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Background Purpose Methods Results Conclusion Microwave ablation (MWA) is a minimally invasive alternative for the treatment of unresectable liver tumors. To verify the effectiveness and safety of MWA, it is critical to measure the temperature variation and assess the regions of the microwave‐induced thermal lesions.Recent studies have indicated that the locations of optimally matched Gabor atoms (LOMGA) from ultrasound radiofrequency (RF) echo signals allow accurate and stable scatterer spacing estimation. Herein, a harmonic‐based LOMGA method is proposed to estimate the scatterer spacing for improving the assessment of microwave‐induced thermal lesions.The mean scatterer spacing (MSS) is estimated via the LOMGA method incorporating the selection of concise atoms from separated second‐harmonic RF echo signals with the pulse‐inversion algorithm for thermal lesion evaluation. In vitro experiments, 10 fresh porcine liver samples were ablated at different time nodes during the ablation period, and 200 sets of second‐harmonic and fundamental RF echo signals were randomly selected from the regions of interest in the coagulated liver samples for MSS estimation. The means and standard deviations of the MSSs, as well as the linear regression for the mean MSSs, were calculated from fundamental and second‐harmonic signals for comparison and evaluation, the receiver operating characteristic (ROC) curves for the 200 sets of fundamental‐based and harmonic‐based MSS estimates from the 10 liver samples at five pairs of adjacent time nodes were calculated, and one‐way analysis of variance (ANOVA) tests were performed for the five pairs of adjacent time nodes. The fundamental and harmonic‐based p‐values in the ANOVA tests and the areas under the ROC curves (AUCs) were calculated to statistically analyze the differences in the MSSs between adjacent time nodes.The harmonic‐based increments in the intensity variation and coherent components were larger than the fundamental‐based increments with the increasing ablation time. The harmonic‐based MSSs from the 10 liver samples at five pairs of adjacent time nodes were found to be highly statistically significant (p < 0.01). Thus, the harmonic‐based MSSs had greater variations. Compared with the fundamental‐based results, for the five preset ST values, the average increment in the harmonic‐based mean slopes was 69.22% and the average decrement in the mean standard deviations was 11.67% for the linear‐fitting MSS results, and the results were statistically significant (p < 0.05).Harmonic‐based MSSs are more sensitive and robust to variations in coagulated tissues, which is advantageous for the assessment of microwave‐induced thermal lesions. [ABSTRACT FROM AUTHOR]

Focused ultrasound on the thalamus is employed to relieve tremor in movement disorders. The treatment efficacy is currently assessed by subjective observation of patient tremor during hand movements. The purpose of this study was to create a simple quantitative method to assess tremor based on traditional pen-and-paper spiral drawing that could be employed during treatment and follow-up visits. Image processing was employed to automatically and rapidly clean scanned spiral and line drawings and to isolate the minimal part of the drawing that could provide sufficient data for tremor detection. Sobel edge detection and easily-interpretable statistics of the distribution of edge-angle orientations in the spirals relative to the spiral centres were used as tremor measures. The method was applied to scanned drawings of 122 tremor patients before treatment and in follow-up visits. Discrimination analysis of the tremor measure between drawings in the session before treatment and the follow-up sessions was carried out. The processing yielded correctly cropped and clean spirals. The tremor measure demonstrated consistent tremor reduction in the treated hand drawings in all five follow-up visits during the first year following treatment date in 76 patients from the study cohort. In this study we developed a computational method that rapidly computed tremor severity estimate in an explainable and clinically useful manner from a traditional pen-and-paper spiral drawing. This rapid quantitative and objective tremor assessment can easily be implemented during focused ultrasound treatment and replace the subjective assessment currently employed.

The objective of this work was to study microbubble-enhanced temperature elevation with high-intensity focused ultrasound (HIFU) at different acoustic pressures and under image guidance. The microbubbles were administered with either local or vascular injections (that mimic systemic injections) in perfused and non-perfused ex vivo porcine liver under ultrasound image guidance. Porcine liver was insonified for 30 s with a single-element HIFU transducer (0.9 MHz, 0.413 ms, 82% duty cycle, focal pressures of 0.6–3.5 MPa). Contrast microbubbles were injected either locally or through the vasculature. A needle thermocouple at the focus measured temperature elevation. Diagnostic ultrasound (Philips iU22, C5-1 probe) guided placement of the thermocouple and delivery of microbubbles and monitored the procedure in real time. At lower acoustic pressures (0.6 and 1.2 MPa) in non-perfused liver, inertial cavitation of the injected microbubbles led to greater temperatures at the focus compared with HIFU-only treatments. At higher pressures (2.4 and 3.5 MPa) native inertial cavitation in the tissue (without injecting microbubbles) resulted in temperature elevations similar to those after injecting microbubbles. The heated area was larger when using microbubbles at all pressures. In the presence of perfusion, only local injections provided a sufficiently high concentration of microbubbles necessary for significant temperature enhancement. Local injections of microbubbles provide a higher concentration of microbubbles in a smaller area, avoiding acoustic shadowing, and can lead to higher temperature elevation at lower pressures and increase the size of the heated area at all pressures. [ABSTRACT FROM AUTHOR]

