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

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5th International Conference on
Electromagnetic Fields Health and EnvironmentEHE2014
Tony R. Almeida(1), Carlos L. Antunes(2)
(1) Department of Electrical and Computer Engineering, University of Coimbra
(2) Rianda Research – Centro de Investigação em Energia, Saúde e Ambiente
Session OD.1.3
Modelling of a bipolar stent-based electrode
for thermal radio frequency ablation of
tumours located in hollow organs

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Overview
• Introduction
• Numerical models
– Coupled thermal electrical problem
– Geometry of the model
– Stent based electrode models considered
• Results
• Conclusion

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Introduction
• Radiofrequency thermal ablation is a medical
procedure that has been considered as an effective
therapy for the treatment of tumours located in soft
tissue organs

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Introduction
• Self-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

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Introduction
• Previous 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

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Introduction
• Connection to the RF power
generator through a BI-PAL
endoscopic biopsy forceps

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Introduction
• However, numerical and experimental analysis on
this solution showed that the tissue adjacent to the
ends of the stent-based electrode is heated
preferentially

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Goal
• Numerical 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

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Overview
• Introduction
• Results
• Conclusion

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Mathematical modelling
• The numerical models considered consist in the
analysis of a coupled thermal electrical problem
 

       
 b b b
T
c k T c T
t
J E u
 ( ) 0  T V
 ( ) 0  T V
 ( ) 0  T V
     ( ) 0T V
Chen and Holmes model
Laplace’s equation

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Geometry of the model
• Simplified model of the porta hepatis (bile
duct, portal vein and hepatic artery) with a 4 cm long
tumour located in the bile duct

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Models of the electrodes
• The stent-based electrode is approximated by a set
of tubular segments with radius of 2 mm. It was also
considered a tube of 0.25 mm thickness.
3 models:
Spacing of
5, 10 and 15mm
5mm
5mm
5mm
5mm
5mm
5mm

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Material properties considered
Blood
Vessel
Blood Perfusion
[ml/min]
Portal vein 327.55
Hepatic
artery
20.5
Material ρ [kg/m3]
c
[J/kg·K]
k
[W/m·K]

[S/m]
Nitinol 6450 480 18 1·108
Air 1.202 1 0.025 0
Liver 1060 3600 0.512 l (T)
Tumour 1060 360 0.512 t (T)
Blood 1000 4180 0.543 0.667
C. Antunes et al., Using a tubular electrode for radiofrequency ablation: numerical and experimental analysis, COMPEL,
vol. 31, no. 4, pp. 1077-1086, 2012
C. 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, 2006
D. Haemmerich et al., Hepatic bipolar radio-frequency ablation between separated multiprong electrodes, IEEE T Bio-Med
Eng, vol. 48, no. 10, pp. 1145-1152, 2001
D. Haemmerich et al., In vivo electrical conductivity of hepatic tumours, Physiol Meas, vol. 24, no. 2, pp. 251-260, 2003

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Material properties considered
• Both electrical conductivities for healthy, l (T), and
tumorous tissue, t (T), were considered
temperature-dependent, increasing 2% per degree
Celsius
• At 100ºC, 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 value
• The electrical conductivity of the liver was set to
0.13 S/m at 25ºC, and, for the tumorous tissue, it
was assumed equal to 0.269 S/m, also at 25ºC

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Finally…
• Numerical simulations were performed considering
constant voltages of 20, 22, 24, 26, 28 and 30 V applied
to the boundary of the active elements.
• The external boundary of the model was considered as
electrically insulating
• The 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 37ºC
• All models created and solved for 300 seconds (5
minutes) with finite element analysis software COMSOL
Multiphysics 4.1 (COMSOL, Inc. Burlington, MA, USA)

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Overview
• Introduction
• Results
• Conclusion

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Results
• As expected, the volume of the induced thermal
lesion is proportional to the power delivered to the
tissue through the electrode
26V

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First electrode, 5mm spacing @20V

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First electrode, 5mm spacing @20V

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Results for 5mm spacing
20V, 80s 26V, 40s 30V, 30s

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20V, 300s 20V, 110s 30V, 75s

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22V, 300s 26V, 185s 30V, 110s

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Second electrode model - Results
20V, 70s 26V, 35s 30V, 25s

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Third electrode model, 26V

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Third electrode model - Results
20V, 65s 26V, 35s 30V, 25s

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Overview
• Introduction
• Results
• Conclusion

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Conclusions
• Besides 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 solution

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Conclusions
• Results 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 damaged
• The results obtained with this electrode were very
similar, independently of the applied voltage
• Experimental work should be considered to support
these numerical results.

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

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