2. “Finite Element Modeling of Radiofrequency Cardiac and Hepatic Ablation” SUPAN TUNGJITKUSOLMUN “Finite Element Modeling of Radiofrequency Cardiac and Hepatic Ablation” SUPAN TUNGJITKUSOLMUN Dept. Of Electrical and Computer Engineering University of Wisconsin-Madison Advisor: Professor John G. Webster

3. Goal Use Finite Element Modeling (FEM) to Improve the Efficacy of Current RF Ablation Technologies and to Design New Electrodes

4. Introduction: RF ablation & FEM Overview: Finite element modeling process 1. Effects of changes in myocardial properties 2. Needle electrode creates deep lesions 3. Uniform current density electrodes 4. Bipolar phase-shifted multielectrode catheter 5. Use FEM to predict lesion dimensions 6. FEM of hepatic ablation Outline

5. 95% success rate in curing Supraventricular tachycardias Low success rate for hepatic ablation Development for VT (Large lesions) Development for AFIB (long thin lesions) Introduction What Is Ablation? Modes of operation ~500 kHz, < 50 W Temperature-controlled Power-controlled Present Technology Heating of cardiac tissue to cure rhythm disturbances and of liver tissue to cure cancer What Is Ablation? Modes of operation

6. System for Cardiac Ablation

7. Common cardiac ablation sites AV Node Above the tricuspid valves Above and underneath the mitral valves Ventricular walls Right ventricular outflow tract Etc.

8. Tip Electrode RF generator

9. Energies Involved in RF Ablation Process

10. Bioheat Equation Heat transfer coefficient Blood temperature Density Specific heat Thermal conductivity Time Temperature Current density Electric field intensity heat loss to blood perfusion VARIABLES Heat Change MATERIAL PROPERTIES Electrical conductivity Density Specific heat Thermal conductivity Time Temperature Current density Electric field intensity heat loss to blood perfusion Heat Conduction Joule Heat

11. Finite Element Analysis Divide the regions of interest into small “elements” Partial differential equations to algebraic equations 2-D (triangular elements, quadrilateral elements, etc.) 3-D (tetrahedral elements, hexahedral elements, etc.) Nonuniform mesh is allowed Software & Hardware PATRAN 7.0 (MacNeal-Schwendler, Los Angeles ) ABAQUS 5.8 (Hibbitt, Karlsson & Sorensen, Inc., Farmington Hills, MI) HP C-180, 1152 MB of RAM, 34 GB Storage

12. Process for FEM Generation  Geometry  Material Properties  Initial Conditions  Boundary Cond.  Mesh Generation Preprocessing (PATRAN 7.0) Solution (ABAQUS/STANDARD 5.8)  Duration  Production  Adjust Loads  Check for desired parameters Postprocessing (ABAQUS/POST 5.8)  Temperature Distribution  Current Density  Determine Lesion Dimensions (from 50  C contour) Convergence test (for optimal number of elements )

13. Modes of RF Energy Applications Maintain the tip temperature at a preset value Adjust voltage applied to the electrode Temperature controlled ablation Power controlled ablation Maintain power delivered at a preset value Adjust voltage applied to the electrode

14. 1. Effects of changes in myocardial properties to lesion dimensions* *Tungjitkusolmun, S., Woo, E. J., Cao, H., Tsai, J.-Z., Vorperian, V. R.,and Webster, J. G.., Thermal-electrical finite element modeling for radio-frequency cardiac ablation: effects of changes in myocardialproperties, Med. Biol. Eng. Comput., accepted, 2000. 1.1 Electrical conductivity 1.2 Thermal conductivity 1.3 Specific heat (Density) Material Properties For each case: Temperature independent Temperature dependent Increase by 50%, or 100% Decrease by 50%

15. FEM results Lesion growth over time (Red is 50  C or higher)

16. Temperature distribution after 60 s Maximum temperature ~ 95  C Highest temperature

17. Maximum changes in Lesion Size PropertyCase% Volume Change Electrical conductivity  50%  58.6 Thermal conductivity +100%  60.7 Specific heat  50% +43.2 Power controlled

18. PropertyCase% Volume Change Electrical conductivity  50% +12.9% Thermal conductivity  50%  21.0% Specific heat+100%  29.4% Temperature controlled

19. Conclusion Temperature dependent properties are important Errors in Power-Controlled Mode are higher Better measurement techniques are needed

20. 2. Needle electrode design for VT* E. J. Woo, S. Tungjitkusolmun, H. Cao, J.-Z. Tsai, J. G. Webster, V. R. Vorperian, and J. A. Will, “A new catheter design using needle electrode for subendocardial RF ablation of ventricular muscles: finite element analysis and in-vitro experiments,” IEEE Trans. Biomed. Eng., vol. 47, pp. 23  31, 2000.

