J. Alba

Polytechnical University of Valencia, Valencia, Valencia, Spain

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Publications (5)2.59 Total impact

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    ABSTRACT: To study the relationship between roll-off (sudden increase in impedance) and spatial distribution of dehydrated tissue during RF ablation using a cooled electrode (temperatures around 100°C). We used a double approach: (1) theoretical modelling based on the finite element method, and (2) 20 ablations using an experimental study on ex vivo excised bovine liver in which we measured impedance progress and temperature at three points close to the electrode surface: 0.5 (T1), 1.5 (T2) and 2.5 (T3) mm from the tip. T2 was located exactly at the centre of the 30 mm long electrode. Temperatures at T1 and T3 quickly rose to 100°C (at ≈20 and 40 s, respectively), while at the rise at T2 was somewhat slower, stabilized around 50 s and reached a maximum value of 99°C at about 60 s. Impedance reached a minimum of 65 Ω (plateau), began increasing at 50 s and continued rising throughout the procedure, reaching a value equal to the initial value at 70 s. Likewise, computed impedance dropped to ≈73 Ω (plateau), began increasing at 50 s and reached an impedance value equal to the initial value at ≈78 s, which approximately coincided with the time when the entire zone surrounding the electrode was within the 100°C isotherm. There is a close relationship between the moment at which roll-off occurs and the time when the entire electrode is completely encircled by the dehydrated tissue. The mid-electrode zone is the last in which tissue desiccation occurs.
    International Journal of Hyperthermia 01/2012; 28(1):62-8. · 2.59 Impact Factor
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    ABSTRACT: The Cool-tip is one of the most widely employed electrodes in radiofrequency (RF) ablation (RFA) of hepatic tumors. This electrode creates reliable geometry and coagulation zones. Despite the advantages of this electrode, during the ablation is produced a phenomenon called roll-off in which impedance increases, energy deposition completely stops and the lesion size cannot be increased. Consequently, the thermal lesion size is smaller and the tumors which can be ablated are smaller too. In this research we studied theoretical and experimentally the electrical-thermal performance of the Cool-tip electrode during RFA of hepatic tissue. Mainly, we were interested in the occurrence of the roll-off and its relationship with the tissue temperatures around the electrode. The theoretical model included the vaporization of the tissue and the variation of the thermal and electrical conductivities with temperature. The model was solved numerically using COMSOL Multiphysics software. For the experimental part we conducted a study in ex vivo liver tissue. The experimental and theoretical results showed that the roll-off is totally related when temperatures around 100 °C surrounds the tissue close to the center of the Cool-tip. The knowledge of this fact brings a powerful tool to analyze alternative methods or techniques to avoid the roll-off.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 08/2011; 2011:314-7.
  • [Show abstract] [Hide abstract]
    ABSTRACT: Although some types of bipolar electrodes have been broadly employed in clinical practice to coagulate biological tissue by means of radiofrequency (RF) currents, there is still scanty available information about their electrical-thermal behaviour. We are focused on internally cooled bipolar electrodes. The goal of our study was to know more about the behavior of this kind of electrodes. For that, we planned an experimental and theoretical model. The experimental study was based on bovine hepatic ex vivo tissue and the theoretical model was based on the Finite Element Method (FEM). In order to check the feasibility of the theoretical model, we assessed both theoretically and experimentally the effect of the internal cooling characteristics of the bipolar electrode (flow rate and coolant temperature) on the impedance progress during RF heating and coagulation zone dimensions. The experimental and theoretical results were in good agreement, which suggests that the theoretical model could be useful to improve the design of cooled bipolar electrodes.
    Conference proceedings: ... Annual International Conference of the IEEE Engineering in Medicine and Biology Society. IEEE Engineering in Medicine and Biology Society. Conference 08/2011; 2011:6878-81.
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    ABSTRACT: Radiofrequency ablation has been used to treat some types of cardiac arrhythmias. We have previously proposed an ARMAX model (non structural) to estimate the temperature in the tissue during ablation. Computer modeling has allowed us to study the temperature distribution by means of solving numerically theoretical models based on partial differential equations, which represent physical phenomena. Now, our objective is to consider the biological tissue as a system with an input (applied voltage) and output (tissue temperature), and search for a transfer function between these variables. The final aim is to have a simple model that could estimate the temperature at each point of the tissue. We solved the model using the finite element method and identified the transfer function between the temperature at 4 mm depth and an applied voltage using a 7Fr and 4 mm electrode. We used COMSOL Multiphysics to solve the electro-thermal problem and MATLAB to obtain the transfer function. The results showed that the variation in the electrical conductivity of cardiac tissue affected only the static gain of the system, while the variation in the specific heat produced a change only in the dynamic system response. However, the variation in thermal conductivity modified both the static gain and the dynamic system response. These results are a first step towards the development of a macroscopic model based in physical principles, which would lead to better temperature estimation during ablation.
    Computing in Cardiology, 2010; 10/2010
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    ABSTRACT: Radiofrequency (RF) ablation is currently used to treat some types of cardiac arrhythmias such as Atrial Fibrillation (AF). Currently, the most used protocol type of delivering RF energy is controlled-temperature. The commercially available RF generators employ a PI controller. This controller has fixed parameters denominated Kp and Ki. There are no experimental or theoretical studies on the behavior of this controller against to variations in electrical and thermal characteristics of the tissue, electrode design, tissue penetration, and circulating blood flow. We built a theoretical 2D model which we solved using the Finite Element Method (FEM). We used COMSOL Multiphysics for implementing thermalelectric coupling problem and MATLAB for implementing the temperature control loop. The results suggest that the values of Kp and Ki do not affect generally the dynamic response of the controller. However, for some values of tissue characteristics, oscillations in target temperature were observed.

Publication Stats

6 Citations
2.59 Total Impact Points

Institutions

  • 2011
    • Polytechnical University of Valencia
      • Department of Electronic Engineering
      Valencia, Valencia, Spain
  • 2010
    • University of Valencia
      Valenza, Valencia, Spain