Microwave Ablation Antenna Research Articles

A 4.9 GHz Low-Power Microwave Ablation Antenna With Bidirectional Localized Ablation Zone

A 4.9 GHz Low-Power Microwave Ablation Antenna With Bidirectional Localized Ablation Zone

Assessment of thermochromic phantoms for characterizing microwave ablation devices.

Thermochromic gel phantoms provide a controlled medium for visual assessment of thermal ablation device performance. However, there are limited studies reporting on the comparative assessment of ablation profiles assessed in thermochromic gel phantoms against those in ex vivo tissue. The objective of this study was to compare microwave ablation zones in a thermochromic tissue-mimicking gel phantom and ex vivo bovine liver and to report on measurements of the temperature-dependent dielectric and thermal properties of the phantom. Thermochromic polyacrylamide phantoms were fabricated following a previously reported protocol. Phantom samples were heated to temperatures in the range of 20°C-90°C in a temperature-controlled water bath, and colorimetric analysis of images of the phantom taken after heating was used to develop a calibration between color changes and the temperature to which the phantom was heated. Using a custom, 2.45GHz water-cooled microwave ablation antenna, ablations were performed in fresh ex vivo liver and phantoms using 65W applied for 5min or 10min (n=3 samples in each medium for each power/time combination). Broadband (500MHz-6GHz) temperature-dependent dielectric and thermal properties of the phantom were measured over the temperature range of 22°C-100°C. Colorimetric analysis showed that the sharp change in gel phantom color commences at a temperature of 57°C. Short and long axes of the ablation zone in the phantom (as assessed by the 57°C isotherm) for 65W, 5min ablations were aligned with the extents of the ablation zone observed in ex vivo bovine liver. However, for the 65W, 10min setting, ablations in the phantom were on average 23.7% smaller in the short axis and 7.4 % smaller in the long axis than those observed in ex vivo liver. Measurements of the temperature-dependent relative permittivity, thermal conductivity, and volumetric heat capacity of the phantom largely followed similar trends to published values for ex vivo liver tissue. Thermochromic tissue-mimicking phantoms provides a controlled, and reproducible medium for comparative assessment of microwave ablation devices and energy delivery settings. However, ablation zone size and shapes in the thermochromic phantom do not accurately represent ablation sizes and shapes observed in ex vivoliver tissue for high energy delivery treatments (65W, 10min). One cause for this limitation is the difference in temperature-dependent thermal and dielectric properties of the thermochromic phantom compared to ex vivobovine liver tissue, as reported in the present study.

Importance of the enhanced cooling system for more spherical ablation zones: Numerical simulation, ex vivo and in vivo validation

IntroductionThis study aimed to investigate the efficacy of a small-gauge microwave ablation antenna (MWA) with an enhanced cooling system (ECS) for generating more spherical ablation zones. MethodsA comparison was made between two types of microwave ablation antennas, one with ECS and the other with a conventional cooling system (CCS). The finite element method was used to simulate in vivo ablation. Two types of antennas were used to create MWA zones for 5, 8, 10 min at 50, 60, and 80 W in ex vivo bovine livers (n = 6) and 5 min at 60 W in vivo porcine livers (n = 16). The overtreatment ratio, ablation aspect ratio, carbonization area, and other characteristcs of antennas were measured and compared using numerical simulation and gross pathologic examination. ResultsIn numerical simulation, the ECS antenna demonstrated a lower overtreatment ratio than the CCS antenna (1.38 vs 1.43 at 50 W 5 min, 1.19 vs 1.35 at 50 W 8 min, 1.13 vs 1.32 at 50 W 10 min, 1.28 vs 1.38 at 60 W 5 min, 1.14 vs 1.32 at 60 W 8 min, 1.10 vs 1.30 at 60 W 10 min). The experiments revealed that the ECS antenna generated ablation zones with a more significant aspect ratio (0.92 ± 0.03 vs 0.72 ± 0.01 at 50 W 5 min, 0.95 ± 0.02 vs 0.70 ± 0.01 at 50 W 8 min, 0.96 ± 0.01 vs 0.71 ± 0.04 at 50 W 10 min, 0.96 ± 0.01 vs 0.73 ± 0.02 at 60 W 5 min, 0.94 ± 0.03 vs 0.71 ± 0.03 at 60 W 8 min, 0.96 ± 0.02 vs 0.69 ± 0.04 at 60 W 10 min) and a smaller carbonization area (0.00 ± 0.00 cm2 vs 0.54 ± 0.06 cm2 at 50 W 5 min, 0.13 ± 0.03 cm2 vs 0.61 ± 0.09 cm2 at 50 W 8 min, 0.23 ± 0.05 cm2 vs 0.73 ± 0.05 m2 at 50 W 10 min, 0.00 ± 0.00 cm2 vs 1.59 ± 0.41 cm2 at 60 W 5 min, 0.23 ± 0.22 cm2 vs 2.11 ± 0.63 cm2 at 60 W 8 min, 0.57 ± 0.09 cm2 vs 2.55 ± 0.51 cm2 at 60 W 10 min). Intraoperative ultrasound images revealed a hypoechoic area instead of a hyperechoic area near the antenna. Hematoxylin-eosin staining of the dissected tissue revealed a correlation between the edge of the ablation zone and that of the hypoechoic area. ConclusionsThe ECS antenna can produce more spherical ablation zones with less charring and a clearer intraoperative ultrasound image of the ablation area than the CCS antenna.

