تجزیه و تحلیل حساسیت پارامتری از فرسایش فرکانس رادیویی با طراحی تجربی کارآمد

The aim of this research is to analyze the efficacy of radiofrequency ablation (RFA) for varying electrothermal parameters. An attempt has been made to study the RFA for the effect of thermal conductivity, electrical conductivity and blood perfusion rate with Taguchi's design of experiments methodology. Their combined effect was analyzed quantitatively in different tissues. It was found that ablation volume for temperature control algorithm is mostly affected by blood perfusion followed by electrical conductivity and thermal conductivity. Smallest ablation volume was observed in kidney tissue while largest lesion volume was obtained in muscle tissue. Based on the results some insightful corollaries were drawn which may be translated as qualification of RFA for the respective tissue treatment protocol. Moreover, quantification of parameter sensitivity translates to efficient design of control algorithm for power delivery. It is intended that these conclusions will help the radiologist in the treatment planning stage and would serve as broad guidelines for the application of RFA in varying biological environment.

Radiofrequency ablation (RFA) has been a relatively new form of therapy in the armamentarium of cancer treatment. It belongs to a group of therapies known as thermal therapies which rely on the tissue properties to generate the heating effect [1]. In these therapies the lethal effect of heat is harnessed and temperature of the tissue is raised above a threshold to cause lethal effects. Juxtaposedly, the lethal consequence of heat has a remedial effect for cancer and results in death of cancer cells. Mild temperatures (>42 °C) known as hyperthermic cause cell death by apoptosis [2]. Mostly these are used as an adjunct therapy to increase the effectiveness of other therapies like radiation therapy and chemotherapy [3] and [4]. Temperatures above 45 °C are known as thermoablative. Prolonged exposure at such high temperature destroys the cells via coagulation necrosis [5]. RFA has been used for treatment of cancer in various tissues like bone, fat, heart, liver, kidney, etc [6], [7], [8], [9], [10], [11] and [12]. Goetz et al. used RFA to treat painful non-treatable metastatic tumours to relieve pain [6]. Bitsch et al. studied the effect of vascular perfusion on the lesion size in bovine livers [13]. Berjano et al. simulated RFA for atrial tissue using finite element model [10]. Yoon et al. reported that efficacy of RFA can be increased with renal artery occlusion for treatment of VX2 tumours [14]. Steinke et al. treated large tumours in lung metastases. They showed that large tumours are associated with high risk of recurrence owing to difficulty in achieving complete ablation [15]. Biological tissues vary in terms of electrical conductivity, thermal conductivity and blood perfusion as shown in Table 1. Bone and fat have lesser blood perfusion while kidney is the one with highest perfusion. Fat has lowest thermal conductivity while liver has the highest thermal conductivity of the tissues analyzed. Similarly, muscle tissue has the highest electrical conductivity and fat is the one with lowest electrical conductivity. This variation encountered in the properties of different tissues is bound to have varied repercussions for the output of the therapy. Table 1. Electrical and thermal properties of various tissues used for analysis. Parameter Bone Fat Lung Liver Kidney Muscle Heart (myocardial) Electrical conductivity σ [S/m] 0.022 0.01998 0.122 0.148 0.226 0.3 0.541 Thermal conductivity k [W/(m K)] 0.4 0.22 0.302 0.564 0.54 0.49 0.531 Blood perfusion rate [1/s] 0.000833 0.00035 0.0033 0.0167 0.0667 0.00045 0 Reference [9] [42] [9] [9] [42] [43] [44] and [45] Ablation volume Vi [m3] 8.74E-05 1.08E-04 5.66E-05 3.44E-05 1.85E-05 1.02E-04 1.17E-04 Applied voltage V [V] 31.8 25.5 12.4 15.5 13.15 9.4 7.15 Table options The heating mechanism in RFA is mainly resistive. Applying potential difference between the active electrode and grounding pad results in current flow through the tissue and accompanying resistance to the current flow causes heating of the biological tissue. The power deposition is regulated with the help of control algorithm. Moreover, the control algorithm which ensures to prevent charring and overheating of the tissue also responds differently to this variation. This is due to the fact that each tissue behaves differently in terms of temperature increase and evolution of ablation volume to the external impetus provided by the RFA system. Apart from many factors involved which may contribute towards selection of a particular therapy from a large pool of therapies available, the efficacy of RFA can be assessed by the ablation volume/lesion size produced in the tissue. Based on the heterogeneity of electrothermal properties encountered in biological tissues, the aim of this study was firstly to see the effect of critical parameters like thermal conductivity, electrical conductivity and blood perfusion on the radiofrequency ablation. Furthermore, instead of independent variation, the changes in parameter values across the various tissues are simultaneous. Further analysis was thus carried out to evaluate the cumulative effect of these parameters in various tissues. Ablation volumes for RFA were obtained in bone, fat, lung, liver, kidney, muscle and heart (myocardium) tissues. The next section outlines the relevant mathematical background necessary for mathematical modelling of RFA process. Subsequent sections are dedicated to simulation setup and parametric analysis approach. Findings and observations are included in Discussion of results section and Conclusions are presented in the last section.

Radiofrequency ablation is increasing being used for treatment of various tissues. Each tissue possesses its own characteristic parameter values and RFA efficacy in one particular tissue cannot be integrated to all the tissues. To tackle this variability based on tissue type, the effect of thermal conductivity, electrical conductivity and blood perfusion was firstly quantified using the efficient Taguchi's design of experiment approach. It was concluded that higher value of blood perfusion tends to decrease the final ablation volume whereas higher thermal conductivity has an additive effect for the ablation volume. The effect of electrical conductivity was small over the range of values studied in this study. RFA results were obtained for different tissue types and it was concluded that least ablation volume resulted for kidney tissue whereas highest volume was achieved for heart tissue. The effect of individual parameters and a comparative analysis of various tissues provides an insight to application of RFA for different tissues. Furthermore, it would prove helpful in designing of optimal control system for various tissues which regulates the input parameters based on the system parameter sensitivity. Moreover, the results of this study would prove useful for future studies in case of more complex geometries and parameter sensitivity under automatic temperature control. These extended studies would be the topic of our future publications.