1. Electrosurgery and ablation
John G. Webster
Department of Biomedical Engineering
University of Wisconsin
Madison WI USA

2. Electrosurgery works by cutting, fulguration or desiccation.

3. (a) Block diagram for an electrosurgical unit
(a) Block diagram for an electrosurgical unit. High-power, high-frequency oscillating currents are generated and coupled to electrodes to incise and coagulate tissue. (b) Three different electric voltage waveforms available at the output of electrosurgical units for carrying out different functions.

4. In electrosurgery, crest factor is the peak voltage divided by the rms voltage. (a) represents a cutting waveform with low crest factor. The desiccating output in (b) has a relatively greater crest factor than seen in the cutting waveform. Fulguration (c) has a crest factor even higher than that for desiccation. Adapted from Duffy and Gobb (1995).

5. Block diagram of a typical electrosurgical unit

6. Components of a modern electrosurgical system
Components of a modern electrosurgical system. The waveform selection and signal generating stage provide the desired waveforms for cutting or coagulation. The power output stage employs power transistors such as MOSFETs to amplify the waveforms and output them through an output isolation transformer. This is then applied through a system of electrodes (monopolar mode shown), where the current usually takes the path from the active electrode and back through the return electrode, or alternatively flows through other undesired low impedance pathways such as lead wires attached to ECG electrodes.

7. (a) An electrosurgical generator with the necessary controls for cut and coagulation. (a) Electrosurgical footswitches for choosing the mode of operation. From Valleylab (2001).

8. (a) Bipolar electrosurgery (b) Monopolar electrosurgery.

9. Advantages of Monopolar and Bipolar electrosurgical methods.
Can perform several techniques, especially coagulation much faster than bipolar methods.
Eliminates the possibility of return electrode burns due to its safe and precise effect.
Allows easy repositioning of electrodes to cover the required regions.
Increased operational safety due to the use of low power levels.
Monopolar
Bipolar
Can be used for a wide variety of electrosurgical procedures involving cutting and coagulation

10. Elastomeric silicone-coated cutting electrodes. From Valleylab (2001).

11. Some of the basic shapes and standard sizes of electrodes used in electrosurgery (a) Blade (b) Ball (c) Loop (d) Square (e) Conization (f) Fine wire electrodes. Courtesy of Anthony Products (2001).

12. Bipolar forceps used widely for coagulation
Bipolar forceps used widely for coagulation. Both the active and return electrodes are together unlike monopolar electrodes, which comprise only the active electrode with the inactive electrode being located at some remote site. Courtesy of Elmed.

13. The technique for removal of plantar warts involves inserting a cutting loop and rotating it to sever the wart from the plantar fascia. Adapted from Pearce (1986).

14. (a) Structure of the commonly used electrodes in laparoscopy (b), (c) &(d) some of the most commonly used electrosurgical tips in laparoscopic procedures. Courtesy: AEM Laparoscopic Instruments.

15. Simplified diagram of the typical resectoscope used in TURP procedures
Simplified diagram of the typical resectoscope used in TURP procedures. Adapted from Duffy and Cobb (1995).

16. (a), (b) and (c) show different views of a cutting electrode of rectangular shape used in dental surgery. Adapted from: US patent No

17. An aluminum foil Neutralect pregelled disposable metal foil return electrode.

18. (a) A conductive adhesive type dispersive electrode and the current distribution under it, leading to what is known as the ‘edge effect’, which causes heating a burning at the edges. (b) Impedance vs. frequency for a conductive adhesive electrode.

19. A capacitive contact electrode with its basic parts and current distribution under the electrode. Edge effect is smaller. Adapted from Pearce (1986).

20. (a) A capacitive contact dispersive electrode and (b) Impedance Vs
(a) A capacitive contact dispersive electrode and (b) Impedance Vs. frequency for a capacitive contact electrode. It acts like a parallel plate capacitor with the metal plate and the skin forming the two plates and Mylar as the dielectric between them. Adapted from US patent

21. (a) The temperature profile as a function of electrode surface area for pediatric dispersive electrodes for a power of 40 W for 1 min (b) Temperature profile as a function of the Power applied to the pediatric electrode with surface area of 36. Adapted from Kim &Webster (1986).

