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

AC measurements and simulations of hepatic radiofrequency ablation

INTRODUCTION

The radiofrequency ablation (RFA) of liver cancer is a desirable treatment option, as it is minimally invasive. An accurate numerical simulation can greatly help physicians better plan their surgical protocols. Previously, the displacement current in the RFA process was considered negligible, and therefore RFA simulation was modeled as a direct current (DC) system instead of an alternating current (AC) system. Our study investigated the hypothesis that the displacement current in the RFA process should not always be considered negligible.

METHODS

AC measurements of ex vivo bovine liver ablation were performed, and numerical simulations were also conducted to test the hypothesis that the relative permittivity would significantly decrease after the liver tissue reached a high temperature.

RESULTS

The displacement current was observed to be a sizable fraction of the conduction current, especially before the onset of the first pause. The simulation results indicated that the relative permittivity is likely to decrease to several hundred or lower at elevated temperatures.

CONCLUSIONS

Our study results suggest that the DC model may be inadequate, especially before the first roll-off and that additional information could be available during RFA treatment by considering the AC nature of RFA, which could lead to improved numerical simulation. Additional measurements of tissue parameters are needed to reach the full potential of the AC model for further development of ablation control.

Characterisation of Ex Vivo Liver Thermal Properties for Electromagnetic-Based Hyperthermic Therapies

Electromagnetic-based hyperthermic therapies induce a controlled increase of temperature in a specific tissue target in order to increase the tissue perfusion or metabolism, or even to induce cell necrosis. These therapies require accurate knowledge of dielectric and thermal properties to optimise treatment plans. While dielectric properties have been well investigated, only a few studies have been conducted with the aim of understanding the changes of thermal properties as a function of temperature; i.e., thermal conductivity, volumetric heat capacity and thermal diffusivity. In this study, we experimentally investigate the thermal properties of ex vivo ovine liver in the hyperthermic temperature range, from 25 °C to 97 °C. A significant increase in thermal properties is observed only above 90 °C. An analytical model is developed to model the thermal properties as a function of temperature. Thermal properties are also investigated during the natural cooling of the heated tissue. A reversible phenomenon of the thermal properties is observed; during the cooling, thermal properties followed the same behaviour observed in the heating process. Additionally, tissue density and water content are evaluated at different temperatures. Density does not change with temperature; mass and volume losses change proportionally due to water vaporisation. A 30% water loss was observed above 90 °C.

Method for estimating the temperature distribution associated with the vessel cooling effect in radio frequency ablation

Recently, radio frequency ablation (RFA) has become one of the most popular thermal treatments for liver cancer. RFA is minimally invasive and effective in inducing tumor coagulation, however, because use the procedure depends on the experience of the physician, consistent accuracy cannot be guaranteed. In particular, when the tumor is close to a large vessel, a suboptimal ablation margin can result in tumor recurrence. To improve the accuracy of RFA treatment, we have developed an RFA supporting system, which was constructed by using finite element method and operated by means of a model-based control method. In this study, we focused on the cooling effect of flow volume inside a large vessel during RFA, and analyzed heat transfer between the large vessel and liver tissue using a model. We derived the heat transfer parameter (the Nusselt number (Nu)) between the large vessel and liver tissue during RFA by using a finite-element method (FEM). When the Nu for FEM analysis had a value of 3, the FEM analysis model was representative of the actual ablation objective, and the maximum error between FEM analysis and the measurement results was within 2.0[°C]. Thus, it was suggested that the Nu was effective for FEM analysis regarding heat transfer between a large vessel and tissue. However, according to the differences between the results of FEM analysis and measurements concerning the three livers, the heat transfer volume was determined by the Nu, which is different individually in common with other thermal properties. In conclusion, it is necessary to consider the individual differences in the heat transfer volume parameter for FEM analysis.

Real-time temperature control system based on the finite element method for liver radiofrequency ablation: effect of the time interval on control

Radiofrequency (RF) ablation is increasingly being used to treat liver cancer because it is minimally invasive. However, it is difficult for operators to control the size of the coagulation zones precisely, because no method has been established to form an adequate and suitable ablation area. To overcome this limitation, we propose a new system that can control the coagulation zone size. The system operates as follows: 1) the liver temperature is estimated using a temperature-distribution simulator to reduce invasiveness; 2) the output power of the RF generator is controlled automatically according to the liver temperature. To use this system in real time, both the time taken to calculate the temperature in the simulation and the control accuracy are important. We therefore investigated the relationship between the time interval required to change the output voltage and temperature control stability in RF ablation. The results revealed that the proposed method can control the temperature at a point away from the electrode needle to obtain the desired ablation size. It was also shown to be necessary to reduce the time interval when small tumors are cauterized to avoid excessive treatment. In contrast, such high frequency feedback control is not required when large tumors are cauterized.