A polyacrylamide polysaccharide hydrogel (PASG) containing a nonionic surfactant of the polyoxyethylene nonylphenyl ethers series (NP14) has been adapted to the fabrication of a reusable cost-effective ultrasonic tissue-mimicking phantom for real-time visualization of the thermal lesions by high intensity focused ultrasound (HIFU) irradiation. The constructed NP14 (40% in w/v) PASG is optically transparent at room temperatures, and it turns out to be opaque white as heated over the clouding points of about 55 °C and returns to its original transparent state after cooling. The acoustic property of the proposed phantom is similar to those of human liver tissues, which includes the acoustic impedance of 1.68 Mrayls, the speed of sound of 1595 ± 5 m/s, the attenuation coefficient of 0.52 ± 0.05 dB cm−1 (at 1 MHz), the backscatter coefficient of 0.21 ± 0.09 × 10−3 sr−1 cm−1 (at 1 MHz), and the nonlinear parameter B/A of 6.4 ± 0.2. The NP14-PASG was tested to assess the characteristic information (sizes, shapes, and locations) of the thermal lesions visualized when exposed to typical HIFU fields (1.1 MHz, focal pressure up to 20.1 MPa, focal intensity 4075 W/cm2). The proposed NP14-PASG is expected to replace the existing costly BSA-PASG used for more effective testing of the performance of therapeutic ultrasonic devices based on thermal mechanisms. [ABSTRACT FROM AUTHOR]

Background: Radiofrequency (RF) lesion creation is related to the heat propagation induced by RF application on tissues. Thermocouple embedded in the RF antenna are not able to predict deep tissue temperature at various level.Objectives: This study aims to investigate the influence of power delivered on radiofrequency catheter ablation (RFCA) effects by means of high resolved 2D temperature maps.Methods: Three trials of four ablations (12 applications) were executed on each specimen of healthy excised swine myocardium in different application points at four RF power values (30 W, 40 W, 50 W, and 60 W) for a fixed treatment time. All the data provided by the fiber Bragg gratings (FBGs) were analyzed. Temperature variations (ΔT) in time recorded in the 28 sites of measurements were reported. Also, temperature maps showing the ΔT spatial distribution reached within the tissue at the end of the RFCA were produced and displayed, together with the representation of the lethal isotherm. Moreover, the time of achievement of the lethal isotherm at different tissue depths (from 1 to 8 mm) was evaluated for the four power settings.Results: Temperature trends reported comparable profiles across the different power settings. ΔT values and ΔT rising times showed dependence on the sensors' proximity to the RF energy source and on the set RF power. Temperature maps confirmed that heat propagation occurs preferentially along the width of the tissue than in the depth. Also, for the adjusted treatment time, no power setting guarantees lesions thicker than 6 mm.Conclusions: ΔT maximal values and ΔT rising time strongly depends on the proximity of the tissues to RF energy source, as well as on the RF power setting. A plateau is reached in lesion size, regardless of the power setting. A first correlation between lesion size, power setting, and time to achieve lethal isotherms has been established. [ABSTRACT FROM AUTHOR]

According to the traditional method of high intensity focused ultrasound (HIFU) treatment, the acoustic and thermal characteristic parameters of constant temperature (room temperature or body temperature) are used to predict thermal lesion. Based on the nonlinear spherical beam equation (SBE) and Pennes bio-heat transfer equation, and a new acoustic-thermal coupled model is proposed. The constant and temperature-dependent acoustic and thermal characteristic parameters are used to predict thermal lesion, and the predicted lesion area are compared with each other. Moreover, the relationship between harmonic amplitude ratio (P2/P1) and thermal lesion is studied. Combined with the known experimental data of acoustic and thermal characteristic parameters of biological tissue and data fitting method, the relationship between acoustic and thermal characteristic parameters and temperature is obtained; and the thermal lesion simulation calculation is carried out by using the acoustic and thermal characteristic parameters under constant temperature and temperature- dependent acoustic and thermal characteristic parameters, respectively. The simulation results show that under the same irradiation condition, the thermal lesion predicted by temperature-dependent acoustic and thermal characteristic parameters is larger than that predicted by traditional method, and the thermal lesion increases with the decrease of harmonic amplitude ratio.

A polyacrylamide polysaccharide hydrogel (PASG) containing a nonionic surfactant of the polyoxyethylene nonylphenyl ethers series (NP14) has been adapted to the fabrication of a reusable cost-effective ultrasonic tissue-mimicking phantom for real-time visualization of the thermal lesions by high intensity focused ultrasound (HIFU) irradiation. The constructed NP14 (40% in w/v) PASG is optically transparent at room temperatures, and it turns out to be opaque white as heated over the clouding points of about 55 °C and returns to its original transparent state after cooling. The acoustic property of the proposed phantom is similar to those of human liver tissues, which includes the acoustic impedance of 1.68 Mrayls, the speed of sound of 1595 ± 5 m/s, the attenuation coefficient of 0.52 ± 0.05 dB cm−1 (at 1 MHz), the backscatter coefficient of 0.21 ± 0.09 × 10−3 sr−1 cm−1 (at 1 MHz), and the nonlinear parameter B/A of 6.4 ± 0.2. The NP14-PASG was tested to assess the characteristic information (sizes, shapes, and locations) of the thermal lesions visualized when exposed to typical HIFU fields (1.1 MHz, focal pressure up to 20.1 MPa, focal intensity 4075 W/cm2). The proposed NP14-PASG is expected to replace the existing costly BSA-PASG used for more effective testing of the performance of therapeutic ultrasonic devices based on thermal mechanisms.

