A dual-slot microwave antenna for more spherical ablation zones: ex vivo and in vivo validation

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

INTRODUCTION

This 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.

METHODS

A 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.

RESULTS

In 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.

CONCLUSIONS

The 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.

Numerical study of the induction of intratumoral apoptosis under microwave ablation by changing slot length of microwave coaxial antenna

Recent advances in technology have led to an increase in the detection of previously undetected deep-located tumor tissue. As a result, the medical field is using a variety of methods to treat deep-located tumors, and minimally invasive treatment techniques are being explored. In this study, therapeutic effect of microwave ablation (MWA) on tumor generated inside liver tissue was analyzed through numerical analysis. The distribution of electromagnetic fields in biological tissues emitted by microwave coaxial antenna (MCA) was calculated through the wave equation, and the thermal behavior of the tissue was analyzed through the Pennes bioheat equation. Among various treatment conditions constituting MWA, tumor radius and the slot length inside the MCA were changed, and the resulting treatment effect was quantitatively confirmed through three apoptotic variables. As a result, each tumor radius has optimal power condition for MWA, 2.6W, 2.4W, and 3.0W respectively. This study confirmed optimal therapeutic conditions for MWA. Three apoptotic variables were used to quantitatively identify apoptotic temperature maintenance inside tumor tissue and thermal damage to surrounding normal tissue. The findings of this study are expected to serve as a standard for treatment based on actual MWA treatment.

Development of a novel MR-conditional microwave needle for MR-guided interventional microwave ablation at 1.5T

PURPOSE

To develop an MR-conditional microwave needle that generates a spherical ablation zone and clear MRI visibility for MR-guided microwave ablation.

METHODS

An MR-conditional microwave needle consisting of zirconia tip and TA18 titanium alloy tube was investigated. The numerical model was created to optimize the needle's geometry and analyze its performance. A geometrically optimized needle was produced using non-magnetic materials based on the electromagnetics simulation results. The needle's mechanical properties were tested per the Chinese pharmaceutical industry standard YY0899-2013. The MRI visibility performance and ablation characteristics of the needle was tested both in vitro (phantom) and in vivo (rabbit) at 1.5T. The RF-induced heating was evaluated in ex vivo porcine liver.

RESULTS

The needle's mechanical properties met the specified requirements. The needle susceptibility artifact was clearly visible both in vitro and in vivo. The needle artifact diameter (A) was small in in vivo (A_shaft: 4.96 ± 0.18 mm for T1W-FLASH, 3.13 ± 0.05 mm for T2-weighted fast spin-echo (T2W-FSE); A_tip: 2.31 ± 0.09 mm for T1W-FLASH, 2.29 ± 0.08 mm for T2W-FSE; tip location error [TLE]_: -0.94 ± 0.07 mm for T1W-FLASH, -1.10 ± 0.09 mm for T2W-FSE). Ablation zones generated by the needle were nearly spherical with an elliptical aspect ratio ranging from 0.79 to 0.90 at 30 W, 50 W for 3, 5, 10 min duration ex vivo ablations and 0.86 at 30 W for 10 min duration in vivo ablations.

CONCLUSION

The designed MR-conditional microwave needle offers excellent mechanical properties, reliable MRI visibility, insignificant RF-induced heating, and a sufficiently spherical ablation zone. Further clinical development of MR-guided microwave ablation appears warranted.

Evaluation of the performance of designed coaxial antennas for hyperthermia using simulation and experimental methods

Introduction: Antenna geometries and tissue properties affect microwave energy distributions during microwave ablation procedures. There is paucity information on the potential of antenna fabricated from a thick semi-rigid coaxial cable in the field of microwave thermal therapy. This study aimed at comparing the performance of two dual-slot antennas designed from different semi-rigid coaxial cables for the ablation of a liver tumour using numerical simulation and experimental validation methods.

Materials and Methods: COMSOL Multiphysics software was used for designing dual-slot antennas and as well as to evaluate microwave energy deposition and heat distribution in the liver tissue. Experimental validations were conducted on the ex-vivo bovine livers to validate the simulation results.

Results: Thick antenna developed in this study produced a higher sphericity index, larger ablation diameter and reduced backward heating along the antenna shaft than the existing one. The experimental validation results also indicate significant differences between the two antennas in terms of ablation diameters (p = 0.04), ablation lengths (p = 0.02) and aspect ratios (p = 0.02).

