““Finite Element Modeling of Finite Element Modeling of Radiofrequency Cardiac and Radiofrequency Cardiac and

Hepatic Ablation”Hepatic Ablation”

SUPAN TUNGJITKUSOLMUNSUPAN TUNGJITKUSOLMUNDept. Of Electrical and Computer Engineering

University of Wisconsin-Madison

Advisor: Professor John G. Webster

GoalGoal

Use Finite Element Modeling (FEM) to Improve the Efficacy of

Current RF Ablation Technologies and to Design New Electrodes

Introduction: RF ablation & FEMOverview: Finite element modeling process1. Effects of changes in myocardial properties2. Needle electrode creates deep lesions3. Uniform current density electrodes4. Bipolar phase-shifted multielectrode catheter5. Use FEM to predict lesion dimensions6. FEM of hepatic ablation

OutlineOutline

95% success rate in curing Supraventricular tachycardiasLow success rate for hepatic ablationDevelopment for VT (Large lesions)Development for AFIB (long thin lesions)

IntroductionIntroduction

What Is Ablation?Modes of operation

~500 kHz, < 50 WTemperature-controlledPower-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 AblationSystem for Cardiac Ablation

RF generator

Handle

Reference patchelectrode on the

dorsal side

Catheter body

Ablationelectrode

Common cardiac ablation sitesCommon cardiac ablation sites AV Node Above the tricuspid valves Above and underneath the

mitral valves Ventricular walls Right ventricular outflow tract Etc.

Tip ElectrodeTip Electrode RF generatorRF generator

Energies Involved in RF Energies Involved in RF Ablation ProcessAblation Process

Catheterbody

Myocardium

Blood

Convective coolingfrom blood

Electrode

Joule heat

Conduction tomyocardium

Conduction toelectrode

50 °C after1 s

50 °C after60 s

Bioheat EquationBioheat Equation

)( blb TThT

k n

Heat transfer coefficient Blood temperature

Density

Specific heat

Thermal conductivity

Time

Temperature

Current density

Electric field intensity

heat loss to blood

perfusionVARIABLES

Heat Change

MATERIAL PROPERTIES

Electrical conductivity

Density

Specific heat

Thermal conductivity

Time

Temperature

Current density

Electric field intensity

heat loss to blood

perfusion

heat loss to blood

perfusion

Heat Conduction

Joule Heat

Finite Element AnalysisFinite 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 GenerationProcess 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 ApplicationsModes of RF Energy Applications

Maintain the tip temperature at a preset valueAdjust voltage applied to the electrode

Temperature controlled ablationTemperature controlled ablation

Power controlled ablationPower controlled ablation

Maintain power delivered at a preset valueAdjust voltage applied to the electrode

1. Effects of changes in myocardial 1. Effects of changes in myocardial properties to lesion dimensions*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 conductivity1.2 Thermal conductivity1.3 Specific heat (Density)

Material Material PropertiesProperties

For each case:For each case: Temperature independentTemperature dependentIncrease by 50%, or 100%Decrease by 50%

FEM resultsFEM results

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

Temperature distribution after 60 sTemperature distribution after 60 s

Maximum temperature ~ 95 C

Highest temperature

Maximum changes in Lesion SizeMaximum changes in Lesion Size

Property Case % Volume Change

Electrical conductivity

50% 58.6

Thermal conductivity

+100% 60.7

Specific heat 50% +43.2

Power controlled

Property Case % Volume Change

Electrical conductivity

50% +12.9%

Thermal conductivity

50% 21.0%

Specific heat +100% 29.4%

Temperature controlled

ConclusionConclusion

Temperature dependent properties are important

Errors in Power-Controlled Mode are higher

Better measurement techniques are needed

2. Needle electrode design for VT*2. Needle electrode design for VT*

20

40 4010

1.3r

2

d

z

r

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. 2331, 2000.

MethodsMethods

Both FEM & in vitro experimentsVary needle diametersVary insertion depthsVary RF ablation durationChange temperature settingsCompare lesion dimensions

FEM ResultsFEM Results

Insertion depth (mm) Lesion width (mm) Lesion depth (mm)2.0 3.24 2.804.0 4.52 4.906.0 5.30 6.908.0 5.60 9.10

Needle Diameter (insertion = 8 mm)

Insertion Depth (diameter = 0.5 mm)

Diameter of needle (mm) Lesion width (mm) Lesion depth (mm)

