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Purpose:Microwave ablation (MWA) applicators capable of creating directional heating patterns offer the potential of simplifying treatment of targets in proximity to critical structures and avoiding the need for piercing the tumour volume. This work reports on improved directional MWA antennas with the objectives of minimising device diameter for percutaneous use (≤ ∼13 gauge) and yielding larger ablation zones. Methods:Two directional MWA antenna designs, with a modified monopole radiating element and spherical and parabolic reflectors are proposed. A 3D-coupled electromagnetic heat transfer with temperature-dependent material properties was implemented to characterise MWA at 40 and 77 W, for 5 and 10 min. Simulations were also used to assess antenna impedance matching within liver, kidney, lung, bone and brain tissue. The two antenna designs were fabricated and experimentally evaluated with ablations inex vivotissue at the two power levels and treatment durations (n = 5 repetitions for each group). Results:The computed specific absorption rate (SAR) patterns for both antennas were similar, although simulations indicated slightly greater forward penetration for the parabolic antenna. Based on simulations for antennas inserted within different tissues, the proposed antenna design appears to offer good impedance matching for a variety of tissue types. Experiments inex vivotissue showed radial ablation depths of 19 ± 0.9 mm in the forward direction for the applicator with spherical reflector and 18.7 ± 0.7 mm for the applicator with parabolic reflector. Conclusion:These results suggest the applicator may be suitable for creating localised directional ablation zones for treating small and medium-sized targets with a percutaneous approach. [ABSTRACT FROM PUBLISHER]

Background and Purpose: 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., Methods: 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., Results: 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., Conclusion: 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.

Introduction: Percutaneous microwave ablation (MWA) is clinically accepted for the treatment of lung tumors and oligometastatic disease. Bronchoscopic MWA is under development and evaluation in the clinical setting. We previously reported on the development of a bronchoscopy-guided MWA system integrated with clinical virtual bronchoscopy and navigation and demonstrated the feasibility of transbronchial MWA, using a maximum power of 60 W at the catheter input. Here, we assessed the performance of bronchoscopy-guided MWA with an improved catheter (maximum power handling of up to 120 W) in normal porcine lung in vivo (as in the previous study)., Methods: A total of 8 bronchoscopy-guided MWA were performed ( n = 2 pigs; 4 ablations per pig) with power levels of 90 W and 120 W applied for 5 and 10 min, respectively. Virtual bronchoscopy planning and navigation guided transbronchial or endobronchial positioning of the MWA applicator for ablation of lung parenchyma. Following completion of ablations and post-procedure CT imaging, the lungs were harvested and sectioned for gross and histopathologic ablation analysis., Results: Bronchoscopy-guided MWA with applied energy levels of 90 W/5 min and 120 W/10 min yielded ablation zones with short-axis diameters in the range of 20-28 mm (56-116% increase) as compared to ∼13 mm from our previous study (60 W/10 min). Histology of higher-power and previous lower-power ablations was consistent, including a central necrotic zone, a thermal fixation zone with intact tissue architecture, and a hemorrhagic periphery. Catheter positioning and its confirmation via intra-procedural 3D imaging (e.g., cone-beam CT) proved to be critical for ablation consistency., Conclusion: Bronchoscopy-guided MWA with an improved catheter designed for maximum power 120 W yields large ablations in normal porcine lung in vivo., Competing Interests: J.S., W.L.B., M.A.H., N.H., D.S.H., D.S.B., R.A.M., and S.L. have no relevant disclosures. S.G. and H.W. were employees of phenoMapper at the time of this study. A.S. and K.-C.Y. were employees of Broncus Medical Inc., at the time of this study. P.P. reports grants from the National Institutes of Health during the conduct of the study; and grants from the National Institutes of Health, National Science Foundation, and Jano.Life, outside the submitted work. In addition, P.P. is an inventor on issued patents and pending patent applications on microwave ablation technology., (© 2024 The Author(s). Published by S. Karger AG, Basel.)

Thermal ablation techniques are increasingly used for the treatment of symptomatic uterine fibroids. Thermal protection of myometrial tissue adjacent to the fibroid from ablation is critical to maximally preserve the uterus. This study presents a bench top experimental setup, using ex vivo bovine muscle as a surrogate tissue, for evaluating collateral thermal damage in tissues during fibroid ablation. The study reports on the effect of applicator insertion angles (67.5° and 90°) into a mock fibroid on the efficacy of treatment. 6 experiments were performed (3 for each insertion angle) with 30 W applied power at 2.45 GHz. The heating duration was restricted to the time at which a thermal dose of 10 cumulative equivalent minutes at 43 °C (10 CEM 43) was accrued at the boundary of the mock fibroid. Results showed that the volume of ablation inside the mock fibroid dropped considerably from 66% to 17% when the applicator insertion angle was changed from 90º to 67.5º, suggesting that insertion angle plays an important role during microwave ablation of fibroid. The proposed setup provides a method for validating computational models for accurate and safe delivery of ablation to target tissues in fibroid treatment.