Conclusion: Based on the findings in this study, antenna fabricated from a thick coaxial cable has a higher potential of localizing microwave energy in the liver than conventional antennas.

How large is the periablational zone after radiofrequency and microwave ablation? Computer-based comparative study of two currently used clinical devices

Purpose:

To compare the size of the coagulation (CZ) and periablational (PZ) zones created with two commercially available devices in clinical use for radiofrequency (RFA) and microwave ablation (MWA), respectively.

Methods:

Computer models were used to simulate RFA with a 3-cm Cool-tip applicator and MWA with an Amica-Gen applicator. The Arrhenius model was used to compute the damage index (Ω). CZ was considered when Ω> 4.6 (>99% of damaged cells). Regions with 0.6<Ω< 2.1 were considered as the PZ (tissue that has undergone moderate sub-ablative hyperthermia). The ratio of PZ volume to CZ volume (PZ/CZ) was regarded as a measure of performance, since a low value implies achieving a large CZ while keeping the PZ small.

Results:

Ten-min RFA (51 W) created smaller periablational zones than 10-min MWA (11.3 cm³ vs. 17.2 22.9 cm³, for 60 100 W MWA, respectively). Prolonging duration from 5 to 10 min increased the PZ in MWA more than in RFA (2.7 cm³ for RFA vs. 8.3–11.9 cm³ for 60–100 W MWA, respectively). PZ/CZ for RFA were relatively high (65–69%), regardless of ablation time, while those for MWA were highly dependent on the duration (increase of up to 25% between 5 and 10 min) and on the applied power (smaller values as power was raised, 102% for 60 W vs. 81% for 100 W, both for 10 min). The lowest PZ/CZ across all settings was 56%, obtained with 100 W-5 min MWA.

Conclusions:

Although RFA creates smaller periablational zones than MWA, 100 W-5 min MWA provides the lowest PZ/CZ.

A pilot study of the shapes of ablation lesions in the canine prostate by laser, radiofrequency and microwave and their clinical significance

To explore the shape characteristics of ablation lesions created via laser ablation (LA), radiofrequency ablation (RFA) and microwave ablation (MWA) in canine prostates and the clinical significance of these characteristics, six adult male beagles were randomly assigned to the LA, RFA, and MWA groups. These ablations were performed with common parameters applied in clinical practice (LA, 3 W/1200 J; RFA and MWA, 30 W/120 s). One ablation lesion was created in each lobe of the prostate via the ablation technique, resulting in a total of twelve ablation lesions. Transrectal ultrasound (TRUS) was used as guidance during puncture and to monitor changes in the ablation lesions. Finally, the ablation efficacy was assessed using transrectal contrast-enhanced ultrasonography (CEUS), and the transverse diameter (TRD), anteroposterior diameter (APD) and longitudinal diameter (LD) of each ablation lesion were measured. The volume (V) and the ratio (R) value were calculated. R reflects the shape characteristic of the ablation lesion (the R value close to 1.0 indicates a more spherical shape). The R values of the ablation lesions were 0.89 ± 0.02, 0.72 ± 0.01, and 0.65 ± 0.03 for RFA, MWA and LA, respectively, and they were significantly different (P = 0.027). The volumes of the ablation lesions were 2.17 ± 0.10 ml, 1.51 ± 0.20 ml, and 0.79 ± 0.07 ml for MWA, LA and RFA, respectively, and they were also significantly different (P = 0.001). The three abovementioned thermal ablation techniques with common parameters in clinical practice can be used for ablation in the prostate. The shapes and volumes of the ablation lesions of the three techniques were varied: The RFA-created lesions had the lowest volumes and were more spherical in shape, demonstrating that RFA could be used for the treatment of relatively small lesions or tumours adjacent to vital organs. The MWA lesions had the largest size with a spherical shape, which could be advantageous for the ablation of tumours with relatively large sizes. The sizes of the ablation lesions created via LA were between those of RFA and MWA but presented more oval in shape, suggesting that this method is highly appropriate for the ablation of benign prostatic hyperplasia (BPH).

Antenna Designs for Microwave Tissue Ablation

Microwave (MW) ablation has emerged as a minimally invasive therapeutic modality and is in clinical use for treatment of unresectable tumors and cardiac arrhythmias, neuromodulation, endometrial ablation, and other applications. Components of image-guided MW ablation systems include high-power MW sources, ablation applicators that deliver power from the generator to the target tissue, cooling systems, energy-delivery control algorithms, and imaging guidance systems tailored to specific clinical indications. The applicator incorporates a MW antenna that radiates MW power into the surrounding tissue. A variety of antenna designs have been developed for MW ablation with the objective of efficiently transferring MW power to tissue, with a radiation pattern well matched to the size and shape of the targeted tissue. Here, we survey advances in percutaneous, endocavitary, and endoscopic antenna designs as an integral element of MW ablation applicators for a diverse set of clinical applications.

