Computer-aided dynamic simulation of microwave-induced thermal distribution in coagulation of liver cancer

Comparison of endovenous microwave ablation versus radiofrequency ablation for lower limb varicose veins

OBJECTIVE

Endovenous microwave ablation (EMA) is a recently developed thermal ablation technique used in the treatment of lower limb varicose veins. However, its efficacy and safety have been largely understudied. In the present study, we sought to explore the clinical results of EMA and radiofrequency ablation (RFA) in treating lower limb varicose veins.

METHODS

Patients who underwent EMA (n = 65) or RFA (n = 46) at our institute from September 2018 to September 2020 were included in this retrospective investigation. The clinical results and complications were evaluated at 1, 3, 6, and 12 months after the procedure. The effects on disease severity and quality of life were evaluated using the venous clinical severity score and chronic venous insufficiency questionnaire (CIVIQ).

RESULTS

The technical success rate was 100% for both experimental groups. Although the operative time between the two groups was comparable, the EMA technique was associated with lower direct costs (P < .001), although also with prolonged hospitalization (P < .001). We found that the use of EMA correlated with more pain at 48 hours postoperatively. Except for the visual analog scale scores, no statistically significant variations were observed in the occurrence of postoperative complications within the first 48 hours postoperatively between the EMA and RFA groups, including paresthesia, ecchymosis, induration, and phlebitis (P > .05). At 4 weeks postoperatively, significantly less pigmentation was observed in the RFA group than in the EMA group (13.04% vs 32.31%; P = .020). However, the pigmentation had resolved in all patients by 12 months postoperatively. The two groups had a reduction in the venous clinical severity scores and an increase in the CIVIQ scores after the procedure. However, the CIVIQ scores within the RFA group had increased more than had those within the EMA group (P < .05). No significant differences were found in recurrence between the two groups (EMA group, 1.54%; RFA group, 2.17%; P = .804).

CONCLUSIONS

Both ablation techniques are safe and effective. RFA is associated with relatively higher treatment costs but shorter hospitalization and better quality of life improvement.

Thulium Fiber Laser Behavior on Tissue During Upper- and Lower-Tract Endourology

PURPOSE OF REVIEW

To present the latest evidence on thulium fiber laser (TFL) effects on tissue, during lithotripsy and ablation, emphasizing on generated temperatures, thermal damage thresholds, incision depths, areas of coagulation, and laser damage.

RECENT FINDINGS

Lasers are frequently utilized during endoscopic treatment of different urological conditions. The holmium:yttrium-aluminum-garnet (Ho:YAG) is most frequently used for various types of stones and soft tissue. The TFL has been recently introduced, offering several advantages. However, its activity on tissue during upper and lower tract endourology is poorly understood. At equivalent power settings, TFL and Ho:YAG generate similar temperature changes during lithotripsy. TFL has a shallow incision depth during tissue ablation. Compared to SP TFL, (cw) TFL results in a broader coagulation zone, whereas SP TFL gives of Ho:YAG-similar incision, and (cw) TFL offers a quick, precise cut with more carbonization.

Managing Urolithiasis with Thulium Fiber Laser: Updated Real-Life Results-A Systematic Review

Thirty-three years ago, pulsed lasers marked the beginning of a new era in endoscopic lithotripsy, and the one that was highlighted because of its potential was the Holmium: YAG laser, which became and still is the gold standard in endourology. Recently, a new laser technology has been accepted for clinical use in lithotripsy: the thulium fiber laser (TFL), showing appealing characteristics not seen before in several preclinical studies. A review of the literature was performed and all relevant in vitro studies and clinical trials until April 2021 were selected. The search came back with 27 clinical experiences (7 full-text clinical trials and 20 peer-reviewed abstracts) and 33 laboratory studies (18 full-text articles and 15 peer-reviewed abstracts). The clinical experiences confirmed the clinical safety of using the wide parameter range of the TFL. This technology demonstrated the performance at a higher ablation speed, the higher ablation efficiency, and the better dust quality of the TFL, as well as reduced stone retropulsion, thus helping to maintain an optimal visibility. No thermal or radiation damage was found. Given the current evidence, we may be facing the future gold standard laser in endoscopic lithotripsy.

A Preclinical System Prototype for Focused Microwave Breast Hyperthermia Guided by Compressive Thermoacoustic Tomography

OBJECTIVE

As a newly developed technique, focused microwave breast hyperthermia (FMBH) can provide accurate and cost-effective treatment of breast tumors with low side effect. A clinically feasible FMBH system requires a guidance technique to monitor the microwave power distribution in the breast. Compressive thermoacoustic tomography (CTT) is a suitable guidance approach for FMBH, which is more cost-effective than MRI. However, no experimental validation based on a realized FMBH-CTT system has been reported, which greatly hinders the further advancement of this novel approach.

METHODS

We developed a preclinical system prototype for the FMBH-CTT technique, containing a microwave phased antenna array, a microwave source, an ultrasound transducer array and associated data acquisition module.

RESULTS

Experimental results employing homogeneous and inhomogeneous breast-mimicking phantoms demonstrate that the CTT technique can offer reliable guidance for the entire process of the FMBH. In addition, small phase noises do not deteriorate the overall performance of the system prototype.

CONCLUSION

The realized preclinical FMBH-CTT system prototype is capable for noninvasive, accurate and low-side-effect breast tumor treatment with effective guidance.

SIGNIFICANCE

The experimentally validated FMBH-CTT system prototype provides a feasible paradigm for CTT guided FMBH, establishes a practical platform for future improvement of this technique, and paves the way for potential clinical translation.

Temperature changes during laser lithotripsy with Ho:YAG laser and novel Tm-fiber laser: a comparative in-vitro study

AIM

The aim of this study was to compare the thermal effects of Ho:YAG and Tm-fiber lasers during lithotripsy in an in-vitro model via real-time temperature measurement.

METHODS

We compared a Ho:YAG laser (p_av up to 100 W, Lumenis, Yokneam, Israel) and a superpulse Tm-fiber laser (SP TFL, p_av up to 40 W, NTO IRE-Polus, Fryazino, Russia), both equipped with 200 µm bare-ended fibers. The following settings were used: 0.2 J, 40 Hz (nominal p_av 8 W). Power meter FieldMaxII-TO (Coherent, Santa Clara, CA, USA) was used to verify output laser power (p_av). Each laser was fired for 60 s in two setups: (1) thermos-insulated (quasi-adiabatic) cuvette; (2) actively irrigated setup with precise flow control (irrigation rates 0, 10, 35 mL/min).