22. (a) An example of a resistive reusable return electrode (b) Equivalent circuit for the electrode shown in (a). Adapted from US patent No

23. (a) A return electrode monitoring system using the concept of split dispersive electrodes. The arrows indicate direction of current flow. (b) A Neutralect split dispersive electrode showing the two segments, developed to provide safety against electrosurgical burns.

24. Burns due to gel dry-out in some dispersive electrodes
Burns due to gel dry-out in some dispersive electrodes. (a) When there is gel dry-out, intense electric fields are generated, which causes arcing to skin leading to burns. (b) Burns caused by confined currents, and burns due to arcs to the skin during gel dry-out. Adapted from Pearce (1986).

25. (a) Argon beam coagulator with argon gas flowing to the active electrode. Adapted from Duffy and Cobb (1995). (b) Ionized argon beam produces a more conductive medium between the electrode and tissue. Adapted from Absten (2001).

26. Ablation: Method of delivering physical, chemical, or energy treatment to tissue for the purpose of removing, altering, creating scar tissue or causing aposis (cell death). Radiofrequency ablation Cryo-ablation Microwave ablation Ultrasonic ablation Laser ablation Chemical ablation

27. Calculate tissue temperature using the bioheat equation
Where T = final temperature (K)
 = electrical conductivity (S/m)
 = tissue density (kg/m3)
c = tissue specific heat(J/kgK)
J = magnitude of current density (A/m2)
t = duration of activation (s)
T0 = initial temperature (K)
also,
J = E,
Where E = electric field vector (V/m)

28. Example Assume that a uniform tissue has an electrical conductivity of 0.25 S/m, a density of 1000 kg/m3 and specific heat of 4186 J/kgK. If the electric field applied is of the order of 8000 V/m, estimate the time of activation required to reach a tissue temperature of 55 C assuming the initial temperature to be the body temperature (37 C).
Solution:
Given  = 0.25 S/m;  = 1000 kg/m3; c = 4186 J/kgK; E = 8000 V/m;
T0 = 310 K; T = 328 K;
Using the bioheat equation and substituting the given values yields
J = 2000 A/m2
Substituting the calculated value of J yields,
which gives the value of t = 4.7 s

29. Examples of ablation procedures that are currently performed in clinics. C = cryoablation, US = ultrasound ablation, RF = radio-frequency, MW = microwave.
Application
Technique
Cardiology (cardiac arrhythmias) RF
Urology (benign prostatic hyperplasia, gallbladder)
C, US, laser, RF, MW, chemical
Neurology (brain cancer)
US, RF
Oncology (tumors)
MW, RF, C, laser
Dentistry
Laser, chemical
Ophthalmology (cataracted lens, corneal sculpting, astigmatism)
Laser, US

30. A qualitative plot of survival curves of human bone marrow cells
A qualitative plot of survival curves of human bone marrow cells. The survival fraction is on a logarithmic scale, while the time axis is on a linear scale. Adapted from Bromer et al (1982).

31. The Joule heat generated from the catheter tip elevates the temperature of the surrounding tissue. Then the thermal energy is transferred deep into the myocardium by thermal conduction and some heat is lost due to the blood perfusion and conduction to the metal electrode. Flowing blood in the cardiac chamber cools down the surface of the electrode and the myocardium.

32. A typical ablation electrode system with thermistor embedded at the tip. A thermal insulating sleeve surrounding the sensing element blocks the transfer of heat from the electrode to the temperature-sensing element. Thus, the thermistor measures temperature without being affected by the surrounding thermal mass of the electrode. Adapted from Edwards and Stern (1997).

33. (a) 7F 4 mm cardiac ablation catheter (EP Technologies)
(a) 7F 4 mm cardiac ablation catheter (EP Technologies). (b) Four-tine hepatic RF ablation probe (RITA).

34. An ablation catheter is advanced into a cardiac chamber
An ablation catheter is advanced into a cardiac chamber. The RF generator delivers current to the ablation electrode at the tip of the catheter. Adapted from Panescu et al (1995).

35. Catheters in cardiac chambers

36. Fluoroscopy shows catheters

37. The lesion appears white

38. Cross section of the lesion

40. Common cardiac ablation sites
AV Node
Above the tricuspid valves
Above and underneath the mitral valves
Ventricular walls
Right ventricular outflow tract
Etc.

