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Method for electrosurgery with enhanced electric field and minimal tissue damage

Method for electrosurgery with enhanced electric field and minimal tissue damage description/claims

This application is related to and claims priority from U.S. provisional application 60/447,715, filed on Feb. 14, 2003, and hereby incorporated by reference.

The present invention was made with support from the National Institutes of Health, under contract number R01-EY-12888. The government has certain rights in this invention.

The present invention relates generally to an electro-surgical device, and in particular, to the design of efficient electro-surgical probes and waveforms for pulsed plasma-mediated cutting, fragmentation, and evaporation of biological tissue in fluid media.

Plasma-mediated cutting of soft biological tissue in conductive liquid media with sub-microsecond pulses of high voltage is described in the patent of Palanker [U.S. Pat. No. 6,135,998]. Dissection of tissue based on explosive vaporization by short (under few microseconds) pulses of high voltage is described in the patent of Lewis et al. [U.S. Pat. No. 6,352,535]. In these applications an inlaid cylindrical electrode (i.e. a wire embedded into a thick insulator and exposed at its end) is applied to ionize, evaporate and fragment tissue in proximity of electrode using dielectric breakdown or vaporization of water induced by a high electric field. An inlaid cylindrical electrode cannot penetrate into tissue and thus can only produce shallow cuts on its surface. Due to the pulsed regime of application, this device produces a series of perforations in tissue, which often do not merge into a continuous cut. In addition, cavitation bubbles accompanying each pulse create substantial collateral damage in tissue during their growth and collapse phases [Effect of the Probe Geometry on Dynamics of Cavitation, D. Palanker, A. Vankov, and J. Miller, Laser-Tissue Interactions XIII, vol. 4617 SPIE (2002)]. The size of such a damage zone typically far exceeds the size of the electrode and the corresponding zone of initial energy deposition [Effect of the Probe Geometry on Dynamics of Cavitation, D. Palanker, A. Vankov, and J. Miller, Laser-Tissue Interactions XIII, vol. 4617 SPIE (2002)]. Reduction in pulse energy helps to reduce the mechanical damage, but also leads to decreased cutting depth.

A second mechanism of electrosurgical ablation is vaporization of tissue in the proximity of the probe by overheating a conductive medium with either a continuous radio frequency waveform or with sub-millisecond long bursts of pulses. This mechanism is universally applicable to soft and hard biological tissue ranging from membranes and retina to skin and cartilage. In such regimes wire electrodes are typically used, although the use of a device that could provide a uniform electric field along its length would be preferable.

Without considering end effects, the electric field in a conductive liquid at distance r from a cylindrical electrode with potential U and radius ro much smaller than its length L is:

E=U/(r ln(ro/L)),  (1)

assuming that the return electrode is much larger and positioned at infinity. The threshold electric field required for dielectric breakdown in water is on the order of 105-106 V/cm [Jones, H. M. & Kunhardt, E. E. Development of Pulsed Dielectric Breakdown In Liquids. Journal of Physics D-Applied Physics 28, 178-188 (1995); Jones, H. M. & Kunhardt, E. E. Pulsed Dielectric Breakdown of Pressurized Water and Salt Solutions. Journal of Applied Physics 77, 795-805 (1995)]. Such a threshold electric field Eth can be achieved with electric pulses of several kV on a wire electrode with a diameter of several tens of micrometers. The threshold voltage required for ionization of a surface layer of water is:

Uth=Ethro ln(L/ro).  (2)

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