RESULTS

Power measurements performed before the test revealed a 10% power drop in Ho:YAG (up to 7.2 ± 0.1 W) and 6.25% power drop in SP TFL (up to 7.5 ± 0.1). At the second step of our experiment, irrigation reduced the respective temperatures in the same manner for both lasers (e.g., at 35 mL/s SP TFL - 1.9 °C; for Ho:YAG laser - 2.8 °C at 60 s).

CONCLUSION

SP TFL and Ho:YAG lasers are not different in terms of volume-averaged temperature increase when the same settings are used in both lasers. Local temperature rises may fluctuate to some degree and differ for the two lasers due to varying jet streaming caused by non-uniform heating of the aqueous medium by laser light.

Conformal coverage of liver tumors by the thermal coagulation zone in 2450-MHz microwave ablation

Purpose: To optimize treatment schemes using 2450-MHz microwave ablation (MWA), a novel conformal coverage method based on bipolar-angle mapping is proposed that determines whether a liver tumor is completely encompassed by thermal coagulation zones. Materials and methods: Firstly, three-dimensional (3-D) triangular mesh data of liver tumors were reconstructed from clinical computed tomography (CT) slices using the Marching cubes (MC) algorithm. Secondly, characterization models of thermal coagulation zones were established based on finite element simulation results of 40, 45, 50, 55, and 60 W ablations. Finally, coagulation zone models and tumor surface data were mapped and fused on a two-dimensional (2-D) plane to achieve conformal coverage of liver tumors by comparing the corresponding polar radii. Results: Optimal parameters for ablation treatment of liver tumors were efficiently obtained with the proposed conformal coverage method. Fifteen liver tumors were obtained with maximal diameters of 12.329-78.612 mm (mean ± standard deviation, 39.094 ± 19.447 mm). The insertion positions and orientations of the MWA antenna were determined based on 3-D reconstruction results of these tumors. The ablation patterns and durations of tumors were planned according to the minimum mean standard deviations between the ablative margin and tumor surface. Conclusion: The proposed method can be applied to computer-assisted MWA treatment planning of liver tumors, and is expected to guide clinical procedures in future.

Characteristics of a Surgical Snare Using Microwave Energy

Currently, minimally invasive treatments that insert various treatment devices into an endoscope are actively being performed. A high-frequency (HF) snare is commonly used as an energy device inserted into an endoscope. However, using a high-frequency snare, problems usually occur, such as the obstruction of the visual field caused by smoke. On the other hand, microwave heating produces less smoke and provides a better visual field. In this study, a snare using microwave energy inserted into an endoscope is proposed, and its characteristics are evaluated.

Ultrasound-guided microwave ablation in the treatment of benign thyroid nodules in 435 patients

The objective of the present study was to investigate the effectiveness and safety of ultrasound-guided microwave ablation in the treatment of benign thyroid nodules. A total of 474 benign thyroid nodules in 435 patients who underwent ultrasound-guided microwave ablation from September 2012 to August 2015 were included. Nodule volume and thyroid function were measured before treatment and at 1, 3, 6, and 12 months and subsequently after every 6 months. The nodule volume reduction rate and changes of thyroid function were evaluated. The volume of all thyroid nodules significantly decreased after ultrasound-guided microwave ablation. The average volume was 13.07 ± 0.95 ml before treatment, and 1.14 ± 0.26 ml at 12-months follow-up. The mean volume reduction rate was 90% and the final volume reduction rate was 94%. The volume reduction rate of mainly cystic nodules was significantly higher than that of simple solid and mainly solid nodules (all P < 0.05). The pretreatment volume of nodules was positively correlated with the final volume reduction rate at final follow-up ( P = 0.004). No serious complications were observed after treatment. In conclusion, ultrasound-guided microwave ablation is an effective and safe technique for treatment of benign thyroid nodules, and has the potential for clinical applications. Impact statement Ultrasound-guided MWA is an effective and safe technique for the treatment of benign thyroid nodules. It can significantly reduce the nodule volume, improve the patients' clinical symptoms, has less complication, guarantees quick recovery, meets patients' aesthetic needs, and shows less interference on the physiological and psychological aspects of the body. MWA should be a good complement to traditional open surgery and has potentials in clinical applications.

Radiofrequency ablation vs. microwave ablation for patients with benign thyroid nodules: a propensity score matching study

PURPOSE

To compare the efficacy and the safety of radiofrequency ablation and microwave ablation for treatment of benign thyroid nodules using a propensity score matching study design.

METHODS

Two hundred and sixty patients with benign thyroid nodules were studied retrospectively, including 102 patients treated with radiofrequency ablation and 158 treated with microwave ablation. To reduce confounding bias due to retrospective assignment, propensity score matching was performed to balance the preablation data of the two groups. After matching, a total of 102 patient pairs (1:1) were created. The volume reduction ratio, therapeutic success rate, symptom and cosmetic score, and major complication were compared between the two groups at 1, 3, 6, and 12 months after treatment.

RESULTS

Between the well-matched groups, no significant differences were found in all nodule volume-related end points at 6 months (volume reduction ratio: 79.4 vs. 77.2 %, P = 0.108; symptom score: 2.1 vs. 1.9, P = 0.456; cosmetic score: 2.1 vs. 2.3, P = 0.119; therapeutic success rate: 99 vs. 97 %, P = 0.621) and 12 months (volume reduction ratio: 83.6 vs. 81.6 %, P = 0.144; symptom score: 1.5 vs. 1.5, P = 0.869; cosmetic score: 1.6 vs. 1.7, P = 0.409; therapeutic success rate: 100 vs. 100 %, P > 0.99) after treatment. No major complications occurred in either group (P > 0.99).

CONCLUSIONS

With well-matched groups and consistent procedure design, our results demonstrated that the volume reduction ratio, therapeutic success rate, symptom and cosmetic score, and complications related to treatment for the two techniques are equivalent. Radiofrequency ablation and microwave ablation are both effective and safe methods in treating benign thyroid nodules.

