULTS

The results demonstrated that different ADV droplet concentrations produced a variety of thermal lesion shapes and sizes. The lesion volume could be increased using PW ultrasound followed by CW exposure, especially for higher droplet concentrations, e.g. 3.41 × 10⁶/mL yielded a tenfold increase over that seen using CW alone.

CONCLUSION

These findings could lead to optimization of HIFU therapy by selecting a bubble forming strategy and droplet concentrations, especially using lower ultrasound powers which is desirable in clinical applications.

Numerical Study of Bubble Area Evolution During Acoustic Droplet Vaporization-Enhanced HIFU Treatment

Acoustic droplet vaporization has the potential to shorten treatment time of high-intensity focused ultrasound (HIFU) while minimizing the possible effects of microbubbles along the propagation path. Distribution of the bubbles formed from the droplets during the treatment is the major factor shaping the therapeutic region. A numerical model was proposed to simulate the bubble area evolution during this treatment. Using a linear acoustic equation to describe the ultrasound field, a threshold range was defined that determines the amount of bubbles vaporized in the treated area. Acoustic parameters, such as sound speed, acoustic attenuation coefficient, and density, were treated as a function of the bubble size distribution and the gas void fraction, which were related to the vaporized bubbles in the medium. An effective pressure factor was proposed to account for the influence of the existing bubbles on the vaporization of the nearby droplets. The factor was obtained by fitting one experimental result and was then used to calculate bubble clouds in other experimental cases. Comparing the simulation results to these other experiments validated the model. The dynamic change of the pressure and the bubble distribution after exposure to over 20 pulses of HIFU are obtained. It is found that the bubble area grows from a grainlike shape to a "tadpole," with comparable dimensions and shape to those observed in experiments. The process was highly dynamic with the shape of the bubble area changing with successive HIFU pulses and the focal pressure. The model was further used to predict the shape of the bubble region triggered by HIFU when a bubble wall pre-exists. The results showed that the bubble wall helps prevent droplet vaporization on the distal side of the wall and forms a particularly shaped region with bubbles. This simulation model has predictive potential that could be beneficial in applications, such as cancer treatment, by parametrically studying conditions associated with these treatments and designing treatment protocols.

Effects of silibinin-loaded thermosensitive liposome-microbubble complex on inhibiting rabbit liver VX2 tumors in sub-hyperthermia fields

In the present study, the effects of silibinin-loaded thermosensitive liposome-microbubble complex (STLMC) on rabbit liver VX2 tumors in sub-hyperthermia fields were investigated using two-dimensional ultrasonography (2D US), contrast-enhanced US (CEUS), hematoxylin and eosin (H&E) staining, immunohistochemistry and ultrastructure observation. 50 rabbits with VX2 liver tumors were divided into five groups: Sub-hyperthermia microwave ablation group (SHM), STLMC injection group (STLMC), SHM ablation plus STLMC injection group (SHM + STLMC), microbubble injection group and blank control group without any treatment. Rabbits in each group were examined using 2D US and CEUS in order to evaluate the tumor volume and diameter before treatment and at day 7 and 21 after treatment. Morphology, expression of CD163 and CD206, and ultrastructure of the tumors were assessed. The average post-treatment volume of tumors in group SHM + STLMC was 1.17±0.88 cm³ at day 7 and 2.15±0.96 cm³ at day 21, which was significantly decreased compared with all other groups (P<0.05). H&E staining indicated that the number of disordered macrophages in the SHM + STLMC group significantly increased compared with the other groups (P<0.05). Immunohistochemical results demonstrated that in the SHM + STLMC, the expression of CD163 and CD206 significantly decreased compared with all other groups (P<0.05). These results suggested that STLMC has a potential function in preventing tumor growth, which may be due to its inhibitory effect on tumor-associated macrophages in the tumor microenvironment.