Microwave Ablation (MWA) of Pulmonary Neoplasms: Clinical Performance of High-Frequency MWA With Spatial Energy Control Versus Conventional Low-Frequency MWA

OBJECTIVE. The objective of our study was to evaluate the clinical performance of a new high-frequency (HF) microwave ablation (MWA) technology with spatial energy control for treatment of lung malignancies in comparison with a conventional low-frequency (LF) MWA technology. MATERIALS AND METHODS. In this retrospective study, 59 consecutive patients (mean age, 58.9 ± 12.6 [SD] years) were treated in 71 sessions using HF spatial-energy-control MWA. Parameters collected were technical success and efficacy, tumor diameter, tumor and ablation volumes, ablation time, output energy, complication rate, 90-day mortality, local tumor progression (LTP), ablative margin size, and ablation zone sphericity. Results were compared with the same parameters retrospectively collected from the last 71 conventional LF-MWA sessions. This group consisted of 56 patients (mean age, 60.3 ± 10.8 years). Statistical comparisons were performed using the Wilcoxon-Mann-Whitney test. RESULTS. Technical success was 98.6% for both technologies; technical efficacy was 97.2% for HF spatial-energy-control MWA and 95.8% for LF-MWA. The 90-day mortality rate was 5.1% (3/59) in the HF spatial-energy-control MWA group and 5.4% (3/56) in the LF-MWA group; for both groups, there were zero intraprocedural deaths. The median ablation time was 8.0 minutes for HF spatial-energy-control MWA and 10.0 minutes for LF-MWA (p < 0.0001). Complications were recorded in 21.1% (15/71) of HF spatial-energy-control MWA sessions and in 31.0% (22/71) of LF-MWA sessions (p = 0.182); of these complications, 4.2% (3/71) were major complications in the HF spatial-energy-control MWA group, and 9.9% (7/71) were major complications in the LF-MWA group. The median deviation from ideal sphericity (1.0) was 0.195 in the HF spatial-energy-control MWA group versus 0.376 in the LF-MWA group (p < 0.0001). Absolute minimal ablative margins per ablation were 7.5 ± 3.6 mm (mean ± SD) in the HF spatial-energy-control MWA group versus 4.2 ± 3.0 mm in the LF-MWA group (p < 0.0001). In the HF spatial-energy-control MWA group, LTP at 12 months was 6.5% (4/62). LTP at 12 months in the LF-MWA group was 12.5% (7/56). Differences in LTP rate (p = 0.137) and time point (p = 0.833) were not significant. CONCLUSION. HF spatial-energy-control MWA technology and conventional LFMWA technology are safe and effective for the treatment of lung malignancies independent of the MWA system used. However, HF spatial-energy-control MWA as an HF and high-energy MWA technique achieves ablation zones that are closer to an ideal sphere and achieves larger ablative margins than LF-MWA (p < 0.0001).

Evaluation of tissue deformation during radiofrequency and microwave ablation procedures: Influence of output energy delivery

PURPOSE

The purpose of this study was to quantitatively analyze tissue deformation during radiofrequency (RF) and microwave ablation for varying output energy levels.

METHODS

A total of 46 fiducial markers which were classified into outer, middle, and inner lines were positioned into a single plane around an RF or microwave ablation applicator in each ex vivo bovine liver sample (8 cm × 6 cm × 4 cm, n = 18). Radiofrequency (500 kHz; ~35 W average) or microwave (2.4 GHz; 50-100 W output, ~35-70 W delivered) ablation was performed for 10 min (n = 4-6 each setting). CT images were acquired over the entire liver volume every 15 s. Principle strain magnitude and direction were determined from fiducial marker displacement. Normal and shear strain were then calculated such that negative strain denoted contraction and positive strain denoted expansion. Temporal variations, the final magnitudes, and angles of the strain were compared across energy delivery settings, using one-way ANOVA with post hoc Tukey's tests.