Ultrasound-Guided Percutaneous Microwave Ablation for Solid Benign Thyroid Nodules: Comparison of MWA versus Control Group

BACKGROUND

The aim of this research is to investigate the feasibility of percutaneous ultrasound-guided microwave ablation (MWA) for benign solid thyroid nodules.

METHODS

Ultrasound-guided percutaneous microwave ablation was performed for 90 benign solid thyroid nodules in 75 patients. The volume changes of the nodules were evaluated before and after microwave ablation, and the cosmetic grading and clinical symptoms were assessed as well.

RESULTS

The volume of all the 90 benign thyroid nodules obviously decreased after microwave ablation at 3-, 6-, 9-, and 12-month follow-ups (p < 0.01), while that of the control group increased at the follow-up of 12 months (p < 0.01). The volume reduction rate (VRR) at 3-, 6-, 9-, and 12-month follow-ups was 55.98%, 69.31%, 76.65%, and 84.67% in the MWA group, respectively. The cosmetic problems and clinical symptoms were also improved in the MWA group. All the patients are well tolerated to the procedure. Hoarseness occurred in 2 cases (2.7%) and Horner syndrome in 1 case (1.3%), and 1 patient (1.3%) developed slight burn on cervical skin.

CONCLUSIONS

Ultrasound-guided percutaneous microwave ablation is a practical method for treating benign solid thyroid nodules, and the complications were acceptable. The trial is registered with clinicaltrials.gov with the registration number NCT03057925.

Treatment planning in microwave thermal ablation: clinical gaps and recent research advances

Microwave thermal ablation (MTA) is a minimally invasive therapeutic technique aimed at destroying pathologic tissues through a very high temperature increase induced by the absorption of an electromagnetic field at microwave (MW) frequencies. Open problems, which are delaying MTA applications in clinical practice, are mainly linked to the extremely high temperatures, up to 120 °C, reached by the tissue close to the antenna applicator, as well as to the ability of foreseeing and controlling the shape and dimension of the thermally ablated area. Recent research was devoted to the characterisation of dielectric, thermal and physical properties of tissue looking at their changes with the increasing temperature, looking for possible developments of reliable, automatic and personalised treatment planning. In this paper, a review of the recently obtained results as well as new unpublished data will be presented and discussed.

Efficacy of Ablation Therapy for Secondary Hyperparathyroidism by Ultrasound Guided Percutaneous Thermoablation

The objective of this study was to explore the value of ultrasound-guided percutaneous microwave thermoablation to treat secondary hyperparathyroidism (SHPT). One hundred and thirty-eight parathyroid glands from 56 patients with SHPT were ablated in this study. All the parathyroid glands were evaluated by real-time contrast-enhanced ultrasound before, during and after ablation. Changes in serum parathyroid hormone (sPTH) levels were measured before treatment and at 1 h, 1 wk, 1 mo and 6 mo after thermoablation treatment. All 56 cases had a 1-mo follow-up, and 34 cases had a 6-mo follow-up. The sPTH level of the 54 cases 1 mo after ablation was significantly lower than that before (p < 0.05). In the 34 cases that had a 6-mo follow-up, the sPTH levels were also significantly lower at 6 mo after ablation than before (p < 0.05). Bone pain in patients improved post-operatively (p < 0.05), but itchiness and insomnia did not improve (p > 0.05). Ultrasound-guided percutaneous microwave thermoablation is a feasible and effective non-surgical alternative treatment for SHPT patients.

Locoregional Therapies for Primary and Secondary Hepatic Malignancies

Management of hepatic malignancies is a multidisciplinary task with the involvement of hepatologists, medical/surgical oncologists, transplant surgeons, and interventional radiologists. The patients should be selected for a specific targeted therapy after multidisciplinary consensus. Interventional oncology has established its role in the management of hepatic malignancies. Image-guided locoregional therapies decrease the rate of systemic toxicity without compromising tumoricidal effect.

GNRs@SiO₂-FA in combination with radiotherapy induces the apoptosis of HepG2 cells by modulating the expression of apoptosis-related proteins

The aim of the present study was to examine the apoptosis of the hepatocellular carcinoma cell line, HepG2, induced by treatment with folic acid-conjugated silica-coated gold nanorods (GNRs@SiO₂-FA) in combination with radiotherapy, and to determine the involvement of apoptosis-related proteins. An MTT colorimetric assay was used to assess the biocompatibility of GNRs@SiO₂-FA. The distribution of GNRs@SiO₂-FA into the cells was observed using transmission electron microscopy (TEM). HepG2 cells cultured in vitro were divided into the following 4 groups: i)the control group (untreated), ii) the GNRs@SiO₂-FA group, iii) the radiotherapy group (iodine 125 seeds) and iv) the combination group (treated with GNRs@SiO₂-FA and iodine 125 seeds) groups. The apoptosis of the HepG2 cells was detected by flow cytometry. The concentration range of <40 µg/ml GNRs@SiO₂-FA was found to be safe for the biological activity of the HepG2 cells. GNRs@SiO₂-FA entered the cytoplasm through endocytosis. The apoptotic rates of the HepG2 cells were higher in the GNRs@SiO₂-FA and radiotherapy groups than in the control group (P<0.05). The apoptotic rate was also significantly higher in the combination group than the GNRs@SiO₂-FA and radiotherapy groups (P<0.05). Taken together, these findings demonstrate that the combination of GNRs@SiO₂-FA and radiotherapy more effectively induces the apoptosis of HepG2 cells. These apoptotic effects are achieved by increasing the protein expression of Bax and caspase-3, and inhibiting the protein expression of Bcl-2 and Ki-67. The combination of GNRs@SiO₂-FA and radiotherapy may thus prove to be a new approach in the treatment of primary liver cancer.