Purpose: Atrial fibrillation (AF) is the most common heart rhythm disorder in the world. Radiofrequency catheter ablation (RFCA) has become the preferred method of treatment for drug-refractory AF. One of the rare (< 0.2%) but deadly (≈ 80%) complications of RFCA is Atrioesophageal fistula (AEF). Although the exact pathophysiological events in developing AEF are not fully understood, one hypothesis is that the underlying cause may be thermal damage to the mucosa (the esophagus lumen). Method: The present study reports on a computer model of RFCA in the posterior wall of the left atrium (LA) which is in close proximity to the esophagus. A novel systematic approach was taken by considering a range of anatomical variations (obtained from clinical data) to study the spatial and temporal temperature data when RF energy was applied to cause a threshold temperature of 50 °C in the mucosa. The model is also used to investigate the spatial and temporal changes in mucosal temperature that may affect the reliability of the readings from esophageal temperature monitoring devices if they are not positioned accurately. Results: The results suggest evidence of transmural esophageal lesions in all the anatomies except one, if the 50 °C temperature threshold is the only criteria used for identification of thermal damage. However, by taking into consideration the effect of time (temperature-time integral), only some anatomies were identified as being partially damaged. Investigating the temperature and the temperature gradient data during the ablation revealed that the increases in both the temperature and the temperature gradient were time, location and anatomy dependent. This finding may have significance in the design and development of next-generation temperature monitoring devices that will provide a temperature map rather than single point measurements. Conclusion: Studies such as the present work may provide more convenient platforms for investigating the effect of the many factors involved in the RF procedure and how they may link to the development of AEF. [ABSTRACT FROM AUTHOR]

This study considers the temperature-dependent thermal parameters (specific heat capacity, thermal diffusivity and thermal conductivity) used when predicting the temperature rise of tissue exposed to high-intensity focused ultrasound (HIFU). Numerical analysis was performed using the Khokhlov-Zabolotskaya-Kuznetsov equation coupled with a bioheat transfer function. The thermal parameters were set as the functions of temperature using experimental data. The results revealed that, for liver tissue exposed to HIFU with a focal intensity of 3000 W/cm2 for 10 s, the predicted focal temperature rise was 23% lower and the thermal lesion area 41% smaller than those predicted without considering the temperature dependence. The prediction was validated by experimental observations on thermal lesions visualized in a tissue-mimicking phantom. The present results suggest that temperature-dependent thermal parameters should be considered in the prediction of HIFU-induced temperature rise to avoid lowering ultrasonic output settings for HIFU surgery. The aim of the present study was to investigate how significantly the temperature dependence of the thermal parameters affects the thermal dose imposed on the tissue by a typical clinical HIFU device. A numerical simulation was performed using a thermo-acoustic algorithm coupling the non-linear Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation (Meaney et al. 1998; Filonenko and Khokhlova 2001) and a bio-heat transfer (BHT) equation (Pennes 1948). Thermal parameters of liver tissue were modeled in the present study as functions of temperature and were incorporated into the BHT equation to compensate for the variations in thermal parameters with temperature. Experimental validation was achieved by comparing the predictions with the thermal lesions formed in the tissue-mimicking phantoms. [ABSTRACT FROM AUTHOR]

Background: High‐intensity focused ultrasound (HIFU) operating at 20 MHz is new and applicable to skin. Details of use and instrumentation are not documented. Materials and Methods: A GLP compliant 12‐week study of Göttingen minipigs (n = 3) was undertaken. Effects of HIFU treatment at different focal depths, energy levels and field size (single shot vs 5 × 5 multiple shots) were studied. Clinical scoring and histology of treated sites were made. Results: High‐intensity focused ultrasound showed instant and initial effects with wheal and flare responses followed by delayed inflammatory reactions associated with outer skin necrosis, depending on energy dose. HIFU treatment was tunable in the range 0.3‐1.5 J, ablative at higher energy level. Transducers with deeper focal points gave more profound effects, while epidermal effects were comparable. Multiple doses of 5 × 5 shots produced stronger reactions than single dose indicating that nearby applied shots were synergistic. Recovery from single doses was faster than in multidose areas. Clinical scarring at the end point was not seen despite occasional fibrous change of dermis. Findings illustrated intended therapeutic use; no special safety issues of concern were raised. Conclusion: The new 20 MHz HIFU was reproducible, tunable and produced targeted effects in the outer skin, for example instant wheal and flare followed by inflammation and possibly necrosis depending on energy setting. Reactions recovered during the study with only minor findings at study end. No special safety concerns were raised. The method can be controlled and modulated, and it is ready for clinical testing of dermatological disease indications including conditions presently treated with lasers. [ABSTRACT FROM AUTHOR]