Derivation of the relationship between the rate of temperature rise and viscoelasticity for constructing a coagulation model for liver radio frequency ablation

Radio frequency ablation (RFA) is usually conducted using ultrasound (US) imaging to monitor the insertion procedure and the coagulation extent of liver tissue which is contiguous to the RFA electrode. However, when RFA surgery is started, the US image becomes unclear because of water vapor. This disadvantage of RFA can lead to excessive and insufficient RFA thereby diminishing the advantages of the procedure. In the present study, we proposed a simulation system which shows the progress status of coagulation for liver RFA. To derive the coagulation characteristics in liver RFA, we used the viscoelasticity of liver tissue as the coagulation indicator to investigate coagulation development for liver RFA. This paper shows the acquisition procedures for analyzing the relationship between the rate of temperature and viscoelasticity. We measured the complex modulus of porcine liver tissue under different rate of temperature in RFA by controlling the output power. We showed that the viscoelasticity of liver tissue depended on temperature previous temperature increase above 60°C. This result indicates that in RFA, controlling the output power is important to completely coagulate the tumor.

Validation of accuracy of liver model with temperature-dependent thermal conductivity by comparing the simulation and in vitro RF ablation experiment

Radiofrequency (RF) ablation is increasingly used to treat cancer because it is minimally invasive. However, it is difficult for operators to control precisely the formation of coagulation zones because of the inadequacies of imaging modalities. To overcome this limitation, we previously proposed a model-based robotic ablation system that can create the required size and shape of coagulation zone based on the dimensions of the tumor. At the heart of such a robotic system is a precise temperature distribution simulator for RF ablation. In this article, we evaluated the simulation accuracy of two numerical simulation liver models, one using a constant thermal conductivity value and the other using temperature-dependent thermal conductivity values, compared with temperatures obtained using in vitro experiments. The liver model that reflected the temperature dependence of thermal conductivity did not result in a large increase of simulation accuracy compared with the temperature-independent model in the temperature range achieved during clinical RF ablation.

Estimation of intraoperative blood flow during liver RF ablation using a finite element method-based biomechanical simulation

Radiofrequency ablation is increasingly being used for liver cancer because it is a minimally invasive treatment method. However, it is difficult for the operators to precisely control the formation of coagulation zones because of the cooling effect of capillary vessels. To overcome this limitation, we have proposed a model-based robotic ablation system using a real-time numerical simulation to analyze temperature distributions in the target organ. This robot can determine the adequate amount of electric power supplied to the organ based on real-time temperature information reflecting the cooling effect provided by the simulator. The objective of this study was to develop a method to estimate the intraoperative rate of blood flow in the target organ to determine temperature distribution. In this paper, we propose a simulation-based method to estimate the rate of blood flow. We also performed an in vitro study to validate the proposed method by estimating the rate of blood flow in a hog liver. The experimental results revealed that the proposed method can be used to estimate the rate of blood flow in an organ.

A method for deriving the coagulation boundary of liver tissue using a relational model of viscoelasticity and temperature in radio frequency ablation

Recently radiofrequency (RF) ablation has become increasingly important in treating liver cancers. RF ablation is ordinarily conducted using elastographic imaging to monitor the ablation procedure and the temperature of the electrode needle is displayed on the RF generator. However, the coagulation boundary of liver tissue in RF ablation is unclear and unconfident. This can lead to both excessive and insufficient RF ablation thereby diminishing the advantages of the procedure. In the present study, we developed a method for determining the coagulation boundary of liver tissue in RF ablation. To investigate this boundary we used the mechanical characteristics of biochemical components as an indicator of coagulation to produce a relational model for viscoelasticity and temperature. This paper presents the data acquisition procedures for the viscoelasticity characteristics and the analytical method used for the coagulation model. We employed a rheometer to measure the viscoelastic characteristics of liver tissue. To determine the model functional relationship between viscoelasticity and temperature, we used a least-square method and the minimum root mean square error was calculated to optimize the model functional relations. The functional relation between temperature and viscoelasticity was linear and non-linear in different temperature regions. The boundary between linear and non-linear functional relation was 58.0°C.