Assessment of thermochromic phantoms for characterizing microwave ablation devices.

Thermochromic gel phantoms provide a controlled medium for visual assessment of thermal ablation device performance. However, there are limited studies reporting on the comparative assessment of ablation profiles assessed in thermochromic gel phantoms against those in ex vivo tissue. The objective of this study was to compare microwave ablation zones in a thermochromic tissue mimicking gel phantom and ex vivo bovine liver, and to report on measurements of the temperature dependent dielectric and thermal properties of the phantom. Thermochromic polyacrylamide phantoms were fabricated following a previously reported protocol. Phantom samples were heated to temperatures in the range of 20 - 90 °C in a temperature-controlled water bath, and colorimetric analysis of images of the phantom taken after heating were used to develop a calibration between color changes and temperature to which the phantom was heated. Using a custom, 2.45 GHz water-cooled microwave ablation antenna, ablations were performed in fresh ex vivo liver and phantoms using 65 W applied for 5 min or 10 min ( n = 3 samples in each medium for each power/time combination). Broadband (500 MHz - 6 GHz) temperature-dependent dielectric and thermal properties of the phantom were measured over the temperature range 22 - 100 °C. Colorimetric analysis showed that the sharp change in gel phantom color commences at a temperature of 57 °C. Short and long axes of the ablation zone in the phantom (as assessed by the 57 °C isotherm) for 65 W, 5 min ablations were aligned with extents of the ablation zone observed in ex vivo bovine liver. However, for the 65 W, 10 min setting, ablations in the phantom were on average 23.7% smaller in short axis and 7.4 % smaller in long axis than those observed in ex vivo liver. Measurements of the temperature dependent relative permittivity, thermal conductivity, and volumetric heat capacity of the phantom largely followed similar trends to published values for ex vivo liver tissue. Thermochromic tissue mimicking phantoms provide a controlled, and reproducible medium for comparative assessment of microwave ablation devices and energy delivery settings, though ablation zone size and shapes may not accurately represent ablation sizes and shapes observed in ex vivo liver tissue under similar conditions.

Design of Microwave Ablation Antenna for Flexible Omni-directional/Directional Ablation Zone Control

Design of Microwave Ablation Antenna for Flexible Omni-directional/Directional Ablation Zone Control

A Flexible Microwave Ablation Antenna for Lung Cancer Treatment

A Flexible Microwave Ablation Antenna for Lung Cancer Treatment

Microwave Antenna Design for Cardiovascular Applications: A Comparison Between a Balanced and Unbalanced Microwave Ablation Antenna

Microwave Antenna Design for Cardiovascular Applications: A Comparison Between a Balanced and Unbalanced Microwave Ablation Antenna

Emprint Microwave Thermoablation System: Bridging Thermal Ablation Efficacy between Human Patients and Porcine Models through Mathematical Correlation.

