“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

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

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

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

System for Cardiac Ablation

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

Tip Electrode RF generator

Energies Involved in RF Ablation Process

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

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

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 )

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

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, 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%

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

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

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

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

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

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.

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

FEM Results Insertion depth (mm)Lesion width (mm)Lesion depth (mm) Needle Diameter (insertion = 8 mm) Insertion Depth (diameter = 0.5 mm) Diameter of needle (mm)Lesion width (mm)Lesion depth (mm)

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

Needle electrode designs

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 , January “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

FEM results Hot spot at the edge of the metal electrode

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

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

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

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

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

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

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

Changes in thermal conductivity

Electrode spacing (2mm vs. 3mm)

Simplified Control system

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.

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 Position PositionContactBlood flow 1. Above the mitral valve1.3 mm embeddedHigh 2. Underneath the mitral valve3.0 mm embeddedLow

Temperature Controlled RF Lesion volume vs. time

Power controlled RF Lesion volume vs. time

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., Hepatic Ablation: Use RF probe to destroy tumor cancer, or cirrhosis Minimally invasive Present: -High recurrence rate -Small lesions

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

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

Blood vessel at 5 mm

Bifurcated blood vessel

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

Bipolar Hepatic Ablation BipolarUnipolar