Introduction: Deep brain stimulation (DBS) is a well-established treatment of movement disorders; but recently there has been an increasing trend toward the ablative procedure magnetic resonance-guided focused ultrasound (MRgFU). DBS is an efficient neuromodulatory technique but associated with surgical complications. MRIgFUS is an incision-free method that allows thermal lesioning, with fewer surgical complications but irreversible effects. Areas covered: We look at current and prospective aspects of both techniques. In DBS, appropriate patient selection, improvement in surgical expertise, target accuracy (preoperative and intraoperative imaging), neurophysiological recordings, and novel segmented leads need to be considered. However, increased number of older patients with higher comorbidities and risk of DBS complications (mainly intracranial hemorrhage, but also infections, hardware complications) make them not eligible for surgery. With MRgFUS, hemorrhage risks are virtually nonexistent, infection or hardware malfunction are eliminated, while irreversible side effects can appear. Expert commentary: Comparison of the efficacy and risks associated with these techniques, in combination with a growing aged population in developed countries with higher comorbidities and a preference for less invasive treatments, necessitates a review of the indications for movement disorders and the most appropriate treatment modalities. [ABSTRACT FROM AUTHOR]

High-intensity focused ultrasound (HIFU) has been used to ablate solid tumors and cancers. Because of the hypervascular structure of the tumor and circulating blood inside it, the interaction between the HIFU burst and vessel is a critical issue in the clinical environment. Influences on lesion production and the potential of vessel rupture were investigated in this study for the efficiency and safety of clinical ablation. An extracted porcine artery was embedded in a transparent polyacrylamide gel phantom, with bovine serum albumin (BSA) as an indicator of the thermal lesion, and degassed water was driven through the artery sample. The HIFU focus was aligned to the anterior wall, middle of the artery, and posterior wall. After HIFU ablation, the produced lesion was photographically recorded, and then its size was quantified and compared with that in the gel phantom without artery. In addition, the bubble dynamics (i.e., generation, expansion, motion, and shrinkage of bubbles and their interaction with the artery) were captured using high-speed imaging. It was found that the presence of the artery resulted in a decrease in lesion size in both the axial and lateral directions. The characteristics of the lesion are dependent on the focus alignment. Acoustic and hydrodynamic cavitation play important roles in lesion production and interaction with the artery. Both thermal and mechanical effects were found on the surface of the artery wall after HIFU ablation. However, no vessel rupture was found in this ex vivo study.

Background: Arterial epistaxis is commonly seen in the rhinology clinic and can have a significant impact on quality of life. Previous studies have explored various therapies to address epistaxis. Methods: Patients were treated in the outpatient clinic or operating room by microwave ablation (MWA) with a 2450‐MHz cooled‐shaft antenna to treat an epistaxis bleeding point. Results: Of the 481 patients with arterial epistaxis, bleeding was controlled within 1 to 2 minutes and only 2 patients with recurrent bleeding required re‐ablation. The shape of the thermal lesion was elliptical and it was approximately 2 mm in length, 1 mm in width, and 0.5 to 1 mm in penetration depth. Of the 481 patients, 167 (34.7%) complained of minor nasal pain on the same day of ablation. No patients complained of nasal pain or obstruction during the follow‐up period. In addition, 139 (28.9%) patients showed a minor increase in rhinorrhea during the first postoperative week. Of the 481 patients, 469 (97.5%) were followed‐up at 6 months with no severe MWA‐related complications, such as septal perforation, synechiae formation, or orbit and brain complications. Conclusion: MWA is a technically feasible alternative method for patients with arterial epistaxis. [ABSTRACT FROM AUTHOR]

To improve wound healing, low-level laser therapy (LLLT) is used increasingly in clinical routine. We treated 6 children, ages 1 to 7, with superficial thermal injuries (scalding) of the face immediately upon admission and daily thereafter with a Ga:As-diode laser. All patients showed rapid healing with excellent cosmetic results. [ABSTRACT FROM AUTHOR]

In this study, a microwave-induced ablation zone (thermal lesion) monitoring method based on ultrasound echo decorrelation imaging was proposed. A total of 15 cases of ex vivo porcine liver microwave ablation (MWA) experiments were carried out. Ultrasound radiofrequency (RF) signals at different times during MWA were acquired using a commercial clinical ultrasound scanner with a 7.5-MHz linear-array transducer. Instantaneous and cumulative echo decorrelation images of two adjacent frames of RF data were calculated. Polynomial approximation images were obtained on the basis of the thresholded cumulative echo decorrelation images. Experimental results showed that the instantaneous echo decorrelation images outperformed conventional B-mode images in monitoring microwave-induced thermal lesions. Using gross pathology measurements as the reference standard, the estimation of thermal lesions using the polynomial approximation images yielded an average accuracy of 88.60%. We concluded that instantaneous ultrasound echo decorrelation imaging is capable of monitoring the change of thermal lesions during MWA, and cumulative ultrasound echo decorrelation imaging and polynomial approximation imaging are feasible for quantitatively depicting thermal lesions.