Cell death, perfusion and electrical parameters are critical in models of hepatic radiofrequency ablation

Purpose: A sensitivity analysis has been performed on a mathematical model of radiofrequency ablation (RFA) in the liver. The purpose of this is to identify the most important parameters in the model, defined as those that produce the largest changes in the prediction. This is important in understanding the role of uncertainty and when comparing the model predictions to experimental data. Materials and methods: The Morris method was chosen to perform the sensitivity analysis because it is ideal for models with many parameters or that take a significant length of time to obtain solutions. A comprehensive literature review was performed to obtain ranges over which the model parameters are expected to vary, crucial input information. Results: The most important parameters in predicting the ablation zone size in our model of RFA are those representing the blood perfusion, electrical conductivity and the cell death model. The size of the 50 °C isotherm is sensitive to the electrical properties of tissue while the heat source is active, and to the thermal parameters during cooling. Conclusions: The parameter ranges chosen for the sensitivity analysis are believed to represent all that is currently known about their values in combination. The Morris method is able to compute global parameter sensitivities taking into account the interaction of all parameters, something that has not been done before. Research is needed to better understand the uncertainties in the cell death, electrical conductivity and perfusion models, but the other parameters are only of second order, providing a significant simplification.

Safety assessment and therapeutic efficacy of percutaneous microwave ablation therapy combined with percutaneous ethanol injection for hepatocellular carcinoma adjacent to the gallbladder

OBJECTIVE

This study sought to evaluate the safety and efficacy of ultrasound-guided (US-guided) percutaneous microwave (MW) ablation combined with percutaneous ethanol injection (PEI) to treat liver tumours adjacent to the gallbladder.

MATERIALS AND METHODS

A total of 136 patients with hepatocellular carcinoma (HCC) adjacent to the gallbladder, who underwent ultra-sonographically-guided percutaneous MW ablation, which was combined with PEI in 132 patients, were retrospectively assessed. The patient population characteristics, tumour features, local tumour progression and treatment were compared and analysed. The safety and efficacy of the therapy were assessed by clinical data and imaging in follow-up examinations.

RESULTS

All patients were completely treated with two sessions; 120 patients underwent one session, 16 patients underwent two sessions. The primary technique was effective in 95.6% of the cases, according to the computed tomography (CT) or magnetic resonance imaging (MRI) in the one-month follow-up (132 of 138 sessions). PEI and other therapies were performed in the patients who had been incompletely treated (all six patients underwent PEI, and some underwent other therapies, including one transcatheter arterial chemoembolisation (TACE), one liver transplantation and two liver resections). There was a median follow-up period of 30.1 months and a range of 4 to 68 months. None of the patients had major complications. There were no treatment-related deaths. Twenty-six patients died of primary disease progression that was not directly attributable to MW ablation (19.1%, 26/136). Local tumour progression was noted in five patients (3.7%, 5/136), who had completely ablated tumours at follow-up. The patients with locally progressing tumours underwent additional therapy (three patients underwent PEI, one patient TACE, and one liver resection).

CONCLUSION

Ultrasound-guided percutaneous MW ablation, in combination with percutaneous ethanol injection and thermal monitoring, is a safe and effective treatment for HCC adjacent to the gallbladder.

Comparison of 3D ultrasound and magnetic resonance imaging for microwave ablation in the canine splenomegaly model

PURPOSE

Microwave ablation is used for the treatment of hypersplenism. Image guidance and ablation volume assessment is important to ensure that the ablation is successful. The accuracy of 3D ultrasound (US) and magnetic resonance imaging (MRI) in determining the parameters for microwave ablation were compared in a canine splenomegaly model.

METHODS

Microwave ablation of the spleen was performed on 13 dogs with congestive splenomegaly. Several combinations of power output and ablation time were used: 60 W for 300 s, 50 W for 360 s and 40 W for 450 s. The ablation zone volume was measured by 3D US and 3D MRI immediately after microwave ablation, and at 1, 2 and 8 weeks thereafter.

RESULTS

Compared with 3D MRI, the ablation zone reconstruction rate was lower with 3D US (92 vs. 100%). However, there was no significant difference was found in the ablation volume calculated soon after the treatment and 1 week and 2 months later.

CONCLUSION

3D US may be useful for quantifying the volume of microwave ablation zones in the spleens of experimental animals and appears promising as an alterative modality to MRI for clinical examinations.

Chemothermal therapy for localized heating and ablation of tumor

Chemothermal therapy is a new hyperthermia treatment on tumor using heat released from exothermic chemical reaction between the injected reactants and the diseased tissues. With the highly minimally invasive feature and localized heating performance, this method is expected to overcome the ubiquitous shortcomings encountered by many existing hyperthermia approaches in ablating irregular tumor. This review provides a relatively comprehensive review on the latest advancements and state of the art in chemothermal therapy. The basic principles and features of two typical chemothermal ablation strategies (acid-base neutralization-reaction-enabled thermal ablation and alkali-metal-enabled thermal/chemical ablation) are illustrated. The prospects and possible challenges facing chemothermal ablation are analyzed. The chemothermal therapy is expected to open many clinical possibilities for precise tumor treatment in a minimally invasive way.

Monopolar electrosurgical thermal management for minimizing tissue damage

In this study, a novel thermal management system (TMS) is developed for the minimization of thermal spread created by a monopolar electrosurgical device, the most commonly used surgical instrument. The phenomenon of resistive heating of tissue is modeled using the finite-element method (FEM) to analyze the electrical potential and temperature distributions in biological tissue subjected to heat generation during monopolar electrosurgery. Ex vivo experiments are used to validate the FEM by comparing the model predicted and experimentally measured temperatures. The predicted FEM maximum temperature 1.0 m adjacent to the electrode is within 1% of the experimentally measured maximum temperature using a standard monopolar pencil electrode. A TMS consisting of adjacent cooling channels produces coagulation volumes 80% that of standard monopolar procedures while maintaining comparable temperatures in the targeted tissue below the electrode. In vivo temperatures using a device incorporating a TMS at distances of 2 and 3 m adjacent to the electrode edge are maintained below temperatures known to damage tissue.

Temperature dependence of thermal conductivity of liver based on various experiments and a numerical simulation for RF ablation

Radiofrequency ablation (RFA) for liver cancer has increasingly been used over the past few years because RFA is minimally invasive treatment for patients. However, precise control of the formation of coagulation zones is difficult for operators due to inadequate imaging modalities. With this in mind, we have proposed a model-based robotic ablation system using numerical simulation to analyze temperature distributions in the organ to overcome this deficiency. The objective of our work is to develop a temperature-dependent thermophysical organ model to construct a precise numerical simulator for RFA. However, no standard methods exist for obtaining the thermophysical properties of biological tissues, as detailed evaluations of the accuracy of properties obtained from various experiments have not been completed. The purpose of this study was thus to measure and model the temperature dependence of thermal conductivity in hog liver from three representative methods, and to compare these results using our developed numerical simulator to reveal differences in temperature distributions stemming from differences in thermal conductivities.