21. Methods Both FEM & in vitro experiments Vary needle diameters Vary insertion depths Vary RF ablation duration Change temperature settings Compare lesion dimensions

22. FEM Results Insertion depth (mm)Lesion width (mm)Lesion depth (mm) 2.03.242.80 4.04.524.90 6.05.306.90 8.05.609.10 Needle Diameter (insertion = 8 mm) Insertion Depth (diameter = 0.5 mm) Diameter of needle (mm)Lesion width (mm)Lesion depth (mm) 0.55.609.1 0.66.069.1 0.76.249.1 0.86.509.1 0.96.779.2 1.07.049.3

23. Conclusion Lesion depths are 1  mm deeper than the insertion depth Lesion width increases with increasing diameter and duration Confirmed by in vitro experiments Good contact

24. Needle electrode designs

25. 3. Uniform current density electrodes* *Tungjitkusolmun, S., Woo, E. J., Cao, H., Tsai, J.-Z., Vorperian, V. R., and Webster, J. G., Finite element analyses of uniform current density electrodes for radio-frequency cardiac ablation, IEEE Trans. Biomed. Eng., 47, pp. 32-40, January 2000. “hot spot” at the edge of the conventional electrode Uniform current density electrode by – Recession depth – contour on the surface of the electrode (  is the parameter for the shape function). – Filled with coating material

26. FEM results Hot spot at the edge of the metal electrode

27. Current densities at the edge of the tip electrode  is the shape function

28. Cylindrical electrodes Changing conductivities Changing the curvatures  (S/m)  is for the shape function)

29. Current density distributions Cardiac tissue Catheter body Electrode Highest current density +0.00E+00 +2.50E  01 +5.00E  01 +7.50E  01 +1.00E+00 ECDM VALUE C SCALE = 144. Flat Catheter body Cardiac tissue Coating Uniform current density +0.00E + 00 +2.50E  01 +5.00E  01 +7.50E  01 +1.00E + 00 C SCALE = 582. ECDMVALUE Recessed

30. 4. Bipolar phase-shifted multielectrode catheter ablation* *S. Tungjitkusolmun, H. Cao, D. Haemmerich, J.-Z. Tsai, Y. B. Choy, V. R. Vorperian, and J. G. Webster, “Modeling bipolar phase-shifted multielectrode catheter ablation,” in preparation, IEEE Trans. Biomed. Eng., 2000 TeTe TmTm

31. Method A. 3-D Unipolar Multielectrode Catheter (MEC) B. Optimal phase-shifted for a system with fixed myocardial properties Optimal phase-shift Optimal phase-shift: T e / T m = 1 C. Effects of changes in myocardial properties on the optimal phase-shift D. Optimal phase-shift for MEC with 3 mm spacing

32. FEM results Phase = 0  Phase = 26.5  Phase = 45 

33. Phase vs. T e /T m Changes in electrical conductivity

34. Changes in thermal conductivity

35. Electrode spacing (2mm vs. 3mm)

36. Simplified Control system

37. 5. FEM predicts lesion size* Ablation over the mitral valve annulus Ablation underneath the mitral valve leaflets *S. Tungjitkusolmun, V. R. Vorperian, N. C. Bhavaraju, H. Cao, J.-Z. Tsai, and J. G. Webster, “Guidelines for predicting lesion size at common endocardial locations during radio-frequency ablation,” submitted to IEEE.Trans. Biomed. Eng., 1999.

38. Physical conditions LocationBlood velocity (cm/s) h b at blood  myocardium interface [(W/(m 2  K)] h be at blood  electrode interface [W/(m 2  K)] Position 1 11.014174191 Position 2 2.75442197 PositionContactBlood flow 1. Above the mitral valve1.3 mm embeddedHigh 2. Underneath the mitral valve3.0 mm embeddedLow

39. Temperature Controlled RF Lesion volume vs. time

40. Power controlled RF Lesion volume vs. time

41. 6. FEM for Hepatic Ablation* *S. Tungjitkusolmun, S. T. Staelin, D. Haemmerich, J.-Z. Tsai, H. Cao, V. R. Vorperian, F. T. Lee, D. M. Mahvi, and J. G. Webster, “Three-dimensional finite element analyses for radio-frequency hepatic tumor ablation,” submitted to IEEE. Trans. Biomed.Eng., 2000. Hepatic Ablation: Use RF probe to destroy tumor cancer, or cirrhosis Minimally invasive Present: -High recurrence rate -Small lesions

42. Models 4-tine RF Probe Geometry for FEM, 352,353 tetrahedral elements

43. Effect of Blood Vessel Location No Blood VesselBlood Vessel at 1 mm

44. Blood vessel at 5 mm

45. Bifurcated blood vessel

46. Summary 1. Outline a process for FEM creation for RF ablation 2. Show that needle electrode catheter design can create deep lesions by FEM & in vitro studies 3. Uniform current density electrodes reduce “hot spots” 4. Bipolar phase-shifted multielectrode catheter can create long and contiguous lesions 5. We can use FEM to predict lesion formations 6. Apply FEM for RF ablation to hepatic ablation

47. Bipolar Hepatic Ablation BipolarUnipolar