To investigate the in vivo ablation characteristics of a microwave ablation antenna in the livers of humans with tumors, a retrospective analysis of the ablation zones was conducted after applying Emprint microwave ablation systems for treatment. Percutaneous microwave ablations performed between January 2022 and September 2022 were included in this study. Subsequently, immediate post-ablation echography images were subjected to retrospective evaluation to state the long ablated diameter, short ablated diameter, and volume. The calculated ablation lengths and volume indices were then compared between in vivo and ex vivo results obtained from laboratory experiments conducted on porcine liver. The ex vivo data showed a good correlation between energy delivered and both increasing ablated dimensions (both p < 0.001) and volume (p < 0.001). The in vivo data showed a good correlation for dimensions (p = 0.037 and p = 0.019) and a worse correlation for volume (p = 0.142). When comparing ex vivo and in vivo data for higher energies, the ablated volumes grew much more rapidly in ex vivo cases compared to in vivo ones. Finally, a set of correlations to scale ex vivo results with in vivo ones is presented. This phenomenon was likely due to the absence of perfusion, which acts as a cooling system.

Interaction of acoustic and electromagnetic waves for thermoacoustic imaging and vibration sensing

Acoustic wave and electromagnetic wave can interact and lead to rich physical phenomenon to be used for applications such as medical diagnosis, non-destructive material characterization, and remote sensing. In this talk, I will present our research studies on two ways acoustic and electromagnetic waves interact with each other: (1) When pulsed microwave is absorbed by tissue, thermoacoustic waves are generated through thermal expansion. We explored the sensing and processing of thermoacoustic waves generated by a pulsed microwave ablation antenna as a potential way for monitoring the ablation process in real time. (2) Electromagnetic waves such as Wi-Fi signals and light can be modulated by acoustic/mechanical vibrations in the environment. We developed non-contact vibration sensing systems based on such modulations, and also developed a metamaterial-based vibration amplification device that can enhance the vibration magnitude sensitivity over a broad range of vibration frequencies.

Thermoacoustic signals generated from pulsed microwave ablation in liver tissue

Microwave-induced thermoacoustic signals are generated when pulsed microwave energy is absorbed by tissue. Thermoacoustic signals contain information about the electromagnetic, thermal, and acoustic material properties of the tissue. Thus, we propose using microwave-induced thermoacoustic signals to monitor microwave ablation, a thermal therapy used to heat and kill malignant tissue, in real-time. Conventional microwave-induced thermoacoustic signals are excited using an external waveguide. Our approach excites thermoacoustic signals via pulsed microwave energy delivered by the already present interstitial microwave ablation antenna. In this talk, we will present the experimental investigation of the evolution of microwave-induced thermoacoustic signals generated during microwave ablation in bovine liver tissue. Pulsed microwave energy was delivered via a narrow-diameter coaxial ablation antenna to simultaneously ablate tissue and excite thermoacoustic signals. Trials were conducted in room-temperature fresh liver tissue as well as room-temperature boiled liver tissue to investigate the influence of ablative temperature rise and tissue coagulation on thermoacoustic signals. Our experimental results show thermoacoustic signal characteristics change throughout the course of microwave ablation that could potentially be used for monitoring the microwave ablation process.

Broadband Dielectric Spectroscopy with a Microwave Ablation Antenna

Microwave ablation is a technique used to treat tumorous tissue. Its clinical use has been greatly expanding in the last few years. Because the design of the ablation antenna and the success of the treatment greatly depend on the accurate knowledge of the dielectric properties of the tissue being treated, it is highly valuable to have a microwave ablation antenna that is also able to perform in-situ dielectric spectroscopy. In this work, an open-ended coaxial slot ablation antenna design operating at 5.8 GHz is adopted from previous work, and its sensing abilities and limitations are investigated in respect of the dimensions of the material under test. Numerical simulations were performed to investigate the functionality of the floating sleeve of the antenna and to find the optimal de-embedding model and calibration option for obtaining accurate dielectric properties of the area of interest. Results show that, as in the case of the open-ended coaxial probe, the accuracy of the measurement greatly depends on the likeness between the calibration standards' dielectric properties and the material under test. Finally, the results of this paper clarify to which extent the antenna can be used to measure dielectric properties and paves the way to future improvements and the introduction of this functionality into microwave thermal ablation treatments.