Three-dimensional ultrasound image-guided robotic system for accurate microwave coagulation of malignant liver tumours

BACKGROUND

The further application of conventional ultrasound (US) image-guided microwave (MW) ablation of liver cancer is often limited by two-dimensional (2D) imaging, inaccurate needle placement and the resulting skill requirement. The three-dimensional (3D) image-guided robotic-assisted system provides an appealing alternative option, enabling the physician to perform consistent, accurate therapy with improved treatment effectiveness.

METHODS

Our robotic system is constructed by integrating an imaging module, a needle-driven robot, a MW thermal field simulation module, and surgical navigation software in a practical and user-friendly manner. The robot executes precise needle placement based on the 3D model reconstructed from freehand-tracked 2D B-scans. A qualitative slice guidance method for fine registration is introduced to reduce the placement error caused by target motion. By incorporating the 3D MW specific absorption rate (SAR) model into the heat transfer equation, the MW thermal field simulation module determines the MW power level and the coagulation time for improved ablation therapy. Two types of wrists are developed for the robot: a 'remote centre of motion' (RCM) wrist and a non-RCM wrist, which is preferred in real applications.

RESULTS

The needle placement accuracies were < 3 mm for both wrists in the mechanical phantom experiment. The target accuracy for the robot with the RCM wrist was improved to 1.6 +/- 1.0 mm when real-time 2D US feedback was used in the artificial-tissue phantom experiment. By using the slice guidance method, the robot with the non-RCM wrist achieved accuracy of 1.8 +/- 0.9 mm in the ex vivo experiment; even target motion was introduced. In the thermal field experiment, a 5.6% relative mean error was observed between the experimental coagulated neurosis volume and the simulation result.

CONCLUSION

The proposed robotic system holds promise to enhance the clinical performance of percutaneous MW ablation of malignant liver tumours.

Microwave ablation: results with double 915 MHz antennae in ex vivo bovine livers

PURPOSE

To investigate the settings for the optimal microwave ablation geometry with the simultaneous application of double 915 MHz antennae in ex vivo bovine livers, so as to provide the technical basis for treating large liver tumor in one ablation session.

MATERIALS AND METHODS

MWAs were performed on ex vivo bovine livers by simultaneously application of double 915 MHz internally cooled-shaft antennae. Four power settings (50, 60, 70 and 80 W) were used during MWAs, while application time was fixed at 10 min. Three inter-antenna distances (2.0, 2.5 and 3.0 cm) were used. Diameters and shapes of the coagulation zones were observed on gross specimens.

RESULTS

(1) The coagulation shape was related to the inter-antenna distance, which was most spherical at an inter-antenna distance of 2.0 cm. A recess of the coagulation zone was observed at an inter-antenna distances of 2.5 and 3.0 cm. (2) The long-axis and short-axis coagulation diameter enlarged with increasing power output. However, there were no significant differences in the coagulation diameters between 70 and 80 W (P>0.05). More desirable coagulation geometry could be obtained by simultaneous application of double antennae at 70 W for 10 min with an inter-antenna distance of 2.0 cm, the long-axis and short-axis coagulation diameter were 6.95 ± 0.32 cm and 5.30 ± 0.22 cm, respectively.

CONCLUSION

Simultaneous application of double 915 MHz antennae can generate large coagulation zones with desirable shape which may be advantageous for treating large liver tumor in one ablation session.

Practical evaluations on heating characteristics of thin microwave antenna for intracavitary thermal therapy

Microwave thermal therapy is one of the modalities for cancer treatment. There are several schemes of microwave heating. The authors have been studying thin coaxial antenna for intracavitary microwave heating aiming at the treatment of bile duct carcinoma. Up to now, the heating characteristics of the antenna are investigated by numerical simulation and experiment for finding a possibility of the treatment. In this study, in order to consider practical situations of the treatment, heating characteristics of the antenna inserted into a metallic stent is evaluated by numerical simulations. Moreover, the relation between coagulation size of the tissue and the radiation power from the antenna is investigated experimentally. It must be considered, when the input power of the antenna is high (around several tens of watts). From these investigations, some useful results for practical treatments were found.

Finite-element analysis and in vitro experiments of placement configurations using triple antennas in microwave hepatic ablation

This study presents analyses of triple-antenna configurations and designs for microwave (MW) hepatic ablation using 3-D finite-element (FE) analyses verified by in vitro experiments. Treatment of hepatic cancer often requires removal or destruction of large volume lesions. Using multiple antennas offers a potential solution for creating ablation zones with larger dimensions, as well as varied geometrical shapes. We performed both 3-D FE analyses and in vitro experiments using three identical open-tip MW antennas simultaneously, placing them in three types of configurations-"linear array," "triangular," and "T-shaped" arrangements. We compared coagulation volumes created, as well as temperature distribution characteristics, from the three-antenna arrangements after power delivery of 50 W for 60 s. We also performed additional tests using nonidentical antennas (open tip, slot, and slot with insulating jacket) for the three configurations. The results illustrate that arranging antennas in the "T-shaped" pattern destroyed more unwanted tissues than those found when using "linear array" and "triangular" arrangements, with maximum coagulation width and depth of 46 and 81 mm, respectively, and coagulation volume of 30.7 cm(3) . In addition, using nonidentical triple antennas caused variations in coagulation zone characteristics, and thus, the technique could be applied to treatment situations where nonsymmetric coagulation zones are required.

Thermal characteristics of microwave ablation in the vicinity of an arterial bifurcation

PURPOSE

The objective of this research was to reveal the thermal characteristics of microwave ablations in the vicinity of an arterial bifurcation.