41. RF generator
Tip Electrode

42. Bioheat Equation Density Specific heat Thermal conductivity Time
Temperature
Current density
Electric field intensity
heat loss to blood perfusion
heat loss to blood perfusion
Temperature
Thermal conductivity
Specific heat
MATERIAL PROPERTIES
VARIABLES
Current density
Electric field intensity
Density
Time
heat loss to blood perfusion
Heat Conduction
Heat Change
Electrical conductivity
Joule Heat
Heat transfer coefficient
Blood temperature

43. 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

44. Process 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 )

45. 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

46. Temperature distribution after 60 s
Maximum temperature ~ 95 °C
Highest temperature

47. Sinus Rhythm with Surgery- Maze Procedure

48. Picture of Newer Catheters (NASPE)

50. FEM for Hepatic Ablation*
Hepatic Ablation: Use RF probe to destroy tumor cancer, or cirrhosis
Minimally invasive
Present: -High recurrence rate
-Small lesions

51. Radio-frequency probe for liver cancer
Radio-frequency probe for liver cancer. The four wire electrodes have thermocouples for temperature sensing at the tips.

52. Bifurcated blood vessel

53. Bipolar Hepatic Ablation
Unipolar

54. Important Parameters Affecting 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.

55. The laser beam intensity decreases with tissue depth
The laser beam intensity decreases with tissue depth. The mean free paths of CO2, argon and Nd:YAG are 10 m, 30 m and 2.5 mm, respectively. 1, 2 and 3 are the absorption coefficients of CO2, argon and Nd:YAG in blood. Note that this figure is not drawn to scale.

56. A simplified optical diagram of components of the laser system for removing cataracted lens tissue. From L’Esperance (1985).

57. A radio-frequency signal is produced by a signal generator and amplified by a RF power amplifier. A power meter is used to monitor the forward and reflected power in the coaxial cable connected to the transducer’s matching network.

58. (a) Endocare’s cryoprobes
(a) Endocare’s cryoprobes. (b) The argon-based eight-probe Cryocare system. Physicians can set the flowing rate, ablation duration, and the thawing rate, and apply up to eight cryoprobes simultaneously.

59. Internal structure of a typical cryoprobe
Internal structure of a typical cryoprobe. LN2 cryoprobes must have vacuum insulation to prevent freezing up the shaft of the cryoprobe and subsequent destruction of normal tissue. The LN2 changes phase when it hits the warm metal surface of the probe tip. Thus, a thin film of gas bubbles is formed on the metal surface.

60. System for prostate cryoablation
System for prostate cryoablation. (a) The needle trocar and the guide wire are first inserted into the body. (b) The cryoprobe. Adapted from Zippe (1996).

61. A schematic block diagram of a microwave power supply system for an ablation catheter. Adapted from Warner and Grundy (1994).

62. Diagram of helical antenna microwave electrode for cardiac ablation
Diagram of helical antenna microwave electrode for cardiac ablation. The stiffener wire allows better flexure control of the catheter. The electromagnetic shield prevents the intense field in the middle of the coil from the wire and electrodes. The insulating material (e.g. Teflon) helps avoid charring and coagulation. Adapted from Warner and Grundy (1994).

63. (a) Mechanism of tissue heating of RF ablation
(a) Mechanism of tissue heating of RF ablation. (b) microwave ablation produces an electromagnetic field and has a potential to create larger lesions. Adapted from Langberg and Leon (1995).

64. Questions for electrosurgery and ablation: You should be able to: 1 Describe 3 types of electrosurgery. 2 Given a waveform, calculate the crest factor. 3 Distinguish bipolar and monopolar electrosurgery. 4 Describe problems resulting from the edge effect. 5 Describe a safety system for electrosurgical dispersive electrodes. 6 Given the equation, calculate temperature rise in tissue. 7 Describe the reasons for cardiac ablation. 8 Describe terms in the bioheat equation. 9 Describe the process of finite element method modeling. 10 Describe the advantages of bipolar hepatic ablation. 11 Describe equipment and limitations of optical ablation. 12 Describe equipment and advantages of ultrasonic ablation. 13 Describe equipment and advantages of microwave ablation.