RESULTS

On average, tissue strain rates peak at around 1 min and decayed exponentially over time. No evidence of tissue expansion was observed. The tissue strains from RF and 50 W, 75 W, and 100 W microwave ablation at 10 min were -8.5%, -38.9%, -54.4%, and -65.7%, respectively, from the inner region and -3.6%, -23.7%, -41.8%, and -44.3%, respectively, from the outer region. Negative strain magnitude was positively correlated to energy delivery in the inner region (Spearman's ρ  = -0.99). Microwaves at higher powers (75-100 W) induced significantly more strain than at lower power (50 W) or after RF ablation (P < 0.01). Principal strain angles ranged from 0.8° to -8.1°, indicating that tissue deformed more in the direction transverse to the applicator than along the direction of the applicator.

CONCLUSIONS

The influence of output energy on tissue deformation during RF and microwave ablation was analyzed. Microwave ablation created significantly greater contraction than RF ablation with similar energy delivery. During microwave ablation, more contraction was noted at higher power levels and in proximity to the antenna. Contraction primarily transverse to the antenna produces ablation zones that are more elongated than the original tissue volume.

Ex vivo validation of microwave thermal ablation simulation using different flow coefficients in the porcine liver

The purpose of the study was to validate the simulation model for a microwave thermal ablation in ex vivo liver tissue. The study aims to show that heat transfer due to the flow of tissue water during ablation in ex vivo tissue is not negligible. Ablation experiments were performed in ex vivo porcine liver with microwave powers of 60 W to 100 W. During the procedure, the temperature was recorded in the liver sample at different distances to the applicator using a fiber-optic thermometer. The position of the probes was identified by CT imaging and transferred to the simulation. The simulation of the heat distribution in the liver tissue was carried out with the software CST Studio Suite. The results of the simulation with different flow coefficients were compared with the results of the ablation experiments using the Bland-Altman analysis. The analysis showed that the flow coefficient of 90,000 W/(K*m³) can be considered as the most suitable value for clinically used powers. The presented simulation model can be used to calculate the temperature distribution for microwave ablation in ex vivo liver tissue.

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.

Microwave ablation at 915 MHz vs 2.45 GHz: A theoretical and experimental investigation

PURPOSE

The relationship between microwave ablation system operating frequency and ablation performance is not currently well understood. The objective of this study was to comparatively assess the differences in microwave ablation at 915 MHz and 2.45 GHz.

METHODS

Analytical expressions for electromagnetic radiation from point sources were used to compare power deposition at the two frequencies of interest. A 3D electromagnetic-thermal bioheat transfer solver was implemented with the finite element method to characterize power deposition and thermal ablation with asymmetrical insulated dipole antennas (single-antenna and dual-antenna synchronous arrays). Simulation results were validated against experiments in ex vivo tissue.

RESULTS

Theoretical, computational, and experimental results indicated greater power deposition and larger diameter ablation zones when using a single insulated microwave antenna at 2.45 GHz; experimentally, 32±4.1 mm and 36.3±1.0 mm for 5 and 10 min, respectively, at 2.45 GHz, compared to 24±1.7 mm and 29.5±0.6 mm at 915 MHz, with 30 W forward power at the antenna input port. In experiments, faster heating was observed at locations 5 mm (0.91 vs 0.49 °C/s) and 10 mm (0.28 vs 0.15 °C/s) from the antenna operating at 2.45 GHz. Larger ablation zones were observed with dual-antenna arrays at 2.45 GHz; however, the differences were less pronounced than for single antennas.

CONCLUSIONS

Single- and dual-antenna arrays systems operating at 2.45 GHz yield larger ablation zone due to greater power deposition in proximity to the antenna, as well as greater role of thermal conduction.

Microwave ablation in primary and secondary liver tumours: technical and clinical approaches

Thermal ablation is increasingly being utilised in the treatment of primary and metastatic liver tumours, both as curative therapy and as a bridge to transplantation. Recent advances in high-powered microwave ablation systems have allowed physicians to realise the theoretical heating advantages of microwave energy compared to other ablation modalities. As a result there is a growing body of literature detailing the effects of microwave energy on tissue heating, as well as its effect on clinical outcomes. This article will discuss the relevant physics, review current clinical outcomes and then describe the current techniques used to optimise patient care when using microwave ablation systems.

Microwave ablation of ex vivo bovine tissues using a dual slot antenna with a floating metallic sleeve

PURPOSE

To study the efficiency of a dual slot antenna with a floating metallic sleeve on the ablation of different ex vivo bovine tissues.