Several studies have been conducted on the applicability of hyperthermia radiofrequency in the treatment of liver tumors. Many theoretical studies have reported the relevance of various physical parameters in terms of their efficacy in combating tumors and have analyzed the impact of these physical parameters on the temperature profile in the diseased tissue. Parameters such as thermal and electrical conductivities have been investigated during simulations of thermal ablation. Such parameters play an important role in the process of heat transfer in tissues. The purpose of this study is to predict the lesion volume, considering the inclusion of temperature dependence of thermal-electrical properties. This paper introduces a three-dimensional computational model that includes different comparative combinations of tissue thermal-electrical parameters as a mapping of temperature (such as thermal and electrical conductivities and specific heat). The finite-element method is employed for simulating hepatic radiofrequency ablation through the numerical solutions of the bioheat, Laplace, and Navier-Stokes equations. The results suggest that different combinations of tissue temperature-dependent parameters can significantly affect the computed lesion volume and that the temperature dependence of electrical conductivity has a major impact on the computed lesion volume and temperature distribution. [ABSTRACT FROM AUTHOR]

Hot beverage consumption is a risk factor for esophageal squamous cell carcinoma, but the underlying mechanisms are still unknown. We developed an experimental mouse model to understand the mechanism of thermal lesion to esophageal carcinogenesis. Female BALB/c mice were treated by gavage with water at different temperatures three times a week and nitrosamines in the drinking water. Water at 70 °C, but not at lower temperatures, initially induced an esophageal necrosis that healed and became resistant to necrosis after further administrations. However, when 70 °C water was associated with N-nitrosodiethylamine at doses above 1 ppm, there was interference in epithelial regeneration, allowing recurrent thermal injury and inflammation. Recurrent thermal injury resulted in hyper proliferative premalignant lesions being induced earlier (at 4 weeks) and at a higher frequency (4-fold increase at 16 weeks) when compared to mice treated with NDEA only. Ki-67 immunostaining revealed that recurrent thermal injury induced basal cell proliferation resulting in the expansion of epithelial basal cells, confirmed by the increase in cytokeratin 14 positive cells with concomitant reduction of differentiated cytokeratin 5 positive cells. We conclude that recurrent thermal lesion may act as a tumor promoter though a strong proliferation stimulus of esophageal epithelial basal cells. [ABSTRACT FROM AUTHOR]

The ability of N-isopropylacrylamide (NIPAM)-based hydrogel phantoms to mimic tissues with different acoustic and thermal properties under high-intensity focused ultrasound (HIFU) ablation was investigated. These phantoms were designed to model the formation of thermal lesions in tissues above the threshold temperature of protein denaturation. By adjusting the concentration of acrylic acid (AAc) in the NIPAM-based hydrogel phantoms, the cloud point (i.e., lower critical solution temperature) of the phantoms could be tailored to produce HIFU thermal lesions similar to those formed in different swine tissues in terms of size and shape. Additionally, energy thresholds for inducing transient or permanent bubbles in the phantoms during HIFU ablation were also identified to shed light on the onset of cavitation or material damage. The NIPAM-based hydrogel phantoms developed in this study possess major advantages such as transparent, reusable and tailorable properties, and are practical tools for characterizing an ablative device (or treatment) to determine its efficacy and safety. [ABSTRACT FROM AUTHOR]

A recipe was created to improve the tissue-mimicking (TM) bovine serum albumin (BSA) polyacrylamide hydrogel (PAG) reported in our previous study (Choi MJ, Guntur SR, Lee KI, Paeng DG, Coleman AJ. Ultrasound Med Biol 2013; 29:439–448). In that work, the concentration of acrylamide in TM BSA PAG was increased to make its attenuation coefficient the same as that of a tissue. However, this increase made the PAG stiffer and less homogeneous. In addition, the increase in acrylamide caused a significant increase in temperature over the denaturation threshold of BSA during polymerization, which required forced cooling so that the PAG did not become opaque at room temperature after polymerization. To eliminate those shortcomings, we substituted the increased acrylamide with a viscous polysaccharide liquid (corn syrup). The concentration of corn syrup was optimized to 20% (w/v, tested in the volume of 50 mL), so that the acoustic properties of the PAG would be close to those of human liver. The improved TM (iTM) BSA PAG constructed in this study had a speed of sound of 1588 ± 9 m/s, an attenuation coefficient of 0.51 ± 0.06 dB cm −1 at 1 MHz and a backscattering coefficient of 0.22 ± 0.09 × 10 −3 sr −1 cm −1 MHz −1 . The density and acoustic impedance were 1057 kg/m 3 and 1.68 MRayl, respectively, and the non-linear parameter ( B / A ) was 5.9 ± 0.3. The thermal, optical and mechanical properties were almost the same as those of the BSA PAG ( Lafon et al.2005 ). Experimental verification indicated that the thermal lesions visualized in the proposed iTM BSA PAG by high-intensity focused ultrasound were highly reproducible. In conclusion, iTM BSA PAG was proven to eliminate TM BSA PAG shortcomings effectively and is expected to be a promising test phantom for clinical high-intensity focused ultrasound device. [ABSTRACT FROM AUTHOR]

