A New Bipolar Probe for Hepatic Radiofrequency Ablation

1
A New Bipolar Probe for Hepatic Radiofrequency
Ablation

• Concepts, mathematical modeling and in vitro and
  in vivo animal testing

2
Background significance

• Radiofrequency Ablation (RFA) is used in multiple
  clinical settings its use in the treatment of
  liver tumors has increased recently, but there
  are limitations to its widespread use
• Studies have demonstrated both an increased rate
  of recurrence over surgical resection and
  cryosurgical ablation as well as unpredictability
  in lesion morphology due to the heterogeneous
  nature of liver tissue

3
Concepts

• Alterations in probe and circuit designs may help
  improve the limitations of RFA to broaden its use
• Bipolar probe structure has the potential
  advantage of increasing lesion size for the
  amount of power applied and for the length of
  time applied
• Electrical and Thermal characteristics of this
  strategy may help overcome the problems of lesion
  morphology and perivascular recurrence that are
  attributed to blood flow effects

4
Design of current probes

• Monopolar radio-frequency electrical system that
  operates at 460kHz
• Umbrella-like arrays and Cooled-TipTM Probes

5
Monopolar Circuit

• Temperature control mode with feedback to power
  source and distant groundpad

6
Monopolar Finite Element Model

• Mathematical models using FEMs enable probe
  designs to be tested and modified
• In vivo scenarios with umbrella probes show
  unequal current and temperature patterns

7
Prototype Bipolar Probe

• Bipolar current strategy keeps RF current
  concentrated in the area between the umbrella
  arrays

8
Bipolar circuit with single probe feedback

• RITA Model 30 with modification to allow bipolar
  RFA
• Temperature control feedback is via one probe
• Other probe temperature is not monitored

9
Bipolar Finite Element Model

• Volumes generated by bipolar arrays are greater
  than two monopolar volumes added together

10
In vitro Monopolar lesions

• Bovine livers 6 lesions created with the RITA
  model 30 RFA probe
• 12 minute ablation at Temperature control of 95oC
• Mean volume 15.04.6 cm3

11
In vitro Bipolar lesions

• Bovine livers 6 lesions created with two
  modified RITA model 30 RFA probes
• 12 minute ablation at Temperature control of 95oC
• Mean volume 23.56.0 cm3

12
In vivo Monopolar lesions

• Porcine livers 7 lesions created with the RITA
  model 30 RFA probe
• 12 minute ablation at Temperature control of 95oC
• Mean volume 3.931.8 cm3

13
In vivo Bipolar lesions

• Porcine livers 10 lesions created with two
  modified RITA model 30 RFA probes
• 12 minute ablation at Temperature control of 95oC
• Mean volume 12.23.0 cm3

14
Bipolar vs. Monopolar Lesion Volumes in vitro in
bovine livers

• All lesions created at the same RFA settings
• Monopolar lesions mean volume 15.04.6 cm3
• Bipolar lesions mean volume 23.56.0 cm3
• This equals a 57 increase in mean lesion volume
  in vitro

15
Bipolar vs. Monopolar RFA Lesion Volumes in vivo
in porcine livers

• All lesions created at the same RFA settings
• Monopolar lesions mean volume 3.91.8 cm3
• Bipolar lesions (at the same settings) mean
  volume 12.1 3.0 cm3
• This equals a 210, or 3-fold increase in mean
  lesion volume in vivo

16
Potential Pitfalls

• Cold areas may be generated depending on the
  local hemodynamics of an area of liver
• e.g. one array is adjacent to a high flow blood
  vessel, making heating of the two probes unequal
• This can result in inadequate tissue heating
• We can manipulate the circuitry to optimize RFA
  lesions in this and other settings

17
Bipolar lesion with unequal heat distribution
near larger vessel
18
The Tri-polar Circuit
19
Conclusions

• Bipolar RFA using umbrella probes is a novel
  design for administering RF current in the
  setting of liver lesions
• Bipolar arrays benefit from increased current
  density and thermal shielding from the cooling
  effects of blood flow, thus increasing lesion
  size
• Lesions are greater than the sum of their parts
• This modality has the potential benefits of not
  only larger, more predictable lesions, but
  decreased risk of adjacent organ injury as well