John G. WebsterDepartment of Biomedical Engineering

University of WisconsinMadison WI 53706 USAwebster@engr.wisc.edu

Supported by NIH grant HL56143

Electrode design for cardiac Electrode design for cardiac radio-frequency ablationradio-frequency ablation

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

Diagnosis - SVT (Accessory Diagnosis - SVT (Accessory Pathway)Pathway)

Beat By Beat MappingBeat By Beat MappingTechniques Are UsedTechniques Are Used

System records location through constant interrogation of theSystem records location through constant interrogation of the magnetic field generated from the location padmagnetic field generated from the location pad

Records Location 1Records Location 1

Beat by Beat MappingBeat by Beat Mapping

Records location 3Records location 3

• Superimposes point location and local activation Superimposes point location and local activation times times

• Connects neighboring points, creating trianglesConnects neighboring points, creating triangles

10ms10ms

5050msms

100ms100ms

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

Sinus Rhythm with Surgery- Sinus Rhythm with Surgery- Maze ProcedureMaze Procedure

Picture of Newer Catheters Picture of Newer Catheters (NASPE)(NASPE)

Bipolar Hepatic AblationBipolar Hepatic Ablation

Bipolar Unipolar

Four-terminal measurementFour-terminal measurement

101

102

103

104

105

106

100

200

300

400

500

600

Frequency, Hz

Myo

card

ial r

esis

tivity

, oh

m*c

m

1979 van Oosterom: dog 1987 Ellenby: dog, in vitro 1993 Fallert: sheep, in vivo1994 Steendijk: dog 1996 Bragos: pig, in vivo 1997 Cinca: pig, in vivo

VI

Four-terminal resistivity probe

Kcircuit = (Vc / Ic) / (Vv / Vi)

Current source

Tissue

Ve

Ie

+ Vc - Ic

Function generator

Differential amplifier

Current-to-voltage

converter

Measurement circuit and equipment

Vv Vi

Vch1 ,Vch2

Digital oscilloscope

Kosc

Computer (LabView virtual instrument)

Resistancedetector

Data acquisition

unit

ThermistorTeflon coating

Silver

Epoxy

Kcircuit

tissue = Vch1 / Vch2 Kosc Kcircuit Kwire Kprobe

Kosc = (Vv / Vi) / (Vch1 / Vch2)

Kprobe

KwireKwire = (Ve / Ie) / (Vc / Ic)

Kprobe = tissue / (Ve / Ie)

Flow Simulation SystemFlow Simulation System

Depth meter

Catheter

Saline

Pump

Flowmeter

Water bath

Myocardium

Frame

Saline

Tube

Top view offrame Crossbar

Sidebar

Probe

Dispersive electrode

Specification of Flow SystemSpecification of Flow System

Temperature 37 ± 1 °C Flow rate 0 to 6 L/min Solution 0.5% saline Ablation generator: EPT-1000XP Ablation catheter: 7Fr (2.5 mm diameter) Depth meter 0.02 mm accuracy Myocardial size 30 30 15 mm

Temperature MeasurementTemperature MeasurementGoal: Measure the temperature change inside

myocardium during ablation.

Previous Work Labonté: Thermographic camera Kaouk: Fluoroptic thermometer Hynynen: Impedance & power Nakagawa: Fluoroptic thermal probe

Temperature System SetupTemperature System Setup

During ablation, we measure both the catheter tip temperature and thermocouple temperature inside the myocardium.

Thermocouple ProbeThermocouple Probe

IT-21 copper/constantan T-type thermocouple0.08 s response time, 0.41 mm diameterProbe 0.9, 2.0, 2.9 mm from tip

1.5 mm in diameter

Thermocouple

Silver wire

Top view

Silver wire

Side view

Probe Insertion ProcedureProbe Insertion Procedure

Myocardium

Steel needle

Plastic tube

Thermocouple

Catheter

Thermocouple CircuitThermocouple Circuit

cold junctionLT 1025

+5 V0.05 mF

43 kW

0.1 mF

Thermocouple

LM 627

0.1 mF

170 W 430 kW

330 kW

43 kW

Shie ld ingBox

ADCBoard

PersonalComputer

7812

7912

+12 V

12 V

Shie ldedextension cable

S ignal fromthermistor

Minimize the RF InterferenceMinimize the RF Interference

Low pass filters at different stages Grounded shielding box Battery supply to avoid power interference Shielded thermocouple and cables Star network layout to avoid ground loops

Thermistor CircuitThermistor Circuit

Constant current on thermistor from generator

Measure the voltage across the catheter tip.

An LP filter (fc = 95 Hz) to minimize the RF interference.