The lifetime evaluation of vapourised phase-change nano-droplets

Phase-change nano-droplets (PCNDs) are sub-micron particles that are coated with phospholipid and contain liquid-state perfluorocarbons such as perfluoropentane (boiling point=29°C) and perfluorohexane (boiling point=57°C), which can vapourise upon application of ultrasound. The bubbles generated by such reactions can serve as ultrasound contrast agents or HIFU sensitisers. However, the lifetime of bubbles generated from PCNDs on μs-order is not well known. Knowledge of the condition of PCND-derived bubbles on μs-order is essential for producing bubbles customised for specific purposes. In this study, we use an optical measurement system to measure the vapourisation and stability of the bubbles (bubble-lifetime) as well as the stability-controlling method of the nucleated bubbles on μs-order while changing the internal composition of PCNDs and the ambient temperature. PCND-derived bubbles remain in a bubble state when the boiling point of the internal composition is lower than the ambient temperature, but lose their optical contrast after approximately 10μs by re-condensation or dissolution when the boiling point of the internal composition is higher than the ambient temperature. We reveal that the superheating condition significantly affects the fate of vapourised PCNDs and that the bubble-lifetime can be controlled by changing both the ambient temperature conditions and the internal composition of PCNDs.

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.

The impact of vaporized nanoemulsions on ultrasound-mediated ablation

Background

The clinical feasibility of using high-intensity focused ultrasound (HIFU) for ablation of solid tumors is limited by the high acoustic pressures and long treatment times required. The presence of microbubbles during sonication can increase the absorption of acoustic energy and accelerate heating. However, formation of microbubbles within the tumor tissue remains a challenge. Phase-shift nanoemulsions (PSNE) have been developed as a means for producing microbubbles within tumors. PSNE are emulsions of submicron-sized, lipid-coated, and liquid perfluorocarbon droplets that can be vaporized into microbubbles using short (<1 ms), high-amplitude (>5 MPa) acoustic pulses. In this study, the impact of vaporized phase-shift nanoemulsions on the time and acoustic power required for HIFU-mediated thermal lesion formation was investigated in vitro.

Methods

PSNE containing dodecafluoropentane were produced with narrow size distributions and mean diameters below 200 nm using a combination of sonication and extrusion. PSNE was dispersed in albumin-containing polyacrylamide gel phantoms for experimental tests. Albumin denatures and becomes opaque at temperatures above 58°C, enabling visual detection of lesions formed from denatured albumin. PSNE were vaporized using a 30-cycle, 3.2-MHz, at an acoustic power of 6.4 W (free-field intensity of 4,586 W/cm²) pulse from a single-element, focused high-power transducer. The vaporization pulse was immediately followed by a 15-s continuous wave, 3.2-MHz signal to induce ultrasound-mediated heating. Control experiments were conducted using an identical procedure without the vaporization pulse. Lesion formation was detected by acquiring video frames during sonication and post-processing the images for analysis. Broadband emissions from inertial cavitation (IC) were passively detected with a focused, 2-MHz transducer. Temperature measurements were acquired using a needle thermocouple.

Results

Bubbles formed at the HIFU focus via PSNE vaporization enhanced HIFU-mediated heating. Broadband emissions detected during HIFU exposure coincided in time with measured accelerated heating, which suggested that IC played an important role in bubble-enhanced heating. In the presence of bubbles, the acoustic power required for the formation of a 9-mm³ lesion was reduced by 72% and the exposure time required for the onset of albumin denaturation was significantly reduced (by 4 s), provided that the PSNE volume fraction in the polyacrylamide gel was at least 0.008%.

Conclusions

The time or acoustic power required for lesion formation in gel phantoms was dramatically reduced by vaporizing PSNE into bubbles. These results suggest that PSNE may improve the efficiency of HIFU-mediated thermal ablation of solid tumors; thus, further investigation is warranted to determine whether bubble-enhanced HIFU may potentially become a viable option for cancer therapy.

Accumulation of phase-shift nanoemulsions to enhance MR-guided ultrasound-mediated tumor ablation in vivo

Magnetic resonance-guided high intensity focused ultrasound (MRgHIFU) is being explored as a non-invasive technology to treat solid tumors. However, the clinical use of HIFU for tumor ablation applications is currently limited by the long treatment times required. Phase-shift nanoemulsions (PSNE), consisting of liquid perfluorocarbon droplets that can be vaporized into microbubbles, are being developed to accelerate HIFU-mediated heating. The purpose of this study was to examine accumulation of PSNE in intramuscular rabbit tumors in vivo. MR images were acquired before and after intravenous injection of gadolinium-containing PSNE. MR signal enhancement was observed in rabbit tumors up to six hours after injection, indicating that PSNE accumulated in the tumors. In addition, PSNE vaporization was detected in the tumor with B-mode ultrasound imaging, and MR thermometry measurements indicated that PSNE accelerated the rate of HIFU-mediated heating. These results suggest that PSNE could dramatically improve the efficiency and clinical feasibility of MRgHIFU.