MATERIALS AND METHODS

COMSOL Multiphysics® version 4.4 (Stockholm, Sweden), which is based on finite element methods (FEM), was used to design and simulate monopole and dual slot with sleeve antennas. Power, specific absorption rate (SAR), temperature and necrosis distributions in the selected tissues were determined using these antennas. Monopole and dual slot with sleeve antennas were designed, simulated, constructed and applied in this study based on a semi-rigid coaxial cable. Ex vivo experiments were performed on liver, lung, muscle and heart of bovine obtained from a public animal slaughter house. The microwave energy was delivered using a 2.45 GHz solid-state microwave generator at 40 W for 3, 5 and 10 min. Aspect ratio, ablation length and ablation diameter were also determined on ablated tissues and compared with simulated results. Student's t-test was used to compare the statistically significant difference between the performance of the two antennas.

RESULTS

The dual slot antenna with sleeve produces localised microwave energy better than the monopole antenna in all ablated tissues using simulation and experimental validation methods. There were significant differences in ablation diameter and aspect ratio between the sleeve antenna and monopole antenna. Additionally, there were no significant differences between the simulation and experimental results.

CONCLUSIONS

This study demonstrated that the dual slot antenna with sleeve produced larger ablation zones and higher sphericity index in ex vivo bovine tissues with minimal backward heating when compared with the monopole antenna.

Design of a dual slot antenna for small animal microwave ablation studies

This study presents the development of a dual-slot antenna for small animal tumor ablation. By using a dual-slot design at 8 GHz, it was hypothesized that smaller and more spherical ablations can be produced. After computer-aided design optimization, antennas were fabricated and ablations performed at 5-20 W for 15-120 s with the objective of creating ablations with a diameter/length aspect ratio of at least 0.9. The new dual-slot design at 8 GHz created significantly more spherical ablations than a commercial antenna at 2.45 GHz in ex vivo liver tissue (Average Aspect Ratio 0.8081 vs. 0.4532, p <;<; 0.05). In vivo studies confirmed the highly spherical results ex vivo. Initial testing shows that the dual-slot antenna and 8 GHz generator can be used to ablate tumors in mice.

Microwave ablation energy delivery: influence of power pulsing on ablation results in an ex vivo and in vivo liver model

PURPOSE

The purpose of this study was to compare the impact of continuous and pulsed energy deliveries on microwave ablation growth and shape in unperfused and perfused liver models.

METHODS

A total of 15 kJ at 2.45 GHz was applied to ex vivo bovine liver using one of five delivery methods (n = 50 total, 10 per group): 25 W continuous for 10 min (25 W average), 50 W continuous for 5 min (50 W average), 100 W continuous for 2.5 min (100 W average), 100 W pulsed for 10 min (25 W average), and 100 W pulsed for 5 min (50 W average). A total of 30 kJ was applied to in vivo porcine livers (n = 35, 7 per group) using delivery methods similar to the ex vivo study, but with twice the total ablation time to offset heat loss to blood perfusion. Temperatures were monitored 5-20 mm from the ablation antenna, with values over 60 °C indicating acute cellular necrosis. Comparisons of ablation size and shape were made between experimental groups based on total energy delivery, average power applied, and peak power using ANOVA with post-hoc pairwise tests.

RESULTS

No significant differences were noted in ablation sizes or circularities between pulsed and continuous groups in ex vivo tissue. Temperature data demonstrated more rapid heating in pulsed ablations, suggesting that pulsing may overcome blood perfusion and coagulate tissues more rapidly in vivo. Differences in ablation size and shape were noted in vivo despite equivalent energy delivery among all groups. Overall, the largest ablation volume in vivo was produced with 100 W continuous for 5 min (265.7 ± 208.1 cm(3)). At 25 W average, pulsed-power ablation volumes were larger than continuous-power ablations (67.4 ± 34.5 cm(3) versus 23.6 ± 26.5 cm(3), P = 0.43). Similarly, pulsed ablations produced significantly greater length (P ≤ 0.01), with increase in diameter (P = 0.09) and a slight decrease in circularity (P = 0.97). When comparing 50 W average power groups, moderate differences in size were noted (P ≥ 0.06) and pulsed ablations were again slightly more circular.

CONCLUSIONS

Pulsed energy delivery created larger ablation zones at low average power compared to continuous energy delivery in the presence of blood perfusion. Shorter duty cycles appear to provide greater benefit when pulsing.

