Modelling of a bipolar stent-based electrode for thermal radio frequency ablation of tumours located in hollow organs

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Tony R. Almeida(1), Carlos L. Antunes(2) (1) Department of Electrical and Computer Engineering, University of Coimbra (2) Rianda Research Centro de Investigao em Energia, Sade e Ambiente

Session OD.1.3 Modelling of a bipolar stent-based electrode for thermal radio frequency ablation of tumours located in hollow organs www.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014OverviewIntroductionNumerical modelsCoupled thermal electrical problemGeometry of the modelStent based electrode models consideredResultsConclusionnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014IntroductionRadiofrequency thermal ablation is a medical procedure that has been considered as an effective therapy for the treatment of tumours located in soft tissue organs

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014It was first considered as a medical procedure for the treatment of cardiac arrythmias.

It extended to the treatment of primary and metastatic unresectable liver tumours and it has been used for the last 20-30 years, with good results.

It expanded to the treatment of other unresectable tumours located in the kidneys, breast, lung and even bones, and it has being studied its application in the treatment of protate, brain and pancreas tumours.

Well documented in literature, some examples are presented in the paper, but there is a large number of references on the subject.3IntroductionSelf-expandable metallic stents are endo-prosthesis commonly used for the palliative treatment of the symptoms caused by the growth of cancerous tumours inside tubular organs

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014After placing and deploying the stent, this device relieves the stricture problems by mechanical action, applying a radially outward force on the walls of the duct.

Nitinol is an alloy of nickel and titanium that shows unique properties of elasticity and shape memory, besides excellent biocompatibility characteristics4IntroductionPrevious work has shown that self-expandable metallic stents can be regarded as a potential solution for thermal radio frequency ablation of tumours located in hollow organs

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014No esquecer referir a existncia de um elctrodo de retorno5IntroductionConnection to the RF power generator through a BI-PAL endoscopic biopsy forceps

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014IntroductionHowever, numerical and experimental analysis on this solution showed that the tissue adjacent to the ends of the stent-based electrode is heated preferentially

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Increasing the voltage, may lead to a more regular volume lesion, but it will exceed largely the dimension of the bile duct

Also, enhanced saline RF procedure was considered: Although an improvement was achieved, the blood flow in the porta vein and the hepatic artery still has a significant impact on the final obtained volume

Still, the end effect was significant. 7GoalNumerical simulation analysis on a modified based-stent electrode so it can be used as a bipolar electrode, i.e., active and ground electrode are placed in the same device

Three different stent-based electrode models were considered in this study

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014OverviewIntroductionNumerical modelsCoupled thermal electrical problemGeometry of the modelStent based electrode models consideredResultsConclusionnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Mathematical modellingThe numerical models considered consist in the analysis of a coupled thermal electrical problem

Chen and Holmes model Laplaces equationnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Penne equation, obtained when studying the temperature distribution on the forearm10Geometry of the modelSimplified model of the porta hepatis (bile duct, portal vein and hepatic artery) with a 4cm long tumour located in the bile duct

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014The present work studied the numerical simulation of a radio frequency thermal ablation of a tumour located in a bile duct.

The 3D models were created considering an external cylinder, which represents liver tissue, with 200 mm diameter and 100 mm height. The bile duct and the portal vein are cylinders of 5 mm radius and the hepatic artery is a cylinder of 2 mm radius

Thickness of the tumour: 3mm11Models of the electrodesThe stent-based electrode is approximated by a set of tubular segments with radius of 2mm. It was also considered a tube of 0.25mm thickness.