0.5 5.60 9.1

0.6 6.06 9.1

0.7 6.24 9.1

0.8 6.50 9.1

0.9 6.77 9.2

1.0 7.04 9.3

ConclusionConclusion

Lesion depths are 1mm deeper than the insertion depth

Lesion width increases with increasing diameter and duration

Confirmed by in vitro experimentsGood contact

Needle electrode designsNeedle electrode designs

3. Uniform current density electrodes*3. Uniform current density electrodes*r

z

s

L 1

Insulator

1.3 mmd

Electrode

(a)

l

L 2

Electrode

(b)

z

1.3 mm

Insulatord

rCoating

*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

FEM resultsFEM results

BloodCardiac tissue

Hot spot

+3.70E+01+4.12E+01+4.54E+01+4.96E+01+5.38E+01+5.80E+01+6.23E+01+6.65E+01+7.07E+01+7.49E+01+7.91E+01+8.33E+01+8.75E+01

TEMP VALUE

Hot spot at the edge of the metal electrode

Current densities at the edge Current densities at the edge of the tip electrodeof the tip electrode

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 12

3

4

5

6

7

8

9

10x 10

-3 Current density distribution

Distance (mm)

Cu

rre

nt

den

sity

(A

/mm

2 )

Flat

= 20

= 1 = 5

= 2

is the shape function

Cylindrical electrodesCylindrical electrodes

Changing conductivities Changing the curvatures (S/m) is for the shape function)

Current density distributionsCurrent density distributions

Cardiac tissue

Catheter body

Electrode

Highest currentdensity

+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 currentdensity

+0.00E + 00

+2.50E 01

+5.00E 01

+7.50E 01

+1.00E + 00

C SCALE = 582.

ECDM VALUE

Recessed

 

4. Bipolar phase-shifted 4. Bipolar phase-shifted multielectrode catheter ablation*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

Te

Tm

MethodMethod

A. 3-D Unipolar Multielectrode Catheter (MEC)B. Optimal phase-shifted for a system with fixed

myocardial properties

Optimal phase-shiftOptimal phase-shift: Te / Tm = 1C. Effects of changes in myocardial properties on

the optimal phase-shiftD. Optimal phase-shift for MEC with 3 mm

spacing

FEM resultsFEM results

Phase = 0Phase = 26.5Phase = 45

Phase vs. Phase vs. TTee//TTmm

Effect of electrical conductivity

00.20.40.60.8

11.21.41.61.8

0 10 20 30 40 50Phase (°)

Te

/Tm

control

low

high23.5° (high)

26.5° (control)

29.5° (low)

Changes in electrical conductivity

Changes in thermal conductivityChanges in thermal conductivity

Effect of thermal conductivity

00.20.40.60.8

11.21.41.61.8

2

0 10 20 30 40 50Phase (°)

Te

/Tm

control

low

high

26.5°

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

Effect of inter-electrode distance

00.20.40.60.8

11.21.41.61.8

0 10 20 30 40 50Phase (°)

Te

/Tm 2 mm

3 mm

30.5° (3 mm)

26.5° (2 mm)

Simplified Control systemSimplified Control system

5. FEM predicts lesion size*5. FEM predicts lesion size*Ablation over the mitral valve annulusAblation 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 conditionsPhysical conditions

Location Blood velocity (cm/s)

hb at bloodmyocardium

interface [(W/(m2K)]

hbe at bloodelectrode

interface [W/(m2K)]

Position 1

11.0 1417 4191

Position 2

2.75 44 2197

Position Contact Blood flow

1. Above the mitral valve 1.3 mm embedded High

2. Underneath the mitral valve 3.0 mm embedded Low

W

D

1.3 mm

Lesion

MyocardiumBlood

D

W

3 mm

Lesion

Blood

Myocardium

(a) (b)

Temperature Controlled RFTemperature Controlled RF

Lesion volume vs. time

Power controlled RFPower controlled RF

Lesion volume vs. time

6. FEM for Hepatic Ablation*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

ModelsModels

4-tine RF ProbeGeometry for FEM, 352,353 tetrahedral elements

Effect of Blood Vessel LocationEffect of Blood Vessel Location

No Blood Vessel Blood Vessel at 1 mm

Blood vessel at 5 mmBlood vessel at 5 mm

Bifurcated blood vesselBifurcated blood vessel

+37.0

+41.1

+45.2

+49.2

+53.3+57.4+61.5

+65.5+69.6

+73.7

+77.8

+81.9

+85.9

+90.0

TEMP VALUE

Blood vessel

Liver

Probe

ABHot spot

SummarySummary

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 formations6. Apply FEM for RF ablation to hepatic ablation

Bipolar Hepatic AblationBipolar Hepatic Ablation

Bipolar Unipolar