Modeling and validation of microwave ablations with internal vaporization

Numerical simulation is increasingly being utilized for computer-aided design of treatment devices, analysis of ablation growth, and clinical treatment planning. Simulation models to date have incorporated electromagnetic wave propagation and heat conduction, but not other relevant physics such as water vaporization and mass transfer. Such physical changes are particularly noteworthy during the intense heat generation associated with microwave heating. In this paper, a numerical model was created that integrates microwave heating with water vapor generation and transport by using porous media assumptions in the tissue domain. The heating physics of the water vapor model was validated through temperature measurements taken at locations 5, 10, and 20 mm away from the heating zone of the microwave antenna in homogenized ex vivo bovine liver setup. Cross-sectional area of water vapor transport was validated through intraprocedural computed tomography (CT) during microwave ablations in homogenized ex vivo bovine liver. Iso-density contours from CT images were compared to vapor concentration contours from the numerical model at intermittent time points using the Jaccard index. In general, there was an improving correlation in ablation size dimensions as the ablation procedure proceeded, with a Jaccard index of 0.27, 0.49, 0.61, 0.67, and 0.69 at 1, 2, 3, 4, and 5 min, respectively. This study demonstrates the feasibility and validity of incorporating water vapor concentration into thermal ablation simulations and validating such models experimentally.

Creation of short microwave ablation zones: in vivo characterization of single and paired modified triaxial antennas

PURPOSE

To characterize modified triaxial microwave antennas configured to produce short ablation zones.

MATERIALS AND METHODS

Fifty single-antenna and 27 paired-antenna hepatic ablations were performed in domestic swine (N = 11) with 17-gauge gas-cooled modified triaxial antennas powered at 65 W from a 2.45-GHz generator. Single-antenna ablations were performed at 2 (n = 16), 5 (n = 21), and 10 (n = 13) minutes. Paired-antenna ablations were performed at 1-cm and 2-cm spacing for 5 (n = 7 and n = 8, respectively) and 10 minutes (n = 7 and n = 5, respectively). Mean transverse width, length, and aspect ratio of sectioned ablation zones were measured and compared.

RESULTS

For single antennas, mean ablation zone lengths were 2.9 cm ± 0.45, 3.5 cm ± 0.55, and 4.2 cm ± 0.40 at 2, 5, and 10 minutes, respectively. Mean widths were 1.8 cm ± 0.3, 2.0 cm ± 0.32, and 2.5 cm ± 0.25 at 2, 5, and 10 minutes, respectively. For paired antennas, mean length at 5 minutes with 1-cm and 2-cm spacing and 10 minutes with 1-cm and 2-cm spacing was 4.2 cm ± 0.9, 4.9 cm ± 1.0, 4.8 cm ± 0.5, and 4.8 cm ± 1.3, respectively. Mean width was 3.1 cm ± 1.0, 4.4 cm ± 0.7, 3.8 cm ± 0.4, and 4.5 cm ± 0.7, respectively. Paired-antenna ablations were more spherical (aspect ratios, 0.72-0.79 for 5-10 min) than single-antenna ablations (aspect ratios, 0.57-0.59). For paired-antenna ablations, 1-cm spacing appeared optimal, with improved circularity and decreased clefting compared with 2-cm spacing (circularity, 0.85 at 1 cm, 0.78 at 2 cm).

CONCLUSIONS

Modified triaxial antennas can generate relatively short, spherical ablation zones. Paired-antenna ablations were rounder and larger in transverse dimension than single antenna ablations, with 1-cm spacing optimal for confluence of the ablation zone.

Electromagnetic thermotherapy for deep organ ablation by using a needle array under a synchronized-coil system

Thermal ablation by using electromagnetic thermotherapy (EMT) has been a promising cancer modality in recent years. It has relatively few side effects and has therefore been extensively investigated for a variety of medical applications in internal medicine and surgery. The EMT system applies a high-frequency alternating electromagnetic field to heat up the needles which are inserted into the target tumor to cause tumor ablation. In this study, a new synchronized-coil EMT system was demonstrated, which was equipped with two synchronized coils and magnetic field generators to provide a long-range, penetrated electromagnetic field to effectively heat up the needles. The heating effect of the needles at the center of the two coils was first explored. The newly designed two-section needle array combined with the synchronized-coil EMT system was thus demonstrated in the in vitro and in vivo animal experiments. Experimental data showed that the developed system is promising for minimally invasive surgery since it might provide superior performance for thermotherapy in cancer treatment.