A Minimally Invasive Microwave Ablation Antenna With Highly Localized Ablation Zone

This letter proposes a novel inexpensive microwave ablation (MWA) antenna with a highly localized ablation zone. The antenna is designed based on a substrate integrated coaxial line, at the end of which a serpentine structure is used to radiate microwave power. The serpentine structure can effectively shorten the length of the radiation structure, making the ablation zone more localized. Due to its simple structure, a convenient and inexpensive printed circuit board technology can be used to fabricate the disposable ablation probe. In addition, the proposed topology has a small cross section and, thus, is very suitable for minimally invasive surgery. Simulation analysis and validation experiments have shown that the proposed MWA antenna can provide reliable performance, which means that it can serve as a promising candidate for minimally invasive ablation therapy.

Microwave-induced thermoacoustic signal characteristics in a dynamic temperature environment

We conduct a simulation-based study to investigate the impact of a dynamic temperature environment on the characteristics of microwave-induced thermoacoustic signals. We investigate thermoacoustic signals that are generated using an interstitial microwave ablation antenna powered by a microsecond pulsed microwave source. Two temperature regimes are examined: first, a spatially uniform temperature throughout the medium to experimentally validate the simulation model, and second, the realistic, spatially nonuniform temperature profiles that arise during microwave ablation. We employ a multi-physics model that considers electromagnetics, heat transfer, and acoustic physics to simulate the coupled processes of microwave absorption and heating of the medium and thermoacoustic signal generation and propagation. An interstitial coaxial antenna is used to generate microsecond microwave pulses that simultaneously induce microwave heating and excite thermoacoustic signals via microwave pulse absorption. We find that thermoacoustic signal characteristics are highly temperature-dependent and thus change significantly within an environment where temperature varies through space and time. Furthermore, the temperature-dependent properties within the active region of the antenna drive the evolution of thermoacoustic signal characteristics. Temperature-dependent thermoacoustic signal characteristics can be exploited to track the progress of microwave ablation. Consequently, microwave-induced thermoacoustic imaging is a promising method for monitoring microwave ablation in real-time.

Microwave ablation via a flexible catheter for the treatment of nonsurgical peripheral lung cancer: A pilot study.

BackgroundEndobronchial microwave ablation via flexible catheter offers the potential for local therapy for inoperable peripheral lung cancer. The study aimed to evaluate the feasibility and safety of navigation bronchoscopy‐guided water‐cooled microwave ablation catheter for nonsurgical peripheral lung cancer.MethodsThis was a prospective single arm pilot study. Patients with early stage or multiple primary peripheral lung cancer who were nonsurgical candidates for surgery were enrolled in the study. Bronchoscopic microwave ablation was performed via a flexible water‐cooled microwave ablation antenna under the guidance of navigation bronchoscopy. Radial probe endobronchial ultrasound combined with fluoroscopy was used to confirm the position. Treatment outcomes were evaluated based on follow‐up chest CT and positron emission tomography scans. Primary endpoints were technical success and safety. Secondary endpoints were complete ablation rate, 2‐year local control rate, and progression‐free survival.ResultsThirteen patients were enrolled in the study from April 2018 to July 2019. A total of 19 sessions of microwave ablation were performed on 14 tumors under the guidance of navigation bronchoscopy. The technical success was 100%. Treatment‐related complications occurred in two patients. The complete ablation rate was 78.6% (11/14). The 2‐year local control rate was 71.4%. Median progression‐free survival was 33 months for all patients.ConclusionsIn this pilot study, bronchoscopic microwave ablation appears to be feasible with acceptable occurrence of complication in the treatment of peripheral lung cancer under the guidance of navigation bronchoscopy.