Highlights: [•] Ultrasound RF data of normal and coagulated porcine liver tissues is collected. [•] Wavelet analysis is used to compute the mean scatterer spacing (MSS) of normal and coagulated tissues. [•] Microwave-induced thermal lesions are detected by MSS differences between normal tissues and thermal lesions. [•] We propose a new method for evaluation of microwave-induced thermal lesions. [Copyright &y& Elsevier]

Abstract: An optically transparent tissue-mimicking (TM) phantom whose acoustic properties are close to those of tissue was constructed for visualizing therapeutic effects by high intensity focused ultrasound (HIFU). The TM phantom was designed to improve a widely used standard bovine serum albumin (BSA) polyacrylamide hydrogel (PAG), which attenuated ultrasound far less than tissue and, unlike tissue, did not scatter ultrasound. A modified recipe has been proposed in the study by adding scattering glass beads with diameters of 40–80 μm (0.002% w/v) and by raising the concentration of acrylamide (30% v/v). The TM BSA-PAG constructed has an acoustic impedance of 1.67 MRayls, a speed of sound of 1576 m/s, an attenuation coefficient of 0.52 dB/cm at 1 MHz, a backscattering coefficient of 0.242 × 10−3 1/sr/cm at 1 MHz and a nonlinear parameter (B/A) of 5.7. These parameters are close to those of liver. The thermal and optical properties are almost the same as the standard BSA-PAG. The characteristic features of the thermal lesions by HIFU were observed to be more accurately visualized in the TM BSA-PAG than in the standard BSA-PAG. In conclusion, the proposed TM BSA-PAG acoustically mimics tissue better than the standard BSA-PAG and is expected to be preferentially used for assuring if a clinical HIFU device produces the thermal lesion as planned. [Copyright &y& Elsevier]

Background: Pulmonary vein isolation (PVI) is an established treatment option for paroxysmal atrial fibrillation. Recently, a novel endoscopic ablation system (EAS) was introduced. The incidence and severity of ablation-induced esophageal thermal lesions (ETLs) are unknown. Objective: The purpose of this study was to investigate the incidence and severity of ablation-induced ETLs. Methods: This prospective analysis compared two groups: group A underwent EAS-based PVI, and group B received PVI using radio frequency current (RFC). If esophageal temperature measured via an endoluminal temperature probe exceeded 38.5°C, energy application was stopped immediately. Endoscopy was performed 2 days postablation. Results: Forty patients (20 female, mean age 56 ± 10 years) were treated with the EAS system (group A). In 20 patients (seven female, mean age 63 ± 9 years) RFC-based PVI was performed (group B). Esophageal endoscopy was performed 2.1 ± 0.8 (group A) and 2.8 ± 1.2 days postablation (group B), respectively. In 7/40 patients (18%) in group A, thermal lesions (four thermal ulcerations, three minimal thermal lesions) were detected. In group B, 3/20 patients (15%) displayed thermal lesions (three minimal thermal lesions). All thermal lesions resolved upon repeat endoscopy performed 8 ± 6 (group A) and 5 ± 1 days (group B) after initial endoscopy. Conclusion: In the EAS group, ETLs were found in 18% of patients, as compared with in 15% in the RFC group. In patients with ETLs, ulcerations were found in 57% in the EAS group, as compared with 0% in the RFC group. This may indicate that the quality of thermal lesions is more severe in the EAS group. Further investigation is required. [ABSTRACT FROM AUTHOR]

Abstract: Ultrasound B-mode visualization of lesions produced in soft tissues using high-intensity focused ultrasound (HIFU) has been shown to be challenging when there is no cavitation activity and, therefore, no hyperechogenecity in the focal region. We investigated a method for the visualization and localization of HIFU-induced lesions after HIFU delivery was complete based on the change in backscattered radio-frequency (RF) signals. A HIFU transducer was used with focal dimension of 8 mm by 2 mm working at 5 MHz. HIFU was applied at different intensities to produce lesions in ex vivo chicken breast, with or without the generation of hyperecho in B-mode images. We compared lesion locations obtained from our RF-processing method, from measurement of physical lesions after exposure and from the B-mode images, if exposures had resulted in hyperecho. The results showed that the RF amplitude decreased as a function of time immediately after stopping the HIFU exposure. The lesions were clearly visualized in two-dimensional (2-D) images of the decay rate of RF amplitude, no matter with or without hyperecho. In experiments with hyperecho, when comparing to physical lesion locations, there was no statistically significant difference in the localization accuracy between the RF-based and the hyperecho-based method (p = 0.76). In cases without hyperecho, the distance between RF-based locations and measured lesion locations was 3.37 ± 1.59 mm (mean ± standard deviation). The axial and lateral difference were 2.00 ± 2.31 mm and 0.85 ± 2.15 mm, respectively, and no statistically significant difference was found between lesion coordinates (axial: p = 0.37 and lateral: p = 0.15). We demonstrated the feasibility of our proposed RF-based method for the localization of HIFU-induced lesions immediately after HIFU treatment. Using the decay rate in RF amplitude as the signature of lesion formation, our method can detect lesion locations even without the appearance of hyperecho. (E-mail: xlzheng@u.washington.edu) [Copyright &y& Elsevier]

