Qun Nan

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.

Numerical study on thermal field of microwave ablation with water-cooled antenna

Purpose: This research was to reveal the thermal characteristics of a water-cooled microwave ablation antenna in phantom, and the influence of cooling water velocity on ablation pattern by numerical method. In addition, by comparing the numerical results with experimental results, the experimentally obtained SAR was proven to be correct. Methods: The temperature distribution in ablations was simulated by the finite element method. In the FEM, the cooling effect in the region of the water-cooled antenna was introduced by applying the convective coefficient to the related boundaries, and the experimental determined SAR in phantom was applied as heat generation of microwave generator. To study the effect of water flow rate on ablation pattern, three different water velocities were chosen. In addition, the phantom thermal properties changes were considered in simulation when the heating temperature was above 80°C. Results: The ablation pattern could be identified as a pear shape. The temperatures of monitoring points which were located near the antenna could rise more rapidly, and they were more likely to achieve steady heat transfer state within a short time compared with those far away from the antenna. The cooling water effectively decreased the temperature near the antenna, an under-temperature occurred in the cooling region, and the different cooling water flow rate did not affect significantly the ablation pattern. Conclusions: The numerical results compared reasonably well with the experimental results in both heating pattern and temperature of individual monitoring point over the same heating duration. The SAR measured by our previous experiment was also confirmed by this numerical simulation. This method could be employed to study combination thermal field of multi-antennas in future work.

Phantom experimental study on microwave ablation with a water-cooled antenna

Microwave ablation therapy using a water-cooled antenna was studied experimentally in a phantom. The development of the heating pattern induced by the microwave antenna was determined from the thermocouple-measured temperature field, and the influence of the cooling water flow within the antenna on temperature distribution and heating pattern was investigated. The shape of the heating pattern was pear-like, and the enlarging rate of the heating pattern decreased with heating time. Because of strong cooling effect, the heating pattern in the region with Z < 0 (where Z = 0 represents the position of radiator, Z < 0 and Z > 0 represent the backward direction of the antenna with cooling water and forward direction without water, respectively) was smaller in diameter than that with Z > 0, and the heating pattern with Z < 0 was slightly reduced when the velocity of the cooling water increased. The highest ablative temperature occurred with Z > 0 decreasing. Finally, the specific absorption rate distribution was also determined and investigated analytically. The present results can be helpful in clinical ablation therapy practice and will be applicable to multiple applicators for surgical planning.

Analysis to a critical state of thermal field in microwave ablation of liver cancer influenced by large vessels

To study the effect of large blood vessels on the temperature field in invasive microwave ablation, a finite element method was applied based on the convective-type boundary condition on the interface between tissues and blood flow. Whether a large blood vessel is outside of or involved in the lesion area will affect the 54°C effective therapeutic area in different critical conditions. This paper drew the function diagraph on the distance between blood vessel and antenna with the diameter of the blood vessel and put forward the concept of effective therapy radius. It can be used to study the influence of large vessels on the external boundary of the coagulation area and can be used as a theoretical basis to help to decide whether to occlude the large vessels before microwave ablation therapy.

Experimental study on thermal field in the vicinity of arterial bifurcation in microwave ablation therapy

Purpose: The aim of this study is to investigate the effects of an arterial bifurcation on the temperature distribution during microwave ablation in a muscle-equivalent phantom. Methods: Two experiments with water flow rates of 42.39 mL/min and 70.79 mL/min in the typical range of blood flows in the hepatic artery of the human body were implemented. Temperature measurements inside the phantom were performed in the plane of the arterial bifurcation, in each experiment the microwave antenna was placed at three different positions at 10, 15 and 20 mm from the vessel. Results: The heating pattern was not symmetrical around the antenna with large temperature gradients near the blood vessel when the antenna was near to the vessel (10 mm): The higher the blood velocity, the smaller the heated area. The heating pattern was more circular and symmetrical, and the temperature contours with the two given flow rates nearly coincide when the antenna was far away from the blood vessel (20 mm). The temperatures near the recirculation zone in the daughter arteries immediately after the bifurcation hardly vary with blood velocity. Conclusion: These results indicate that the flow rate in the vessel and the distance between the vessel and the antenna can significantly affect the heating pattern during thermal ablation. The effect of blood flow on ablation is negligible if the distance between blood vessel and antenna exceeds 20 mm, and vessel occlusion can be avoided. The present results can be helpful in clinical microwave ablation surgical planning.