METHODS

The temperature distribution after microwave heating of a liver-like material in the close proximity of an arterial bifurcation was simulated using the finite element method. Coupled fluid flow and solid heat transfer were taken into consideration and a three-dimensional analysis was performed. An experimentally determined SAR (specific absorption rate) generated by the absorption of microwaves in liver-like material was used in the analysis instead of utilizing electromagnetic calculations. Several different tests of time-controlled ablations with varying distances between the microwave antenna and the bifurcation were performed and detailed temperature distributions near the bifurcation were obtained.

RESULTS

The interaction between the recirculation flow in the bifurcation and the heat transfer in the surrounding tissue makes the temperature distribution near the bifurcation complicated. Most importantly, after a period of continuous heating with constant microwave output power, the maximum temperatures caused by the ablation did not always increase with the distance between the antenna and the bifurcation.

CONCLUSION

It can be concluded that inadequate ablations can be the result not only from a close proximity between the antenna and the blood vessel, but also from a complicated blood flow in large vessels whose structure causes recirculation flow.

Investigation of the thermal and tissue injury behaviour in microwave thermal therapy using a porcine kidney model

Minimally invasive microwave thermal therapies are being developed for the treatment of small renal cell carcinomas (RCC, d<3 cm). This study assessed the thermal history and corresponding tissue injury patterns resulting from microwave treatment of the porcine renal cortex. Three groups of kidneys were evaluated: (1) in vitro treated, (2) in vivo with 2-h post-treatment perfusion (acute) and (3) in vivo with 7-day post-treatment perfusion (chronic). The kidneys were treated with an interstitial water-cooled microwave probe (Urologix, Plymouth, MN) that created a lesion centered in the renal cortex (50 W for 10 min). The thermal histories were recorded at 0.5 cm radial intervals from the probe axis for correlation with the histologic cellular and vascular injury. The kidneys showed a reproducible 2 cm chronic lesion with distinct histologic injury zones identified. The thermal histories at the edge of these zones were found using Lagrangian interpolation. The threshold thermal histories for microvascular injury and stasis appeared to be lower than that for renal epithelial cell injury. The Arrhenius kinetic injury models were fit to the thermal histories and injury data to determine the kinetic parameters (i.e. activation energy and frequency factor) for the thermal injury processes. The resultant activation energies are consistent in magnitude with those for thermally induced protein denaturation. A 3-D finite element thermal model based on the Pennes bioheat equation was developed and solved using ANSYS (V7.0). The real geometry of the kidneys studied and temperature dependent thermal properties were used in this model. The specific absorption rate (SAR) of the microwave probe required for the thermal modelling was experimentally determined. The results from the thermal modelling suggest that the complicated change of local renal blood perfusion with temperature and time during microwave thermal therapy can be predicted, although a first order kinetic model may be insufficient to capture blood flow changes. The local blood perfusion was found to be a complicated function of temperature and time. A non-linear model based on the degree of vascular stasis was introduced to predict the blood perfusion. In conclusion, interstitial microwave thermal therapy in the normal porcine kidney results in predictable thermal and tissue injury behaviour. Future work in human kidney tissue will be necessary to confirm the clinical significance of these results.

915MHz microwave ablation with high output power in in vivo porcine spleens

OBJECTIVE

The purpose of this study was to evaluate the efficacy of 915 MHz microwave (MW) ablation with high output power in in vivo porcine spleens.

MATERIALS AND METHODS

MW ablations were performed in 9 porcine spleens with an internally cooled 915 MHz antenna. Thermocouples were placed at 5, 10, 15, 20 mm away from the antenna to measure temperatures in real-time during MW emission. The energy was applied for 10 min at high output power of 60 W, 70 W or 80 W. Gross specimens were sectioned and measured to determine ablation size. Representative areas were examined by light microscopy and electron microscopy. Coagulation sizes and temperatures were compared among the three power groups.

RESULTS

Hematoxylin-eosin staining showed irreversible necrosis in the splenic coagulation area after MW ablation. As the power was increased, long-axis diameter enlarged significantly (p<.05). Short-axis diameter also tended to increase, but there were no statistical difference (p>.05). The coagulation size of long-axis and short-axis diameter with 80 W in vivo spleen ablation was 6.43+/-0.52 and 4.95+/-0.30 cm, respectively. With the increase of output power, maximum temperatures at 5, 10, 15, 20 mm from the antenna were increased accordingly (p<.05). The maximum temperature with 80 W at 5 and 20 mm from the antenna reached 146.17+/-6.65 and 72.38+/-4.23 degrees C respectively.

CONCLUSION

With internally cooled antenna and high output power, 915 MHz MW ablation in the spleen could produce irreversible tissue necrosis of clinical significance. MW ablation may be used as a promising minimally invasive method for the treatment of splenic diseases.

Subtraction elastography for the evaluation of ablation-induced lesions: a feasibility study

Different noninvasive or minimally invasive therapeutic ablation procedures can produce tissue necrosis associated with local-stiffness increase. Although elastography has been proved as a potential evaluation tool for many kinds of ablation-induced lesions, the application of subtraction technique in elastography to enhance the visualization of the ablation lesions has rarely been reported. In this paper, subtraction elastography is proposed to evaluate the ablation-induced lesions. Three models are constructed to simulate different kinds of ablated inclusions. The simulation results showed that subtraction elastography is superior to conventional elastography in detecting the ablation-induced lesions with higher signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR). The artifacts induced by elastographic signal processing algorithms can be largely reduced in subtraction elastography. In addition, subtraction elastography is less influenced by the stiff background and can provide more reliable boundary information about the lesion than conventional elastography. Furthermore, the feasibility of subtraction elastography is validated by an in vitro experiment of ethanol-induced hepatic lesions. The preliminary results of this work suggest that subtraction elastography may be a good option for the evaluation of ablationinduced lesions.

Modeling the temperature dependence of thermophysical properties: Study on the effect of temperature dependence for RFA

Radio frequency ablation (RFA) has increasingly been used over the past few years and RFA treatment is minimally invasive for patients. However, it is difficult for operators to control the precise formation of coagulation zones due to inadequate imaging modalities. With this in mind, an ablation system using numerical simulation to analyze the temperature distribution of the organ is needed to overcome this deficiency. The objective of our work is to develop a temperature dependent thermophysical liver model. First, an overview is given of the development of the thermophysical liver model. Second, a simulation to evaluate the effect of temperature dependence of the thermophysical properties of the liver is explained. Finally, the result of the simulation, which indicated that the temperature dependence of thermophysical properties accounts for temperature differences influencing the accuracy of RFA treatment is described.

