1 / 46

Finite Element Modeling of Radiofrequency Cardiac and Hepatic Ablation

Use Finite Element Modeling (FEM) to improve the efficacy of current RF ablation technologies and design new electrodes for cardiac and hepatic ablation procedures.

riesf
Download Presentation

Finite Element Modeling of Radiofrequency Cardiac and Hepatic Ablation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. “Finite Element Modeling of Radiofrequency Cardiac and Hepatic Ablation”SUPAN TUNGJITKUSOLMUNDept. Of Electrical and Computer EngineeringUniversity of Wisconsin-MadisonAdvisor: Professor John G. Webster

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

  3. 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

  4. 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

  5. System for Cardiac Ablation

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

  7. Tip Electrode RF generator

  8. Energies Involved in RF Ablation Process

  9. 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

  10. 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

  11. 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 )

  12. 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

  13. 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.    

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

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

  16. Maximum changes in Lesion Size Power controlled

  17. Temperature controlled

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

  19. 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.

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

  21. 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)

  22. 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

  23. Needle electrode designs

  24. 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.

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

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

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

  28. 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

  29. 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

  30. 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

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

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

  33. Changes in thermal conductivity

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

  35. Simplified Control system

  36. 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.

  37. 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

  38. Temperature Controlled RF Lesion volume vs. time

  39. Power controlled RF Lesion volume vs. time

  40. 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.

  41. 4-tine RF Probe Models Geometry for FEM, 352,353 tetrahedral elements

  42. Effect of Blood Vessel Location No Blood Vessel Blood Vessel at 1 mm

  43. Blood vessel at 5 mm

  44. Bifurcated blood vessel

  45. 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

  46. Bipolar Hepatic Ablation Bipolar Unipolar

More Related