Automatic control of finite element models for temperature-controlled radiofrequency ablation

BACKGROUND: The finite element method (FEM) has been used to simulate cardiac and hepatic radiofrequency (RF) ablation. The FEM allows modeling of complex geometries that cannot be solved by analytical methods or finite difference models. In both hepatic and cardiac RF ablation a common control mode is temperature-controlled mode. Commercial FEM packages don't support automating temperature control. Most researchers manually control the applied power by trial and error to keep the tip temperature of the electrodes constant. METHODS: We implemented a PI controller in a control program written in C++. The program checks the tip temperature after each step and controls the applied voltage to keep temperature constant. We created a closed loop system consisting of a FEM model and the software controlling the applied voltage. The control parameters for the controller were optimized using a closed loop system simulation. RESULTS: We present results of a temperature controlled 3-D FEM model of a RITA model 30 electrode. The control software effectively controlled applied voltage in the FEM model to obtain, and keep electrodes at target temperature of 100°C. The closed loop system simulation output closely correlated with the FEM model, and allowed us to optimize control parameters. DISCUSSION: The closed loop control of the FEM model allowed us to implement temperature controlled RF ablation with minimal user input

Antenna design for microwave hepatic ablation using an axisymmetric electromagnetic model

BACKGROUND: An axisymmetric finite element method (FEM) model was employed to demonstrate important techniques used in the design of antennas for hepatic microwave ablation (MWA). To effectively treat deep-seated hepatic tumors, these antennas should produce a highly localized specific absorption rate (SAR) pattern and be efficient radiators at approved generator frequencies. METHODS AND RESULTS: As an example, a double slot choked antenna for hepatic MWA was designed and implemented using FEMLAB™ 3.0. DISCUSSION: This paper emphasizes the importance of factors that can affect simulation accuracy, which include boundary conditions, the dielectric properties of liver tissue, and mesh resolution

Two-compartment mathematical modeling in RF tumor ablation: New insight when irreversible changes in electrical conductivity are considered

[EN] The objective was to explore variations of temperature distribution and coagulation zone size computed by a two-compartment radiofrequency ablation (RFA) model when including simultaneously reversible changes in the tissue electrical conductivity (sigma) due to temperature and irreversible changes due to thermal coagulation. Two-compartment (tumor and healthy tissue) models were built and simulated. Reversible change of sigma was modeled by a piecewise function characterized by increments of +1.5%/degrees C up to 100 degrees C, and a 100 times smaller value from 100 degrees C onwards. Irreversible changes of sigma were modeled using an Arrhenius model. We assumed that both tumor and healthy tissue had a different initial sigma value (as suggested by the experimental data in the literature) and tended towards a common value as thermal damage progressed (necrotized tissue). We modeled a constant impedance protocol based on 90 V pulses voltage and three tumor diameters (2, 3 and 4 cm). Computer simulations showed that the differences between both models were only 0.1 and 0.2 cm for axial and transverse diameters, respectively, and this small difference was reflected in the similar temperature distributions computed by both models. In view of the available experimental data on changes of electrical conductivity in tumors and healthy tissue during heating, our results suggest that irreversible changes in electrical conductivity do not have a significant impact on coagulation zone size in two-compartment RFA models.This work was supported by the National Council of Science and Technology (CONACYT, Mexico) through a scholarship grant to Dora Luz Castro-Lopez, CVU registration No 446604; and by the Spanish Ministerio de Ciencia, Innovacion y Universidades under "Programma Estatal de I+D+i Orientada a los Retos de la Sociedad", Grant No "RTI2018-094357-B-C21".Castro-López, DL.; Trujillo Guillen, M.; Berjano, E.; Romero-Mendez, R. (2020). Two-compartment mathematical modeling in RF tumor ablation: New insight when irreversible changes in electrical conductivity are considered. Mathematical Biosciences and Engineering. 17(6):7980-7993. https://doi.org/10.3934/mbe.2020405S798079931762. D. Haemmerich, L. Chachati, A. S. Wright, D. M. Mahvi, F. T. Lee Jr, J. G. Webster, Hepatic radiofrequency ablation with internally cooled probes: Effect of coolant temperature on lesion size, IEEE Trans. Biomed. Eng., 50 (2003), 493-500.4. Z. Liu, S. M. Lobo, S. Humphries, C. Horkan, S. A. Solazzo, A. U. Hines-Peralta, et al., Radiofrequency tumor ablation: insight into improved efficacy using computer modeling, AJR Am. J. Roentgenol., 184 (2005), 1347-1352.5. S. M. Lobo, Z. J. Liu, N. C. Yu, S. Humphries, M. Ahmed, E. R. Cosman, et al., RF tumour ablation: computer simulation and mathematical modelling of the effects of electrical and thermal conductivity, Int. J. Hyperth., 21 (2005), 199-213.9. D. Haemmerich, D. J. Schutt, RF ablation at low frequencies for targeted tumor heating: In vitro and computational modeling results, IEEE Trans. Biomed. Eng., 58 (2011), 404-410.17. M. Pop, A. Molckovsky, L. Chin, M. C. Kolios, M. A. Jewett, M. D. Sherar, Changes in dielectric properties at 460 kHz of kidney and fat during heating: importance for radio-frequency thermal therapy, Phys. Med. Biol., 48 (2003), 2509-2525.18. U. Zurbuchen, C. Holmer, K. S. Lehmann, T. Stein, A. Roggan, C. Seifarth, et al., Determination of the temperature-dependent electric conductivity of liver tissue ex vivo and in vivo: Importance for therapy planning for the radiofrequency ablation of liver tumours, Int. J. Hyperth., 26 (2010), 26-33.19. E. G. Macchi, M. Gallati, G. Braschi, E. Persi, Dielectric properties of RF heated ex vivo porcine liver tissue at 480 kHz: measurements and simulations, J. Phys. D Appl. Phys., 47 (2014), 485401.21. E. Ewertowska, R. Quesada, A. Radosevic, A. Andaluz, X. Moll, F. G. Arnas, et al., A clinically oriented computer model for radiofrequency ablation of hepatic tissue with internally cooled wet electrode, Int. J. Hyperth., 35 (2019), 194-204.30. M. Qiu, A. Singh, D. Wang, J. Qu, M. Swihart, H. Zhang, P. N. Prasad, Biocompatible and biodegradable inorganic nanostructures for nanomedicine: Silicon and black phosphorus, Nano Today, 25 (2019), 135-155.33. A. Andreozzi, L. Brunese, M. Iasielllo, C. Tucci, G. P. Vanoli, Modeling heat transfer in tumors: A review of thermal therapies, Ann. Biomed. Eng., 47 (2019), 676-693

