Final PhD Defense

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Final PhD Defense

• FINITE ELEMENT MODELING
• OF
• HEPATIC RADIO-FREQUENCY ABLATION

DIETER HAEMMERICH  UNIVERSITY OF
WISCONSIN-MADISON DEPARTMENT OF BIOMEDICAL
ENGINEERING
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Introduction

• Hepatic tumors belong to the most common types of
  cancer
• Estimated annual mortality in US of 56,000
• Surgical resection is only possible less than 30
  of the cases
• The major clinically used alternative treatment
  methods with the US are RF ablation and cryo
  ablation
• Other ablative methods under investigation are
  microwave ablation, laser ablation, ethanol
  injection, ultrasound ablation

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Cryo Ablation

• Cryo ablation uses cold (- 100C) to kill tumor
  tissue
• Open surgery is necessary (bleeding)
• Local recurrence rates may be lower than RF
  ablation
• Multiple probes can be used simultaneously
• Lesion is more predictable

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Radio Frequency Ablation

• RF ablation was introduced in the 1990s for
  treatment of hepatic tumors
• RF (500 kHz) is used to heat tissue above 50C
  (-gt coagulation necrosis)
• Current is applied via catheter, that can be
  introduced transcutaneously (minimally invasive)
• Patient under local anesthesia and conscious
  sedation, or light general anesthesia
• Current travels between catheter and dispersive
  electrode (i.e. ground pad)

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RF Ablation Catheter Types
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Ablation Procedure
Probe insertion
Extension of prongs
RF current application
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Parameters

• Treatment time is 10-30 minutes for a single
  application
• Multiple applications are usually necessary for
  treatment of a single tumor
• Current max. lesion sizes are 4 cm
• Applied power of 50 200 W
• Temperature Control (95 C) or Impedance Control
  is used to avoid charring

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Shortcomings

• High local recurrence rates ( 40)
• Large vessels act as heat sinks, that may prevent
  destruction of tumor cells close to them
• Limited lesion size makes multiple applications
  necessary
• Insufficient imaging modalities

Multiple applications necessary to cover large
tumor
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FEM models

• We have to solve the Bioheat Equation

J Current Density k Thermal Conductivity E
Electric Field hbl heat transfer coefficient
(determines perfusion) c Heat
Capacity ? Mass Density

• We divide the model (tissue, probe) into small
  tetrahedral elements differential equation -gt
  algebraic equations

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Rita model 30 umbrella probe
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1. Bipolar Ablation
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Modified Catheter
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• Effect due to more homogenous electric field is
  negligible

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Results of in-vivo experiments

• Monopolar lesions had 3.9 1.8 cm3 volume (n7)
• Bipolar lesions had 12.2 3 cm3 volume (n10)

Bipolar Lesion
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• Disadvantage Deposited power cannot be
  controlled independentely at the two probes
• Non-uniform heating can result

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2. Multiple Probe RF Ablation

• One Advantage of cryo ablation is the option to
  use multiple probes simultaneously
• Large Lesions can be covered better
• Multiple tumors can be treated simultaneously

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FEM Results
Alternating Monopolar
Monopolar
Bipolar
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• Bipolar mode creates highest temperature between
  probes, but can only be used with 2 probes.
  Positioning of probes is important. Does not
  allow independent control of power deposition.
• Monopolar mode creates lowest temperature between
  probes due to electric shielding
• Alternating monopolar mode creates slightly lower
  temperature than bipolar. Can be used with any
  number of probes. Allows control of power
  deposition at each probe.

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ex-vivo Experiments
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Experimental Conditions

• 2 pieces of liver tissue (26C, 16C) were
  immersed in 0.9 saline solution
• One probe was inserted into each piece
• 12 min. ablation at 95C was performed

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Results

• Alternating monopolar method allows independent
  control of multiple probes (theoretically
  unlimited number)

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3. Bipolar RF Ablation next to blood vessels

• Local tumor recurrence is associated with tumor
  cell survival next to blood vessels
• Can Bipolar ablation create lesions closer to
  blood vessels ?

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Bipolar Ablation with two 10-prong
probes. Distance from vessel 5 mm
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Results
Monopolar, d2.3 mm
Bipolar asymmetric, d1.8 mm
Bipolar symmetric, d1.0 mm
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Current Density

• Does blood vessel conductivity have significant
  impact on lesion formation ?

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Normal blood conductivity
Altered blood conductivity
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• Lesion shape is not significantly altered by
  modified blood conductivity

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Conclusion

• Bipolar Ablation creates lesions closer to blood
  vessels
• Symmetric Bipolar Configuration shows best
  performance, but may be difficult to perform in
  practice

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4. RF ablation at audio frequencies

• Our previous studies of liver metastases in rats
  showed significant differences in conductivity of
  normal vs. tumor tissue in rats

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• HypothesisIf we perform RF ablation at lower
  frequencies, we might be able to preferentially
  target tumor tissue

Tumor, d40 mm
Tumor, d20 mm
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Current Density for 20 mm diameter tumor
f 500 kHz
f 20 kHz

• There were no significant differences in lesion
  for the 40 mm diameter tumor

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Temperature for 20 mm diameter tumor
f 500 kHz
f 20 kHz
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Additional Lesion for ablation at 20 kHz
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Conclusion

• If probe is completely submerged in tumor, no
  differences in lesion are observed
• If probe is partially submerged in tumor, RF
  ablation at audio frequencies preferentially
  targets tumor tissue
• Applied frequency should be limited to 20 kHz
  to avoid excitation of tissue

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Acknowledgements

• Block, W.F.
• Johnson, C.D.
• Lee jr., F.T.
• Mahvi, D.M.
• Staelin, T.S.
• Tompkins, W.J.
• Tsai, J.Z.
• Tungjitkusolmun, S.
• Van der Weide, D.
• Webster, J.G.
• Wright, A.W.