A Study of Microwave Ablation With Hollow Antenna

To solve the problem of the non-spherical ablation zone in microwave ablation, we propose a hollow microwave ablation antenna. Compared with the microwave ablation antenna with coaxial cable as the main body, we use a hollow structure instead of the coaxial cable in the antenna to reduce the external conductor current and achieve a spherical ablation zone. We match between the antenna and the coaxial cable by an L-type matching network. The simulated results show that the roundness of the ablation zone of the hollow antenna is more than 85 % at 20 W and 600 s compared with the coaxial microwave ablation antenna. The hollow antenna operates at 2.45 GHz in the egg white, and the ablation experiment was carried out at the 20 W power for 600 s. The experimental results were in good agreement with the simulated results, which verified the ability of the antenna to generate a spherical ablation zone. The hollow antenna can effectively avoid damaging normal tissue and improve the accuracy and efficiency of microwave ablation.

Study of Single- and Dual-Frequency Microwave Ablation Antennas Based on Shorted Helical Slot

We present single- and dual-frequency antennas based on the shorted helical slot in hepatic microwave ablation. The single-frequency antenna is designed by lathing a helical slot on shorted traditional semirigid coaxial cable, which is expected to be operated at the frequency of 2450 MHz, and the dual-frequency antenna is designed with the combination of an annular slot and a helical slot, which can be operated at 915 and 2450 MHz. We use full-wave electromagnetic simulation and transient thermal simulation to analyze the performance of the proposed ablation antennas. Both the proposed antennas are fabricated and used in <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ex vivo</i> ablation experiments at different frequencies with a power level of 23 W for 2 or 5 min. We observe that the proposed single-frequency antenna with helical slot has more concentrated specific absorption rate (SAR) pattern than those antennas with conventional slots does at 2450 MHz, and the dual-frequency one has the capability to control the roundness of ablation zone from 0.34 to 0.63 under the different combination of frequencies and operation times. The proposed microwave ablation antennas are expected to lead to a potential solution of curing the liver cancers with various characteristics.

Efficacy of Lung-Tuned Monopole Antenna for Microwave Ablation: Analytical Solution and Validation in a Ventilator-Controlled ex Vivo Porcine Lung Model.

The goal of this study was to optimize a lung-tuned monopole antenna to deliver microwave energy at 2.45 GHz into a novel ventilator-controlled ex vivo lung model. An analytic and parametric approach was utilized to create an optimized monopole antenna that was impedance-matched to aerated lung tissue. This lung-tuned antenna was then fabricated using a copper 0.085" semi-rigid copper coaxial cable. For validation, the lung-tuned antenna was inserted centrally into lobes of a ex vivo porcine lung that was fully inflated to physiologically appropriate volumes. Microwave ablations were then created at 50 and 100 W for 1 minute and 5 minutes. Reflected power, cross sectional ablation sizes and spherical shape of the lung-tuned antenna were compared against a liver-tuned antenna in the ventilator-controlled ex vivo lung tissue. The study showed that the lung-tuned antennas delivered energy significantly more efficiently, with less reflected power, compared to the conventionally-used liver-tuned antennas at 50 W at 1 minute (11.8±3.0 vs 16.3±3.1 W; p value=0.03) and 5 minutes (16.2±2.8 vs 19.4±2.9 W; p value=0.04), although this was only true using 100 W at the 1 minute time point (29.0±3.5 vs 38.0±5.3 W; p value=0.02). While overall ablation zone sizes were comparable between the two types of antenna, the lung-tuned antenna did create a significantly more spherical ablation zone compared to the liver-tuned antenna at the 1 minute, 50 W setting (aspect ratio: 0.43±0.07 vs 0.38±0.04; p value=0.04). In both antenna groups, there was a significant rise in the ablation zone aspect ratio between 1 and 5 minutes, indicating that higher power and time settings can increase the spherical shape of ablation zones when using tuned antennas. Adapting this combined analytic and parametric approach to antenna design can be implemented in adaptive tissue-tuning for real-time microwave ablation optimization in lung tissue.