LARGE TARGET TISSUE NECROSIS OF RADIOFREQUENCY ABLATION USING MATHEMATICAL MODELLING

Radiofrequency ablation (RFA) is a clinic tool for the treatment of various target tissues. However, one of the major limitations with RFA is the ‘small’ size of target tissues that can be effectively ablated. By small it is meant the size of the target tissue is less than 3 cm in diameter of the tissue otherwise ‘large’ size of tissue in this thesis. A typical problem with RFA for large target tissue is the incompleteness of tumour ablation, which is an important reason for tumour recurring. It is widely agreed that two reasons are responsible for the tumour recurring: (1) the tissue charring and (2) the ‘heat-sink’ effect of large blood vessels (i.e. ≥3 mm in diameter). This thesis study was motivated to more quantitatively understand tissue charring during the RFA procedure and to develop solutions to increase the size of target tissues to be ablated. The thesis study mainly performed three tasks: (1) evaluation of the existing devices and protocols to give a clear understanding of the state of arts of RFA devices in clinic, (2) development of an accurate mathematical model for the RFA procedure to enable a more quantitative understanding of the small target tissue size problem, and (3) development of a new protocol based on the existing device to increase the size of target tissues to be ablated based on the knowledge acquired from (1) and (2). In (1), a design theory called axiomatic design theory (ADT) was applied in order to make the evaluation more objective. In (2), a two-compartment finite element model was developed and verified with in vitro experiments, where liver tissue was taken and a custom-made RFA system was employed; after that, three most commonly used internally cooled RFA systems (constant, pulsed, and temperature-controlled) were employed to demonstrate the maximum size of tumour that can be ablated. In (3) a novel feedback temperature-controlled RFA protocol was proposed to overcome the small target tissue size problem, which includes (a) the judicious selection of control areas and target control temperatures and (b) the use of the tissue temperature instead of electrode tip temperature as a feedback for control. The conclusions that can be drawn from this thesis are given as follows: (1) the decoupled design in the current RFA systems can be a critical reason for the incomplete target tissue necrosis (TTN), (2) using both the constant RFA and pulsed RFA, the largest TTN can be achieved at the maximum voltage applied (MVA) without the roll-off occurrence. Furthermore, the largest TTN sizes for both constant RFA and pulsed RFA are all less than 3 cm in diameter, (3) for target tissues of different sizes, the MVA without the roll-off occurrence is different and it decreases with increase of the target tissue size, (4) the largest TTN achieved by using temperature-controlled RFA under the current commercial protocol is still smaller 3 cm in diameter, and (5) the TTN with and over 3 cm in diameter can be obtained by using temperature-controlled RFA under a new protocol developed in this thesis study, in which the temperature of target tissue around the middle part of electrode is controlled at 90 ℃ for a standard ablation time (i.e. 720 s). There are a couple of contributions with this thesis. First, the underlying reason of the incomplete TTN of the current commercially available RFA systems was found, which is their inadequate design (i.e. decoupled design). This will help to give a guideline in RFA device design or improvement in the future. Second, the thesis has mathematically proved the empirical conclusion in clinic that the limit size of target tissue using the current RFA systems is 3 cm in diameter. This has advanced our understanding of the limit of the RFA technology in general. Third, the novel protocol proposed by the thesis is promising to increase the size of TTN with RFA technology by about 30%. The new protocol also reveals a very complex thermal control problem in the context of human tissues, and solving this problem effectively gives implication to similar problems in other thermal-based tumour ablation processes

Investigation of Heat Therapies using Multi-Scale Models and Statistical Methods

Ph.DDOCTOR OF PHILOSOPH