Effects of variation in perfusion rates and of perfusion models in computational models of radio frequency tumor ablation

PURPOSE

Finite element method (FEM) models are commonly used to simulate radio frequency (RF) tumor ablation. Prior FEM models of RF ablation have either ignored the temperature dependent effect of microvascular perfusion, or implemented the effect using simplified algorithms to reduce computational complexity. In this FEM modeling study, the authors compared the effect of different microvascular perfusion algorithms on ablation zone dimensions with two commercial RF electrodes in hepatic tissue. They also examine the effect of tissue type and inter-patient variation of perfusion on ablation zone dimensions.

METHODS AND MATERIALS

The authors created FEM models of an internally cooled and multi-tined expandable electrode. RF voltage was applied to both electrodes (for 12 or 15 min, respectively) such that the maximum temperature in the model was 105 degrees C. Temperature dependent microvascular perfusion was implemented using three previously reported methodologies: cessation above 60 degrees C, a standard first-order Arrhenius model with decreasing perfusion with increasing degree of vascular stasis, and an Arrhenius model that included the effects of increasing perfusion at the ablation zone boundary due to hyperemia. To examine the effects of interpatient variation, simulations were performed with base line and +/-1 standard deviation values of perfusion. The base line perfusion was also varied to simulate the difference between normal and cirrhotic liver tissue.

RESULTS

The ablation zone volumes with the cessation above 60 degrees C perfusion algorithm and with the more complex Arrhenius model were up to 70% and 25% smaller, respectively, compared to the standard Arrhenius model. Ablation zone volumes were up to 175% and approximately 100% different between the simulations where -1 and +1 standard deviation values of perfusion were used in normal and cirrhotic liver tissue, respectively.

CONCLUSIONS

The choice of microvascular perfusion algorithm has significant effects on final ablation zone dimensions in FEM models of RF ablation. The authors also found that both interpatient variation in base line tissue perfusion and the reduction in perfusion due to cirrhosis have considerable effect on ablation zone dimensions.

Expanding the Bioheat Equation to Include Tissue Internal Water Evaporation During Heating

We propose a new method to study high temperature tissue ablation using an expanded bioheat diffusion equation. An extra term added to the bioheat equation is combined with the specific heat into an effective (temperature dependent) specific heat. It replaces the normal specific heat term in the modified bioheat equation, which can then be used at temperatures where water evaporation is expected to occur. This new equation is used to numerically simulate the microwave ablation of bovine liver and is compared to experimental ex vivo results.

Computer simulation for postmortem cooling processes in the outer ear

INTRODUCTION

A simulation using a computer model was undertaken to investigate postmortem cooling patterns in the outer ear.

METHODS

Cooling patterns were analyzed using a 3-dimensional head model built from brain CT images of a volunteer. The simulation was verified with a case subject under constant environmental conditions to obtain an appropriate heat transmission coefficient.

RESULTS

The cooling pattern of the head model agreed with that of the case subject when the heat transmission coefficient was 6W/m(2) degrees C, and it could be approximated to a single exponential curve.

DISCUSSION

This is the first simulative study to show the postmortem cooling pattern of the head of an adult human. Our head model will prove useful to predict the cooling patterns of not only the outer ear but also of the entirety of the head.

Investigation of the influence of blood flow rate on large vessel cooling in hepatic radiofrequency ablation

Radiofrequency (RF) ablation using high-frequency current has become an important treatment method for patients with non-resectable liver tumors. Tumor recurrence is associated with tissue cooling in the proximity of large blood vessels. This study investigated the influence of blood flow rate on tissue temperature and lesion size during monopolar RF ablation at a distance of 10 mm from single 4- and 6-mm vessels using two different approaches: 1) an ex vivo blood perfusion circuit including an artificial vessel inserted into porcine liver tissue was developed; and 2) a finite element method (FEM) model was created using a novel simplified modeling technique for large blood vessels. Blood temperatures at the inflow/outflow of the vessel and tissue temperatures at 10 and 20 mm from the electrode tip were measured in the ex vivo set-up. Tissue temperature, blood temperature and lesion size were analyzed under physiological, increased and reduced blood-flow conditions. The results show that changes in blood flow rate in large vessels do not significantly affect tissue temperature and lesion size far away from the vessel. Monopolar ablation could not produce lesions surrounding the vessel due to the strong heat-sink effect. Simulated tissue temperatures correlated well with ex vivo measurements, supporting the FEM model.

Radiofrequency versus microwave ablation in a hepatic porcine model

PURPOSE

To compare microwave (MW) and radiofrequency (RF) ablation in a hepatic porcine model.

MATERIALS AND METHODS

Institutional animal research committee approval was obtained. Nineteen pigs were divided into groups based on time of sacrifice (group A, immediate; group B, 2 days; group C, 28 days; group D, 28 days). Groups A, B, and C each underwent a combination of RF and MW ablation. Group D underwent either four MW or four RF ablations. Ablation was performed with a prototype MW device (915 Mhz, 40 W, 10 minutes) and a commercial RF system (150 W, 10 minutes, 3-cm deployment). Computed tomography (CT) was performed in groups B and C at 2 days and in group C at 28 days. Group D underwent serial laboratory testing. Specimens were serially sectioned, and short-axis diameter and length of each were measured. The percentage deflection caused by local blood vessels (heat-sink effect) was also measured in group A. Likelihood ratio tests and unpaired t tests were used for statistical analyses as appropriate.

RESULTS

MW ablation zones were longer at days 0, 2, and 28 (P < .05), but short-axis diameter was not different from that with RF ablation at any time point (P > .05). Local blood vessels caused 3.5% +/- 5.3 (standard deviation) deflection at MW ablation compared with 26.2% +/- 27.9 at RF ablation (P < .05). MW and RF ablation zones were indistinguishable at CT or pathologic evaluation. Laboratory test results were similar between RF ablation-only animals and MW ablation-only animals, with the exception of a slightly higher alkaline phosphatase levels at day 2 in RF ablation-only animals (P < .02).