Design of a closed dual‐slot antenna for spherical hepatic microwave ablation

Microwave ablation has been considered as a promising therapy for the treatment of liver tumors. However, many antennas proposed in previous literature generated ellipsoidal ablation and also were not suitable for percutaneous intervention due to the lack of mechanical properties. To address these problems, a closed dual-slot antenna was investigated to generate spherical ablation and designed with zirconia tip, which facilitated its ability to puncture into tissues, and metal tube, which provided mechanical strength for percutaneous puncture. A numerical model was constructed to optimize antenna geometry and analyze antenna performance. The reflection coefficient, specific absorption rate, and temperature distribution of the closed and conventional dual-slot antennas were all simulated by using finite element method. The two antennas were also validated with ex vivo porcine livers, finding a great agreement between simulated and experimental results. The closed dual-slot antenna generated a more spherical ablation zone (34.0 mm × 37.2 mm vs. 29.1 mm × 48.5 mm at 50 W for 10 min) with a lower reflection coefficient (−30.3 vs. −9.5 dB) than did conventional dual-slot antenna, and showed qualified mechanical properties for clinical application, which may be further applied to clinical percutaneous intervention to treat liver tumors effectively.

Nearly spherical ablation zone with minimally invasive microwave ablation: Analysis and experimental characterization of a tapered-tip symmetric dipole antenna

Microwave ablation (MWA) is a widely used technology for tumors treatments. The reported asymmetric dipole antennas were not ideal in ablation zone. Although some methods were proposed to improve the ablation performance, other important characteristics were still not satisfactory enough, such as poor impedance matching, increased invasion diameter, or additional insertion devices. In this paper, a minimally invasive antenna with a nearly spherical ablation zone for microwave ablation is presented by a tapered-tip symmetric dipole structure with a sleeve. The tapered-tip symmetric dipole, fed by a coaxial line, are composed of a tapered-tip cylinder arm and a hollow cylinder arm. Both of the lengths of the two arms are approximately equal to a quarter of wavelength. The proposed antenna provides good matching at resonating frequency without any extra impedance matching network. Moreover, the tapered-tip structure enables the proposed MWA antenna be inserted into the tissues directly without an additional needle-shape catheter. Furthermore, the sleeve structure, electrically isolated from the dipole and the coaxial feed line, is employed to prevent the current and thermal from flowing along the feed line. A minimally invasive antenna with a diameter of only 1.9 mm working at 2.45 GHz is designed and fabricated for verification, in which different input power and heating time are taken into account. The temperature evolution in four locations of the liver tissue is recorded by using a 4-channel temperature monitoring system. The comparison with a commercial MWA antenna of the same diameter is also included. Simulation and experiment results show that the proposed MWA antenna create nearly spherical ablation zones. The proposed MWA antenna would be promising for minimally invasive medical applications.

Design of a novel externally‐tapped intertwining helical antenna for microwave ablation and its statistical analysis on tissue model

Cancer is the leading cause of death in recent centuries and microwave ablation (MWA) has been evolving to thermally treat cancerous tissue through cell necrosis and is also a promising substitute to radio frequency ablation. In this article, a novel normal-mode helical antenna based MWA probe is investigated for the effect on ablation diameter in the presence of five critical parameters. The antenna consists of an intertwining helix structure mounted on a very small ground plane. An external tapping is used to match the antenna impedance. In the proposed design at the center frequency, a reflection coefficient of more than 23 dB without insertion into the tissue model and a reflection coefficient of more than 14 dB with insertion into the tissue model have been observed at 5 GHz frequency. The gain and percentage bandwidth of the antenna is 0.6 dB and 10%, respectively. At 20 W power, a maximum ablation diameter of 32 mm (transversal-T) and 44 mm (axial-A) is reached within 5 min of application time at a distance of 30 mm from the probe. Further, the L27 Taguchi's orthogonal array has been employed to study the effect on ablation diameter by varying five thermo-electrical parameters. The outcome of Taguchi's analysis has been ranked according to the contribution of each parameter. Further, F- and p-value test has been employed using analysis of variance to study the significance of each parameter on ablation diameter.