The present study aims to identify a new recipe for reusable tissue mimicking phantoms that allows the optical visualization of thermal lesions produced in various applications of therapeutic ultrasound where thermal mechanisms are important. The phantom was made of polyacrylamide hydrogel containing a nonionic surface-active agent (NiSAA) as a temperature-sensitive indicator. Threshold temperature above which a thermal lesion is regarded to be formed in the phantom is controlled by selecting an NiSAA. In the present study, three NiSAAs of polyoxyethylene alkyl ether series with nominal clouding points of 66 °C, 70°C, and 80 °C were chosen. Test phantoms were prepared with polyacrylamide hydrogel, corn syrup and NiSAAs [5% (w/v)]. Key acoustic properties of the three NiSAA hydrogels were found to be similar to those of human liver. The phantoms were optically transparent at room temperature (25 °C) and became opaque after exceeding the clouding points. The transparency was recovered on cooling, although the system demonstrated hysteresis. The phantoms were tested both in their ability to provide visualization of thermal lesions produced by high-intensity focused ultrasound and also to examine any characteristic differences in the shape of the lesions formed at different threshold temperatures. The present study suggests that the NiSAA polyacrylamide hydrogel will be of a practical use in quality assurance in various applications of therapeutic ultrasound where thermal mechanisms are important. [ABSTRACT FROM AUTHOR]

A volumetric sonication method is proposed that produces volume ablations by steering the focal point along a predetermined trajectory consisting of multiple concentric outward-moving circles. This method was tested in vivo on pig thigh muscle (32 ablations in nine animals). Trajectory diameters were 4, 12, and 16 mm with sonication duration depending on the trajectory size and ranging from 20 to 73 s. Despite the larger trajectories requiring more energy to reach necrosis within the desired volume, the ablated volume per unit applied energy increased with trajectory size, indicating improved treatment efficiency for larger trajectories. The higher amounts of energy required for the larger trajectories also increased the risk of off-focus heating, especially along the beam axis in the near field. To avoid related adverse effects, rapid volumetric multiplane MR thermometry was introduced for simultaneous monitoring of the temperature and thermal dose evolution along the beam axis and in the near field, as well as in the target region with a total coverage of six slices acquired every 3 s. An excellent correlation was observed between the thermal dose and both the nonperfused (R=0.929 for the diameter and R=0.964 for the length) and oedematous (R=0.913 for the diameter and R=0.939 for the length) volumes as seen in contrast-enhanced T1-weighted difference images and T2-weighted postsonication images, respectively. Histology confirmed the presence of a homogeneous necrosis inside the heated volumes. These results show that volumetric high-intensity focused ultrasound (HIFU) sonication allows for efficiently creating large thermal lesions while reducing treatment duration and also that the rapid multiplane MR thermometry improves the safety of the therapeutic procedure by monitoring temperature evolution both inside as well as outside the targeted volume. [ABSTRACT FROM AUTHOR]

Abstract: The viscoelastic characteristics of thermal lesions in ex vivo animal liver are investigated in this paper. Characterization of the moduli of thermal lesions prepared at several temperatures will provide additional information for the elastographic monitoring of radio frequency ablation of hepatic tumors. In this study, the frequency-dependent complex modulus of thermal lesions prepared at temperatures ranging from 60 to 90°C over a frequency range from 0.1 to 50Hz are presented. Lesions were prepared using either radio frequency ablation or double immersion boiling. It was found that both the magnitude and phase of the modulus increase with frequency, a behavior that has been noted in the literature. A new result reported shows that the modulus dependence on temperature reveals a local maximum around 70–75°C corresponding to the temperature at which tissue has released most of its water content. The modulus values at temperatures higher than 70°C continued to increase, but the extent of increase depend on animal species and other factors. [Copyright &y& Elsevier]

Sonoelastography has been developed as an ultrasound-based elasticity imaging technique. In this technique, external vibration is induced into the target tissue. In general, tissue stiffness is inversely proportional to the amplitude of tissue vibration. Imaging tissue vibration will provide the elasticity distribution in the target region. This study investigated the feasibility of using real-time sonoelastography to detect and estimate the volume of thermal lesions in porcine livers in vivo. A total of 32 thermal lesions with volumes ranging from 0.2 to 5.3 cm3 were created using radiofrequency ablation (RFA) or high-intensity focused ultrasound (HIFU) technique. Lesions were imaged using sonoelastography and coregistered B-mode ultrasound. Volumes were reconstructed from a sequence of two-dimensional scans. The comparison of sonoelastographic measurements and pathology findings showed good correlation with respect to the area of the lesions (r2=0.8823 for RFA lesions, r2=0.9543 for HIFU lesions). In addition, good correspondence was found between three-dimensional sonoelastography and gross pathology (3.6% underestimate), demonstrating the feasibility of sonoelastography for volume estimation of thermal lesions. These results support that sonoelastography outperforms conventional B-mode ultrasound and could potentially be used for assessment of thermal therapies. [ABSTRACT FROM AUTHOR]