Effect of healthy tissue ablation surrounding VX2 rabbit liver tumors by high-intensity focused ultrasound combined with an ultrasound contrast agent

OBJECTIVES

The purpose of this study was to determine the minimum amount of healthy peripheral tissue that should be ablated when treating VX2 liver tumors with high-intensity focused ultrasound combined with an ultrasound contrast agent.

METHODS

Fifty-one rabbits with hepatic tumors were established and randomly divided into the following groups: group A, which only had their tumors ablated; group B, which had their tumors and 2 mm of healthy adjacent tissue ablated; and group C, which had their tumors and 4 mm of healthy adjacent tissue ablated. The pathologic characteristics of the target tissue, serum alanine aminotransferase (ALT) level, presence of intrahepatic and distant metastases, and survival time between different groups were compared after high-intensity focused ultrasound treatment.

RESULTS

After ablation, coagulative necrosis was observed in all targeted tissue. The serum ALT level in group C was the highest and the level in group A was the lowest on the third and fifth days after ablation (P < .05), respectively. Fourteen days later, the serum ALT level in groups B and C decreased to normal, whereas the level in group A was abnormal and significantly higher (P < .05). Compared with group A, the prevalence of metastases in groups B and C was significantly lower (P < .05), and the survival time was significantly longer (P < .05); there appeared to be no statistically significant difference between groups B and C (P > .05).

CONCLUSIONS

Ablation of a tumor along with 2 mm of healthy surrounding tissue is a more effective strategy for treating hepatic cancer with high-intensity focused ultrasound coupled with an ultrasound contrast agent.

The influence of blood supply on high intensity focused ultrasound a preliminary study on rabbit hepatic VX2 tumors of different ages

RATIONALE AND OBJECTIVES

The aim of this study was to explore the effects of blood supply on high-intensity focused ultrasound (HIFU) applied to rabbit hepatic VX2 tumors of different ages.

MATERIALS AND METHODS

Eighteen rabbits with VX2 hepatic tumors were randomly divided into three groups according to the time of sacrifice after tumor implantation: 10, 15, or 20 days. Contrast-enhanced ultrasound was performed immediately before HIFU ablation. The same settings for HIFU dose parameters were used to ablate the central tumor area in each group, and the real-time temperature of the targeted site of the tumor was measured. After HIFU, the coagulation necrosis volumes of tumor tissue and the microvascular density of residual tumor tissue were determined.

RESULTS

Histopathologic analysis showed that the extent of a tumor's blood supply followed the order 10-day group > 15-day group > 20-day group (P < .01). Contrast-enhanced ultrasound showed the same results. There was no statistically significant difference among the three groups in terms of temperature-increase parameters during HIFU treatment (P > .05). However, there were statistically significant differences between the groups in terms of temperature-decrease parameters during HIFU treatment and in terms of necrosis volumes after HIFU treatment (P < .05). Necrosis volume was inversely related to absolute enhanced intensity (r = -0.823, P < .001).

CONCLUSIONS

The extent of a tumor's blood supply had a significant effect on the temperature-decrease phase but not on the temperature-increase phase during HIFU treatment. The longer the temperature-decrease phase, the more slowly heat dissipated after HIFU, resulting in larger coagulation necrosis volumes.

An in vitro study of a phase-shift nanoemulsion: a potential nucleation agent for bubble-enhanced HIFU tumor ablation

Phase-shift nanoemulsions have the potential to nucleate bubbles and enhance high-intensity focused ultrasound (HIFU) cancer therapy. This emulsion consists of albumin-coated dodecafluoropentane (DDFP) droplets with a mean diameter of approximately 260 nm at 37°C. It is known that superheated perfluorocarbon droplets can be vaporized with microsecond long ultrasound pulses if the acoustic pressure exceeds a specific threshold. In addition, it is well documented that particles smaller than 400 nm can extravasate through leaky tumor vessels and accumulate in the tumor interstitial space. Thus, nanoemulsions may passively target solid tumors, thus localizing cavitation nuclei for bubble-enhanced HIFU-mediated heating. In this study, we investigate the acoustic droplet vaporization of a DDFP nanoemulsion in tissue-mimicking gels and demonstrate the ability to nucleate inertial cavitation (IC) and enhance HIFU-mediated heating. The nanoemulsion was dispersed throughout albumin-acrylamide gel phantoms and sonicated with microsecond-length HIFU pulses (f = 2 MHz). The pressure threshold needed to vaporize the nanoemulsion was measured as a function of degree of superheat, pulse length and nanoemulsion concentration. It was determined that the vaporization threshold was inversely proportional with degree of superheat and independent of pulse length and concentration within the range of values tested. It was also shown that the bubbles formed from vaporized nanoemulsions reduced the IC threshold in the gel phantoms. Finally, it was demonstrated that cavitation from vaporized nanoemulsions accelerated HIFU-mediated heating. The results from this study demonstrate that phase-shift nanoemulsions can be combined with HIFU to provide a high degree of spatial and temporal control of bubble-enhanced heating.

