“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
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
Introduction
95%
What
Is Ablation?
Modes
of operation
Present Technology
success rate in
curing
Supraventricular
Heating
of cardiac
tissue
~500 kHz,
< 50
W
tachycardias
to cure
rhythm
disturbances
Temperature-controlled

Low
success and
rate of
for
tissue
to cure
hepatic
ablation
liver
Power-controlled
cancer
Development for VT
(Large lesions)
Development for AFIB
(long thin lesions)
System for Cardiac Ablation
Reference patch
electrode on the
dorsal side
Handle
Ablation
electrode
RF generator
Catheter body
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
n
Blood perfusio
Conduction to
myocardium
Conduction to
Joule heat
electrode
50 °C after
1s
50 °C after
60 s
Electrode
Catheter
body
Blood
Myocardium
Convective cooling
from blood
Bioheat Equation
Specific Temperature Thermal
MATERIAL
VARIABLES
PROPERTIES
conductivity
heat
heat loss
to blood
perfusion
T
c
   kT  JE  Qh
t
Current
heat loss
Electric
field
Heat
Density Heat
density
Joule Heattointensity
blood

T
Time
Conduction
Change k
 hb (T  Tbl ) perfusion
Electrical
n
conductivity
Blood temperature
Heat transfer coefficient
J  E
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

Preprocessing (PATRAN 7.0)
Geometry
Material Properties
Boundary Cond.
Mesh Generation

Initial Conditions
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
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
1. Effects of changes in myocardial
properties to lesion dimensions*
Material Properties

For each case:

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%
*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.
FEM results
Lesion growth over time (Red is 50 C or higher)
Temperature distribution after 60 s
Highest
temperature
Maximum temperature ~ 95 C
Maximum changes in Lesion Size
Power controlled
Property
Electrical
conductivity
Thermal
conductivity
Specific heat
Case
50%
% Volume Change
58.6
+100%
60.7
50%
+43.2
Temperature controlled
Property
Electrical
conductivity
Thermal
conductivity
Specific heat
Case
50%
% Volume Change
+12.9%
50%
 21.0%
+100%
 29.4%
Conclusion

Temperature dependent properties are
important
 Errors in Power-Controlled Mode are higher
 Better measurement techniques are needed
2. Needle electrode design for VT*
r
z
20
r
1.3
d
2
40
10
40
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
Needle Diameter (insertion = 8 mm)
Diameter of needle (mm)
0.5
0.6
0.7
0.8
0.9
1.0
Lesion width (mm)
5.60
6.06
6.24
6.50
6.77
7.04
Lesion depth (mm)
9.1
9.1
9.1
9.1
9.2
9.3
Insertion Depth (diameter = 0.5 mm)
Insertion depth (mm)
2.0
4.0
6.0
8.0
Lesion width (mm)
3.24
4.52
5.30
5.60
Lesion depth (mm)
2.80
4.90
6.90
9.10
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
Needle electrode designs
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

r
s
Insulator
d
1.3 mm
z
L1
Electrode
(a)
r
Coating
l
Insulator
d
1.3 mm
L2
Electrode
(b)
z
*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.
FEM results
TEMP
VALUE
+3.70E+01
+4.12E+01
+4.54E+01
+4.96E+01
+5.38E+01
+5.80E+01
Cardiac tissue
+6.23E+01
+6.65E+01
+7.07E+01
+7.49E+01
+7.91E+01
+8.33E+01
+8.75E+01
Blood
Hot spot
Hot spot at the edge of the metal electrode
Current densities at the edge
of the tip electrode
-3
10
Current density distribution
x 10
Current density (A/mm2)
9
8
7
6
Flat
5
a = 20
a=2
a=5
4
a=1
3
2
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Distance (mm)
a is the shape function
1
Cylindrical electrodes
Current density distribution
-3
Current density (A/mm 2)
3.5
3
-3
7
Flat
 = 0.222
2.5
2
1.5
x 10 Current density distribution of a 15-mm long electrode
6
 = 0.333
Current density (A/mm2)
4
x 10
 = 0.111
 = 0.074
1
5
4
3
a = 0.50
a = 0.95
a = 2.00
2
 = 0.0555
0.5
-2.5
-2
-1.5
-1
-0.5
Distance (mm)
Changing conductivities
 (S/m)
0
1
-8
-7
-6
-5
-4
-3
-2
-1
0
Distance (mm)
Changing the curvatures
(a is for the shape function)
Current density distributions
Catheter body
ECDM
VALUE
ECDM
Catheter body
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
C SCALE = 144.
Cardiac tissue
C SCALE = 582.
Electrode
Uniform current
density
Highest current
density
Coating
Flat
Recessed
4. Bipolar phase-shifted
multielectrode catheter ablation*

Blood
Voltage (V)
Tm
VB
Time
(s)
VA
Te
Te
Tm
A
Myocardium
B
Metal
electrodes
Tm
Te
C
Plastic
Blood
*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
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
FEM results
Phase = 0
45
26.5
Phase vs. Te/Tm
Effect of electrical conductivity
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
26.5° (control)
control
low
high
Te/Tm
29.5° (low)
23.5° (high)
0
10
30
20
Phase (°)
40
50
Changes in electrical conductivity
Changes in thermal conductivity
Effect of thermal conductivity
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
26.5°
Te/Tm
control
low
high
0
10
20
30
Phase (°)
40
50
Electrode spacing (2mm vs. 3mm)
Effect of inter-electrode distance
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
26.5° (2 mm)
Te/Tm
30.5° (3 mm)
0
10
20
30
Phase (°)
40
2 mm
3 mm
50
Simplified Control system
Ground
Myocardium
Maximum
temperature
detector
RF voltage
generator
Phase-shift
Automatic
control unit
RF voltage
generator
5. FEM predicts lesion size*

Ablation over the mitral valve annulus
 Ablation underneath the mitral valve leaflets
Ventricular
outflow tract
Highest velocity
AO
LA
Over leaflets ( 1)
LV
High velocity
Low velocity
MV annulus
Underneath leaflets ( 2)
LV wall
*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
Position
Contact
Blood flow
1. Above the mitral valve
1.3 mm embedded
High
2. Underneath the mitral valve
3.0 mm embedded
Low
Location
Blood D
velocity
(cm/s)
hb at
W
bloodmyocardium
Myocardium
interface
[(W/(m2K)]
1.3 mm
Lesion
hbe at
bloodelectrode
interface
[W/(m2K)]
D
Position
1
11.0 Lesion
Position
2
2.75
Blood
W
1417
44
Blood
Myocardium
(a)
4191
3 mm
(b)
2197
Temperature Controlled RF
Volume vs. time (temperature-controlled)
1000
Lesion volume (mm 3 )
80 C (2)
800
600
70 C (2)
70 C (1)
400
60 C (1)
200
60 C (2)
0
0
20
40
60
Time (s)
80
100
Lesion volume vs. time
120
Power controlled RF
Volume vs. time (power-controlled)
700
Lesion volume (mm 3)
600
Position 2
500
400
300
Position 1
200
100
0
0
20
40
60
Time (s)
80
100
Lesion volume vs. time
120
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.
Models
4 tines of
electrode
Stainless steel
trocar
Liver
B
Blood vessel
(10 mm)
Insulated
trocar
A
4-tine RF Probe
Geometry for FEM, 352,353 tetrahedral elements
Effect of Blood Vessel Location
No Blood Vessel
Blood Vessel at 1 mm
Blood vessel at 5 mm
Bifurcated blood vessel
TEMP
VALUE
+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
Blood vessel
Liver
B
A
Hot spot
Probe
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
Bipolar
Unipolar