3 models:Spacing of 5, 10 and 15mm5mm5mm5mm5mm5mm5mmnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Lengths:1 17.5, 15 and 12.5mm2 10mm3 4mm12Material properties consideredBloodVesselBlood Perfusion[ml/min]Portal vein327.55Hepatic artery20.5Material [kg/m3]c[J/kgK]k[W/mK][S/m]Nitinol6450480181108Air1.20210.0250Liver106036000.512l (T)Tumour10603600.512t (T)Blood100041800.5430.667C. Antunes et al., Using a tubular electrode for radiofrequency ablation: numerical and experimental analysis, COMPEL, vol. 31, no. 4, pp. 1077-1086, 2012C. Tziafalia et al., Echo-Doppler measurements of portal vein and hepatic artery in asymptomatic patients with hepatitis B virus and healthy adults, J Gastrointest Liver, vol. 15, no. 4, pp. 343-346, 2006D. Haemmerich et al., Hepatic bipolar radio-frequency ablation between separated multiprong electrodes, IEEE T Bio-Med Eng, vol. 48, no. 10, pp. 1145-1152, 2001D. Haemmerich et al., In vivo electrical conductivity of hepatic tumours, Physiol Meas, vol. 24, no. 2, pp. 251-260, 2003nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Material properties consideredBoth electrical conductivities for healthy, l (T), and tumorous tissue, t (T), were considered temperature-dependent, increasing 2% per degree CelsiusAt 100C, these values drop to 0.001 S/m, allowing this way to simulate the electrical insulation verified when gas forms next to the electrode above this temperature valueThe electrical conductivity of the liver was set to 0.13 S/m at 25C, and, for the tumorous tissue, it was assumed equal to 0.269 S/m, also at 25Cnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014FinallyNumerical simulations were performed considering constant voltages of 20, 22, 24, 26, 28 and 30V applied to the boundary of the active elements.The external boundary of the model was considered as electrically insulatingThe temperature at the external surfaces of the model, the initial temperature of the tissues and the temperature of blood were set to the temperature value of 37CAll models created and solved for 300 seconds (5 minutes) with finite element analysis software COMSOL Multiphysics 4.1 (COMSOL, Inc. Burlington, MA, USA)nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014OverviewIntroductionNumerical modelsCoupled thermal electrical problemGeometry of the modelStent based electrode models consideredResultsConclusionnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014ResultsAs expected, the volume of the induced thermal lesion is proportional to the power delivered to the tissue through the electrode

26Vnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014The volume of the induced lesion is determined from the temperature distribution, considering an isothermal surface of 50C. Above this threshold, tissue thermal necrosis is achieved in very short time

This assessment of the volume of the lesion from the 50C isothermal surface has been used extensively in literature17First electrode, 5mm spacing @20V

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014First electrode, 5mm spacing @20V

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Reaches the bilu duct in about 80 seconds19Results for 5mm spacing

20V, 80s26V, 40s30V, 30snwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014the lesion is mostly induced in the tissue between the electrodes

Increasing the applied voltage value does not lead to a longer volume lesion

for a longer time and an applied voltage above 24V, it is possible to heat whole extension of the tumour

At 20V, the tumour tissue on the side of the blood vessels is hardly heated

20Results for 10mm spacing

20V, 300s20V, 110s30V, 75snwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Increasing spacing between the electrode elements, it can be observed that it takes much longer to heat up the tumour tissue

21Results for 15mm spacing22V, 300s26V, 185s30V, 110s

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014This becomes even more expressive for the 15mm spacing: in this case, the middle portion of the tumour tissue does not reach the 50C threshold 22Second electrode model - Results20V, 70s26V, 35s30V, 25s

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Shorter timesLonger lesions, but not covering the whole tumourThe volume is less regular in the middle portionThe tissue is preferably heated in the region near the spacing between electrodes, as it was seen for the previous electrode model (for shorter spacing, of course)Above 26V, risk of charring23Third electrode model, 26V

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Duct bile reached after 35s24Third electrode model - Results20V, 65s26V, 35s30V, 25s

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Times are almost identical to those verified with the three-segment electrode, but covering the whole extent of the tumour.Ends are not totally heated up

A final consideration is concerned with the time involved: The 50C isothermal surface is set quite fast, in less than a minute for most cases. This might not be time enough effectively damage the tumour tissue

An alternative could be to consider a volume delimited by 60C isothermal surface as a damage assessment25OverviewIntroductionNumerical modelsCoupled thermal electrical problemGeometry of the modelStent based electrode models consideredResultsConclusionnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014ConclusionsBesides its palliative application, self-expandable metallic stens can be considered as a feasible solution for the radio frequency ablation of tumours located in hollow organs.In order to surpass the some limitations that were recognized in previous works, it was performed the numerical simulation of three electrode arrangements with 2, 3 and 5 elements.Results evinced that the 2- and 3-segment arrangements represent a less acceptable solutionnwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014ConclusionsResults obtained with the 5-segment electrode revealed a regular volume lesion shape that spread along the full length of the tumour. Still, the tumour ends are not completely damagedThe results obtained with this electrode were very similar, independently of the applied voltageExperimental work should be considered to support these numerical results.

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Coimbra UNESCO World Heritage 2013

nwww.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014Tony R. Almeida(1), Carlos L. Antunes(2) (1) Department of Electrical and Computer Engineering, University of Coimbra (2) Rianda Research Centro de Investigao em Energia, Sade e Ambiente

Session OD.1.3 Modelling of a bipolar stent-based electrode for thermal radio frequency ablation of tumours located in hollow organs www.uc.pt5th International Conference onElectromagnetic Fields Health and EnvironmentEHE2014