CONCLUSION

MW and RF ablation zones are similar in pathologic appearance and imaging characteristics. Increased length with MW ablation is likely caused by the length of the radiating segment of the antenna. MW ablation may be less affected by the heat-sink effect that is thought to contribute to local recurrence after RF ablation.

Prognostic factors for survival in patients with hepatocellular carcinoma after percutaneous microwave ablation

PURPOSE

To determine the long-term survival and prognostic factors in patients with hepatocellular carcinoma treated with percutaneous microwave ablation.

MATERIALS AND METHODS

Institutional review board approval and informed consent were obtained. A database of cases of hepatocellular carcinoma in 288 patients (259 men, 29 women; mean age, 54.8 years +/- 11.4 [standard deviation]; age range, 25-82 years) with 477 histologically proved lesions who underwent percutaneous microwave coagulation therapy between May 1994 and October 2002 was retrospectively analyzed. Prognostic factors for survival were evaluated by means of univariate and multivariate analyses.

RESULTS

The mean follow-up period after microwave ablation was 31.41 months +/- 20.43 (range, 5-106 months). The 1-, 2-, 3-, 4-, and 5-year cumulative survival rates among all 288 patients were 93%, 82%, 72%, 63%, and 51%, respectively. Ninety-three patients (32%) died. Local recurrence or new tumors occurred in 100 patients (35%). Age (P = .836), sex (P = .073), preablation serum alpha-fetoprotein level (P = .136), and preablation treatment (P = .256) were not related to prognosis, while tumor number (P = .004), tumor size (P < .001), Child-Pugh classification (P = .003), tumor differentiation (P = .026), and local recurrence or presence of new tumors (P = .004) significantly affected survival at univariate analysis. At multivariate analysis, only tumor size (P < .001), number of nodules (P = .005), and Child-Pugh classification (P = .01) each had a significant effect on survival.

CONCLUSION

With use of microwave ablation, there is a high probability of long-term survival of patients with a single lesion of 4.0 cm or less in maximum diameter and Child-Pugh class A cirrhosis.

Prognostic Factors for Percutaneous Microwave Coagulation Therapy of Hepatic Metastases

OBJECTIVE

Microwave-induced tissue coagulation is a new approach for the local ablation of hepatic metastases. The purposes of the study were to analyze therapeutic results and those influencing factors that might be used to predict survival after percutaneous microwave coagulation therapy. SUBJECTS AND METHODS. From July 1995 to March 2002, 74 patients with 149 hepatic metastases were treated with percutaneous microwave coagulation therapy under sonographic guidance. The largest metastasis in each patient ranged from 0.7 to 6.8 cm (mean, 3.12 cm; SD, 1.81 cm). Survival rates and influencing factors were analyzed.

RESULTS

The cumulative survival rates of all 74 patients were 91.4% at 1 year, 59.5% at 2 years, 46.4% at 3 years, 29% at 4 years, and 29% at 5 years. Patient age (p = 0.46) and sex (p = 0.12) and site of primary malignancies (p = 0.58) were not related to prognosis, whereas tumor grade (p = 0.01), number of metastases (p = 0.00), tumor size (p = 0.03), and local recurrence or new metastasis (p = 0.00) significantly affected survival as single independent factors. Multivariate analysis revealed that tumor grade (p = 0.03), number of metastases (p = 0.025), and local recurrence or new metastasis (p = 0.04) each had a significant effect on survival. No severe complications occurred.

CONCLUSION

The probability of long-term survival for patients with one or two metastases, well-differentiated tumors, and without recurrence or new metastasis may be significantly higher than for other patients.

Percutaneous sonographically guided microwave coagulation therapy for hepatocellular carcinoma: results in 234 patients

OBJECTIVE

The objective of our study was to evaluate the long-term results of percutaneous microwave coagulation therapy for the treatment of hepatocellular carcinoma in a large patient population.

SUBJECTS AND METHODS

Survival rates were determined in 234 patients with 339 nodules of hepatocellular carcinoma who had undergone percutaneous microwave coagulation therapy (208 men, 26 women; mean age, 54.8 years; mean tumor size, 4.1 +/- 1.9 cm; range, 1.2-8.0 cm; mean follow-up period, 27.9 months). Patients were those who had been rejected as candidates for surgery by the surgery department, who fit our study's criteria, and who agreed to participate. After baseline imaging studies were performed, the patients were followed up using the same combination of imaging (sonography, CT, or MR imaging) and posttreatment biopsy.

RESULTS

After percutaneous microwave coagulation therapy, color Doppler flow signals disappeared in 92.0% (263/286) of the lesions. No enhancement was apparent in 89.2% (190/213) and 89.1% (41/46) of the lesions on contrast-enhanced CT and MR imaging, respectively. Posttreatment biopsies of 194 nodules showed no evidence of surviving tumor tissue in 180 nodules (92.8%). Resections of six lesions revealed complete tumor necrosis in five. The 1-, 2-, 3-, 4-, and 5-year cumulative survival rates were 92.70%, 81.60%, 72.85%, 66.37%, and 56.70%, respectively. The relationships between survival curves and the degree of hepatocellular carcinoma tumor differentiation and between survival curves and tumor size were statistically significant (p = 0.021). No severe complications were seen.

CONCLUSION

Sonographically guided microwave coagulation proved to be safe and effective for the treatment of hepatocellular carcinoma. This therapy resulted in a high percentage of cases without evidence of residual tumor and satisfactory long-term results.

Thermal dose optimization method for ultrasound surgery

In this paper, a model-based optimization method is derived to control the thermal dose in biological tissues for ultrasound surgery. The optimization method uses the bioheat equation as a system model and quadratic cost criteria for the desired thermal dose. Time-harmonic quasi-stationary ultrasound fields are used as the heat source. In this method the optimal phase and the amplitude trajectories are found directly by minimizing the associated cost function. The approach also allows for maximum input amplitude constraints. The method is based on the Hamiltonian form of the system and results in a large dimensional nonlinear optimization problem which is solved with a gradient-type iterative scheme. The performance of the optimization method is tested with 2D simulations and it is shown that the approach is able to yield a feasible nominal solution. This nominal evolution would then eventually be sought to be maintained with the help of a feedback controller during the actual sonication.