Electrode design for cardiac radio-frequency ablation

1. Electrode design for cardiac radio-frequency ablation John G. Webster Department of Biomedical Engineering University of Wisconsin Madison WI 53706 USA webster@engr.wisc.edu Supported by NIH grant HL56143

2. Colleagues Vicken Vorperian, MD, Electrophysiologist Supan Tungjitkusolmun, Finite element modeling Hong Cao, Temperature in vitro and in vivo Jang-Zern Tsai, Myocardial resistivity Naresh Bhavaraju, Thermal properties Young Bin Choy, Mechanical compliance Dieter Haemmerich, Liver ablation

4. Diagnosis - SVT (Accessory Pathway)

6. Beat By Beat Mapping Techniques Are Used System records location through constant interrogation of the magnetic field generated from the location pad Records Location 1

7. Beat by Beat Mapping Records location 3

8. Connects neighboring points, creating triangles • Superimposes point location and local activation times 100ms 50ms 10ms

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

13. Outline • 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

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

15. System for Cardiac Ablation

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

17. Tip Electrode RF generator

18. Energies Involved in RF Ablation Process

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

20. 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

21. Preprocessing (PATRAN 7.0) ·Geometry ·Material Properties ·Initial Conditions ·Boundary Cond. ·Mesh Generation • 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) Process for FEM Generation • Convergence test (for optimal number of elements )

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

23. 1. Effects of changes in myocardial properties to lesion dimensions* Material Properties • 1.1 Electrical conductivity • 1.2 Thermal conductivity • 1.3 Specific heat (Density) • Temperature independent • Temperature dependent • Increase by 50%, or 100% • Decrease by 50% For each case: *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.    

24. FEM results Lesion growth over time (Red is 50 °C or higher)

25. Highest temperature Maximum temperature ~ 95 °C Temperature distribution after 60 s

26. Maximum changes in Lesion Size Power controlled

27. Temperature controlled

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

29. 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.

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

31. Insertion depth (mm) Diameter of needle (mm) Lesion width (mm) Lesion width (mm) Lesion depth (mm) Lesion depth (mm) 2.0 0.5 5.60 3.24 9.1 2.80 0.6 4.0 6.06 4.52 4.90 9.1 6.0 0.7 6.24 5.30 6.90 9.1 0.8 8.0 6.50 5.60 9.10 9.1 0.9 6.77 9.2 1.0 7.04 9.3 FEM Results Needle Diameter (insertion = 8 mm) Insertion Depth (diameter = 0.5 mm)

32. Conclusion • Lesion depths are 1-2 mm deeper than the insertion depth • Lesion width increases with increasing diameter and duration • Confirmed by in vitro experiments • Good contact

33. Needle electrode designs

34. 3. Uniform current density electrodes* • “hot spot” at the edge of the conventional electrode • Uniform current density electrode by • Recession depth • contour on the surface of the electrode (a is the parameter for the shape function). • Filled with coating material *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.

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

36. Current densities at the edge of the tip electrode a is the shape function

37. Cylindrical electrodes Changing conductivities Changing the curvatures s (S/m) (a is for the shape function)

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

39. Tm Te 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

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

41. FEM results Phase = 45° Phase = 0° Phase = 26.5°

42. Phase vs. Te/Tm Changes in electrical conductivity

43. Changes in thermal conductivity

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

45. Simplified Control system

46. 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.

47. Location Position Blood velocity (cm/s) Contact hb at bloodmyocardium interface [(W/(m2K)] Blood flow hbe at bloodelectrode interface [W/(m2K)] 1. Above the mitral valve 1.3 mm embedded High Position 1 11.0 1417 4191 2. Underneath the mitral valve 3.0 mm embedded Low Position 2 2.75 44 2197 Physical conditions

48. Temperature Controlled RF Lesion volume vs. time

49. Power controlled RF Lesion volume vs. time

50. 6. FEM for Hepatic Ablation* • Hepatic Ablation: Use RF probe to destroy tumor cancer, or cirrhosis • Minimally invasive • Present: -High recurrence rate -Small lesions *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.