Role of sonography for implantation and sequential evaluation of a VX2 rabbit liver tumor model

OBJECTIVE

We investigated the role of sonography in the implantation process of a VX2 rabbit liver tumor model and sequential evaluation.

METHODS

Fifty rabbits were divided into 2 groups. Animals in group I underwent surgical implantation, whereas those in group II received percutaneous sonographically guided implantation. At 7, 14, 21, and 28 days after implantation, respectively, 5 rabbits in each group were examined with conventional, color Doppler (CD), contrast-enhanced (CE) pulse inversion harmonic (PIH), and CE CD sonography. Pathologic examination was performed with hematoxylin-eosin, nicotinamide adenine dinucleotide phosphate-diaphorase, and succinic dehydrogenase stains.

RESULTS

Twenty-one rabbits with tumors survived in group I, and 22 with tumors survived in group II. The mean duration of implantation +/- SD in group II was 16.9 +/- 3.4 minutes, whereas that in group I was 21.5 +/- 4.1 minutes (P < .05). The tumor volume measured by conventional sonography increased from 0.28 +/- 0.14 cm(3) at 7 days to 16.49 +/- 5.50 cm(3) at 28 days in group I and from 0.31 +/- 0.19 to 19.79 +/- 4.70 cm(3) in group II, whereas no significant difference existed between the groups. On CD, CE PIH, and CE CD sonography, most tumors were hypervascular before 14 days and after 14 days had peripheral vessels and central hypovascular areas, which were shown as necrotic areas by pathologic examination.

CONCLUSIONS

Sonographically guided implantation achieved a good success rate with convenient inoculation performance. Conventional gray scale, CD, CE PIH, and CE CD sonography were useful in sequential evaluation of tumor growth and characteristic vascularity.

Ablation of high-intensity focused ultrasound assisted with SonoVue on Rabbit VX2 liver tumors: sequential findings with histopathology, immunohistochemistry, and enzyme histochemistry

BACKGROUND

We investigated sequential effects of HIFU ablation combined with contrast agent SonoVue by using histopathology examination, immunohistochemistry, and enzyme histochemistry.

MATERIALS AND METHODS

Forty rabbits with VX2 liver tumors were subjected to HIFU ablation. Before ablation, a bolus injection of 0.2 mL SonoVue was administrated in group II (n = 20), and normal saline solution was injected in group I (n = 20). On day 0, 3, 7, and 14 after ablation, 5 animals in each group were sacrificed. The tissue in ablated zone, transient zone (within 3 mm around ablated area), and surrounding zone (beyond 3 mm around ablated area) were collected. Coagulated volume measurement, hematoxylin-eosin staining, immunohistochemistry of Ki 67, Bcl-2, CD54, and MMP-2 to determine cell proliferation and tissue repair, and nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) and succinic dehydrogenase (SDH) staining to evaluate tissue viability were performed.

RESULTS

The coagulated volume in group II at each time point was larger than that in group I (P < .05). After day 3, hematoxylin-eosin staining demonstrated necrosis in ablated zones and increasing surrounding fibra bands in group I and group II, while increasing expression of Ki 67, Bcl-2, CD54, and MMP-2 in transient zones was detected using immunohistochemistry in both groups (P > .05). NADPH-d and SDH staining showed dramatic decrease of enzyme activities in ablated zones immediately after ablation, while residual viable tissues in ablated zones of group II were less than those of group I (P < .05).

CONCLUSION

Contrast agent SonoVue enables improvement of HIFU ablation on rabbit VX2 liver tumors.