Ablation unitEPT-1000XP

Thermistor atcatheter tip

DI220ADC converter

1 k W 1 k W

0.1 mF 0.1 mF

CalibrationCalibration

Calibrate between 25 and 95 CExtrapolate to 100 C

Thermocouple cbVaVT 2

Thermistor dcVbVaVT 23

Polynomial curve fitting

Flow Effect on Lesion FormationFlow Effect on Lesion Formation

Langberg et al.: Different electrode sizes (4, 8, 12 mm) to create lesions (convection surface)

Nakagawa et al.: Saline-irrigated (cold saline ejected from catheter tip) catheter to create the lesion (temperature difference)

Peterson et al. studied the lesion dimensions at different flow rates with a catheter laid down setup

RationaleRationaleFlow effect during temperature modeFlow effect during temperature mode

The cooling effect requires more power from catheter

Current density and Joule heat generation inside the myocardium increase. More tissue exceeds 50 C threshold.

The directly heated rim rises to a higher temperature and becomes larger

Myocardial temperature rises faster. More time to conduct heat further.

Ablation ProcedureAblation Procedure Ablate in temperature mode with thermocouple

probe inside the myocardium Stain myocardium with p-nitro blue tetrazolium

chloride and take pictures using digital camera 6 persons measure independently and average

their results Calculate dimensions (assuming ax symmetric) T: 60 C & 80 C Flow 0, 1 & 3 L/min

8 ablation /case

Lesion DimensionLesion Dimension

Volume

Border: From dark to pink border

CD

AB

V22

2

3

1

23

2

Lesion dimension vs. flowLesion dimension vs. flow

max depth vs flow

0

2

4

6

8

10

-1 0 1 2 3 4

flow(L/min)

dept

h (m

m)

max diameter vs flow

02468

10121416

-1 0 1 2 3 4

flow(L/min)

diam

eter

(mm

)

volume vs flow

0

100

200

300

400

500

600

-1 0 1 2 3 4

flow(L/min)

volu

me

(mm

3 )

Power vs flow

0

10

20

30

40

50

60

-1 0 1 2 3 4

Flow Rate (L/min)

Aver

age

Pow

er

(W)

Higher target temperature requires more power Higher target temperature results in larger lesion

(both depth and diameter) Higher flow requires more power Higher flow rate yields larger lesion (both depth

and diameter)

Temperature recordingTemperature recording

Two recordings of ablation.

Ttip rises faster and maintains at Ttarget.

Myocardial T rises gradually and may exceed Ttip.

60 °C 1 L/min

30

40

50

60

70

80

90

0 40 80 120Time (s)

Tem

pera

ture

(°C

)

T0TC1

TC3

TC2

80 °C 1 L/min

30

40

50

60

70

80

90

0 40 80 120Time (s)

Tem

pera

ture

(°C

) T0 TC1

TC3

TC2

Table of Temperature recordingTable of Temperature recording

 

  0 L/min 1 L/min 3 L/min

60 C t50 Tm t50 Tm t50 Tm

TC1 20 53 8 60 8 67

TC2 50 ~50* 20 55 16 60

TC3 NA 46 46 51 30 55

           

80 C t50 Tm t50 Tm t50 Tm

TC1 7 76 6 83 5 81

TC2 7 73 7 78 6 71

TC3 9 68 7 72 7 64

High flow: smaller t50, higher Tm

80 80 C 3 L/min caseC 3 L/min case80 °C 3 L/min

30

40

50

60

70

80

90

0 40 80 120Time (s)

Te

mp

era

ture

(°C

)

T0TC1

TC3

TC2

Tip temperature is well below the thermocouple temperature inside myocardium.

Slight charring during ablation. Impedance Speculation: Charring covers the thermistor and prevents correct myocardial temperature reading.

Myocardium

Catheterelectrode

BloodTherm istor

Electriccurrent

Impedance, power and temperature of normal ablationImpedance, power and temperature of normal ablation

Impedance, power and temperature of 3 L/min 80 Impedance, power and temperature of 3 L/min 80 °C°C

ConclusionConclusion

Setup of an in vitro system to study RF catheter ablation

Study of the temperature setting on the lesion volume Study flow rate effect on the lesion volume and

temperature change inside the myocardium Tissue charring under high flow rate

Important Parameters Important Parameters Affecting Lesion DimensionAffecting Lesion Dimension

Tissue and blood properties Applied power during ablation.Duration of ablation.Target temperature in temperature mode.Blood flow around catheter.Contact condition such as penetration

depth, contact angle.

Papers and links posted at:Papers and links posted at:

http://rf-ablation.engr.wisc.edu/

John G. WebsterDepartment of Biomedical Engineering

University of WisconsinMadison WI 53706 USAwebster@engr.wisc.edu

Supported by NIH grant HL56143