Thermal distribution of microwave antenna for atrial fibrillation catheter ablation

Abstract Purpose: The aim of this study is to investigate the effects of ablation parameters on thermal distribution during microwave atrial fibrillation catheter ablation, such as ablation time, ablation power, blood condition and antenna placement, and give proper ablative parameters to realise transmural ablation. Materials and methods: In this paper, simplified 3D antenna-myocardium-blood finite element method models were built to simulate the endocardial ablation operation. Thermal distribution was obtained based on the coupled electromagnetic-thermal analysis. Under different antenna placement conditions and different microwave power inputs within 60 s, the lesion dimensions (maximum depth, maximum width) of the ablation zones were analysed. Results: The ablation width and depth increased with the ablation time. The increase rate significantly slowed down after 10 s. The maximum temperature was located in 1 mm under the antenna tip when perpendicular to the endocardium, while 1.5 mm away from the antenna axis and 26 mm along the antenna (with antenna length about 30 mm) in the myocardium when parallel to the endocardium. The maximum temperature in the ablated area decreased and the effective ablation area (with the temperature raised to 50°C) shifted deeper into the myocardium due to the blood cooling. Conclusion: The research validated that the microwave antenna can provide continuous long and linear lesions for the treatment of atrial fibrillation. The dimensions of the created lesion widths were all larger than those of the depths. It is easy for the microwave antenna to produce transmural lesions for an atrial wall thickness of 2–6 mm by adjusting the applied power and ablation time.

Numerical study of the effect of blood vessel on the microwave ablation shape

The existence of large blood vessels seriously impacts the results of microwave ablation on heat transfer of surrounding tissue, and the research of influences about large blood vessels could be essential and significant. The temperature distribution in the tissue was analyzed with a microwave heating source by finite element method. The model, where the blood vessel is parallel to antenna, has different distances from antenna to blood vessel. As distance was greater than 20mm, the effect of blood vessel that was parallel to antenna was ignored and the ablation area was elliptical-like. When distance was less than 10mm, the part of asymmetrical coagulated area was on the right side of blood vessel. Therefore, the temperature contour by different conditions could provide numerical references, which is whether to block blood vessel or not, to achieve the aim of guiding the clinical practice, according to the locations of tumor and blood vessel.

The temperature field simulation of radiofrequency catheter-based renal sympathetic denervation for resistant hypertension.

Renal sympathetic denervation (RSD) by the radiofrequency ablation was used to treat the resistant hypertension in clinic and has achieved curative effect. But the temperature distribution in the artery walls and the blood flow have not been investigated. Finite element method (FEM) based on Comsol Multiphysics 4.3a software was used to simulate the temperature distribution in the renal artery. The results of renal artery temperature distribution as well as blood flow effect on the temperature field were obtained, which demonstrated that the blood velocity is very crucial in the temperature distribution of blood vessel near antenna. When the speed of blood is 0.4 m/s, the highest temperature rise of arterial wall near the antenna is 8.882°C (37°C to 45.882°C) and contralateral artery walls highest temperature rise is about 5°C (37°C to 42°C). This temperature value can damage renal sympathetic nerves to cure the resistant hypertension. Due to the blood flow, the temperature field stretches to the direction of blood flow. The temperature rise of blood is only in a small range (37°C to 41°C) at both ends of the antenna. The simulation of RSD by the radiofrequency ablation can give doctors a better scheme to avoid the vascular injury in different blood flow rates and radiofrequency voltages.

The Comparison of Two Simulation Methods on the Thermal Ablation with Large Blood Vessel

Purpose: The aim of this study is to contrast the coupling algorithm (CEE) and boundary heat exchange coefficients (Nu) used in treatment of the large blood vessel in thermal ablation. Methods: Based on the Pennes bioheat transfer equation, the models with blood vessel parallel to microwave antenna were built with finite element method. In two kind of simulation, blood flow rate was set in 0.2 m/s or boundary heat exchange coefficients was set in 1750 W / (m2 °C), respectively. Results and conclusions : There was no significant difference on shape of effective ablation areas and 54°C temperature contours by using two kinds of simulation methods, especially the place far away from the blood vessel. At the place near the blood vessel, the method of CEE is closer to real condition which considers directivity of blood. Whats more, there are higher temperature by using method of Nu inside effective ablation areas.

Experimental and Numerical Analysis in Vitro with a Water-Cooled Microwave Ablation Antenna

This paper gives a comparison of results obtained from numerical simulation and experimental measurements made in vitro pig liver with a water-cooled microwave ablation antenna. An axis-symmetric finite element model was developed to simulate the temperature field based on the pennes bioheat equation, in which the convection heat transfer of the cooling water was involved. The temperature curves and ablation area in the experiments corresponded well with simulated ones in vitro. The results demonstrated that computer-aided simulation of microwave thermal distribution is an accurate and reliable method which can instruct the clinical operation and surgical planning of microwave ablation in practice.