Microwave (MW) ablation therapy is a local treatment by which tumours are destroyed by coagulation from the passage of MWs into cells. The aim of this review is to examine histological results obtained from preclinical and clinical studies. A literature search was undertaken for all studies focusing on MW therapy and in which lesions were excised for a complete histopathological examination after treatment. Two main zones were described after ablative therapy (central and transitional). Both corresponded to specific microscopic characteristics and evolved over time in a precise manner. No viable cells even up to 6 cm in diameter were demonstrated in 93% of lesions after treatment. Microwave therapy is a reliable technique under a variety of clinical situations. Future investigations are needed to compare MW with other ablative techniques to identify factors that influence the effectiveness of the various techniques and to determine specific indications. [ABSTRACT FROM AUTHOR]

Abstract: Study of coagulative lesion formation by high intensity focused ultrasound (HIFU) in tissue usually requires performing a sequence of experiments under different exposure conditions followed by tissue sectioning. This paper, inspired by the pioneering work of Frederic L. Lizzi, reports on the use of the bovine eye lens as a laboratory model to observe visually the development of HIFU-induced lesions. The first part of this work describes the measurement of the lens shape, density, sound speed and attenuation. The measured values were within the range of previously published values. In the second part, HIFU-induced lesion development was observed in real-time and compared with good agreement with theoretical simulation. Theoretical modeling included acoustic propagation, absorptive heating and thermal dose, as well as the experimentally measured lens characteristics. Thus, the transparent eye lens can be used as a laboratory phantom to facilitate the understanding of HIFU treatment in other tissues. (E-mail: peter@apl.washington.edu) [Copyright &y& Elsevier]

High-intensity focused ultrasound (HIFU) has been used to ablate solid tumors and cancers. Because of the hypervascular structure of the tumor and circulating blood inside it, the interaction between the HIFU burst and vessel is a critical issue in the clinical environment. Influences on lesion production and the potential of vessel rupture were investigated in this study for the efficiency and safety of clinical ablation. An extracted porcine artery was embedded in a transparent polyacrylamide gel phantom, with bovine serum albumin (BSA) as an indicator of the thermal lesion, and degassed water was driven through the artery sample. The HIFU focus was aligned to the anterior wall, middle of the artery, and posterior wall. After HIFU ablation, the produced lesion was photographically recorded, and then its size was quantified and compared with that in the gel phantom without artery. In addition, the bubble dynamics (i.e., generation, expansion, motion, and shrinkage of bubbles and their interaction with the artery) were captured using high-speed imaging. It was found that the presence of the artery resulted in a decrease in lesion size in both the axial and lateral directions. The characteristics of the lesion are dependent on the focus alignment. Acoustic and hydrodynamic cavitation play important roles in lesion production and interaction with the artery. Both thermal and mechanical effects were found on the surface of the artery wall after HIFU ablation. However, no vessel rupture was found in this ex vivo study. [ABSTRACT FROM AUTHOR]

According to the traditional method of high intensity focused ultrasound (HIFU) treatment, the acoustic and thermal characteristic parameters of constant temperature (room temperature or body temperature) are used to predict thermal lesion. Based on the nonlinear spherical beam equation (SBE) and Pennes bio-heat transfer equation, and a new acoustic-thermal coupled model is proposed. The constant and temperature-dependent acoustic and thermal characteristic parameters are used to predict thermal lesion, and the predicted lesion area are compared with each other. Moreover, the relationship between harmonic amplitude ratio (P 2 /P 1 ) and thermal lesion is studied. Combined with the known experimental data of acoustic and thermal characteristic parameters of biological tissue and data fitting method, the relationship between acoustic and thermal characteristic parameters and temperature is obtained; and the thermal lesion simulation calculation is carried out by using the acoustic and thermal characteristic parameters under constant temperature and temperature- dependent acoustic and thermal characteristic parameters, respectively. The simulation results show that under the same irradiation condition, the thermal lesion predicted by temperature-dependent acoustic and thermal characteristic parameters is larger than that predicted by traditional method, and the thermal lesion increases with the decrease of harmonic amplitude ratio.

In this study, a microwave-induced ablation zone (thermal lesion) monitoring method based on ultrasound echo decorrelation imaging was proposed. A total of 15 cases of ex vivo porcine liver microwave ablation (MWA) experiments were carried out. Ultrasound radiofrequency (RF) signals at different times during MWA were acquired using a commercial clinical ultrasound scanner with a 7.5-MHz linear-array transducer. Instantaneous and cumulative echo decorrelation images of two adjacent frames of RF data were calculated. Polynomial approximation images were obtained on the basis of the thresholded cumulative echo decorrelation images. Experimental results showed that the instantaneous echo decorrelation images outperformed conventional B-mode images in monitoring microwave-induced thermal lesions. Using gross pathology measurements as the reference standard, the estimation of thermal lesions using the polynomial approximation images yielded an average accuracy of 88.60%. We concluded that instantaneous ultrasound echo decorrelation imaging is capable of monitoring the change of thermal lesions during MWA, and cumulative ultrasound echo decorrelation imaging and polynomial approximation imaging are feasible for quantitatively depicting thermal lesions. [ABSTRACT FROM AUTHOR]