US 20070213704 A1
A medical instrument coupled to first and second energy means and a computer controller for the controlled volumetric removal of thin tissue layers. The system provides a source for introducing a gas to controllably form and capture transient gas volumes in a microchannel structure at the working surface of the instrument that interfaces with a targeted tissue site. Each of the microchannel features of the working surface carries an electrode element coupled to the electrical source. The energy may be applied to the targeted site in either of two modes of operation, depending in part on voltage and repetition rate of energy delivery. In one mode of energy application, electrical potential is selected to cause an intense electrical arc across the transient ionized gas volumes to cause an energy-tissue interaction characterized by tissue vaporization. In another preferred mode of energy delivery, the system applies selected levels of energy to the targeted site by means of an energetic plasma at the instrument working surface to cause molecular volatilization of surface macromolecules thus resulting in material removal. Both modes of operation limit collateral thermal damage to tissue volumes adjacent to the targeted site. Another preferred embodiment provides and an ultrasound source or other vibrational source coupled to the working end to cause cavitation in fluid about the working end.
1. A medical device for delivering energy to targeted tissue, comprising:
an elongated member having a distal working end;
an interior flow channel having a first cross-section, the flow channel transitioning to at least one channel portion having a second smaller cross-section and terminating in at least one outlet in the distal working end;
at least one electrode element carried in said working end; and
a conductive flow media source for providing a flow through the interior flow channel, and wherein the electrical energy delivery to the flow proximate the at last one outlet creates a plasma for ablation of tissue.
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This application is a continuation of U.S. patent application Ser. No. 11/065,180 (Attorney Docket No. 022356-000310US), filed Feb. 23, 2005, which was a continuation of Ser. No. 10/282,555 (Attorney Docket No. 022356-000300US), filed Oct. 28, 2002, which was a continuation-in-part of U.S. patent application Ser. No. 09/614,163, filed Jul. 11, 2000, titled Electrical Discharge Devices and Techniques for Plasma-Assisted Skin Resurfacing, which was a continuation-in-part of U.S. patent application Ser. No. 09/317,768, filed May 24, 1999, titled Photoionized Gas Enabled Electrical Discharge Technique for Plasma-Mediated Cold Tissue Ablation, the full disclosures which are incorporated herein by reference.
1. Field of the Invention
The invention is related to electrosurgical instruments and techniques for precision application of energy to tissue, and more particularly to a system for thin layer ablation. More in particular, the working surface of a probe carries (i) delivery means for introducing and capturing a gas volume in a microchannel structure at an interface between the probe and the targeted site, (ii) energy delivery means for creating an intense electrical field of micron-dimensioned gas volumes to controllably apply intense energy to the targeted site to cause volumetric tissue removal, and (iii) vibration means for assisting in the creation of micron-scale gas volumes or bubbles.
2. Description of the Background Art
Various electromagnetic and acoustic energy delivery sources have been investigated for surgical tissue ablation or removal, including radiofrequency (Rf) current flow within tissue, high intensity focused ultrasound (HFU) tissue interactions and microwave energy absorption in tissue. In general, at high intensities, the above listed energy sources generate thermal effects that can vaporize tissue as the means of tissue ablation or removal. In other words, the energy sources elevate the temperature of water in intra- and extracellular spaces to above 100° C. thereby explosively vaporizing water to damage or destroy the tissue. The drawback to such purely thermally-mediated ablations is significant collateral damage to tissue volumes adjacent to the targeted site. While in many surgical fields, the above-described collateral thermal damage may be acceptable, in fields such where thin layer ablations are required, such as skin resurfacing, ophthalmology, neurology, and interventional cardiology, there is a need to prevent, or limit, any such collateral damage.
Lasers for Use in Tissue Ablation. Various laser systems have been developed for tissue ablation. The conventional long-pulse laser systems outside the UV range, wherein long-pulse is defined as a system operating in a range of 10's of nanoseconds to microseconds in pulse duration, have been found to be inefficient in volumetric tissue removal without causing extensive collateral damage. In a the conventional long-pulse laser system (e.g., Nd:YAG, Er:YAG, IR lasers), the photonic energy delivered to a targeted site does not directly disrupt the molecular integrity of surface layers of the site, but rather the energy is transferred into surrounding tissue volumes as photothermal energy, or photomechanical energy. These collateral effects propagate through surrounding tissues as heat, and perhaps mechanical shock waves, which manifest themselves as undesirable collateral damage. More specifically, the generally accepted model of volumetric ablation or removal with lasers having a pulse longer than tens or hundreds of picoseconds is described as follows: The energy absorption is chromophore dependent (and/or scattering dependent), and the energy transfer involves the heating of conduction band electrons by the incident beam of coherent photons which is followed by transfer of thermal energy to the structure's lattice. Ablation or damage occurs by conventional heat deposition resulting in vaporization, melting, or fracture of the structure. The rate of volumetric structure removal depends on thermal conduction through the structure lattice and its thermodynamic properties (heat capacity, heat of vaporization, heat of fusion, etc.). Thus, the minimum energy requirements to cause an ablation effect in the structure's properties may be defined by a threshold of incident laser energy per unit of structure volume at the target site, which threshold is directly dependent on pulse duration. It has been found that ablation thresholds generally require relatively long pulse durations, which in turn are the source of undesirable collateral photothermal (or photomechanical) damage.
In certain tissue ablation fields (e.g., corneal ablation in ophthalmology), excimer lasers have been developed that emit high intensity pulses of ultraviolet (UV) light, typically with pulse durations in the 1 ns to 100 ns range. The short wavelengths, as well as sequenced nanosecond pulse regimes, define a substantially non-thermally mediated form of tissue ablation. Short wavelength UV photons are highly energetic and when radiated onto biological tissue can directly break the chemical bonds in surface layer molecules of the tissue. As a consequence, UV excimer lasers can vaporize or breakdown a surface tissue layer with minimal thermal energy transfer to underlying (or adjacent) tissue volumes. The by-product of the breakdown is predominantly a gas that is ejected away from the surface on which the energy was deposited, thus generally leaving the subsurface layer free from substantial collateral thermal damage. Tissue ablation with UV irradiation with can be controlled depth-wise since biological tissues exhibit strong absorption in the UV region of the electromagnetic spectrum (e.g., at c. 1.93 μm). In biological tissue, UV radiation typically only penetrates to a depth of from about 0.25 μm to about 4.0 μm per pulse in the ns duration pulses described above. Thus, to ablate to a certain depth, the system uses a pre-determined number of pulses of ns energy delivery
While UV photonic energy delivery can reduce collateral damage in tissue ablation, there are numerous disadvantages that limit the applicability of UV lasers to biomedical procedures. First, UV photonic energy delivery cannot be easily delivered to a targeted site of a body structure in a fluid environment (e.g., thrombus in a blood vessel) since intervening fluid may absorb energy rather than the target site. For this reason, UV energy delivery is most useful for ablating tissue surfaces exposed to the atmosphere, such as a patient's cornea in a LASIK procedure. Second, the desired lack of collateral damage in UV ablation is known to occur when a single pulse of UV photonic energy irradiates a tissue surface. However, when the UV pulse repetition rate exceeds about 5 to 10 Hz, considerable photothermal collateral damage (as well as photomechanical collateral damage) has been observed. Thus, UV ablation generally may result in low volumetric removals of tissue surfaces per unit of time. Third, while UV photons carry sufficient energy to directly break chemical bonds in surface molecules of tissue, UV wavelengths also may be sufficiently energetic to promote mutagenic effects thus elevating concerns about the long-term health and health of the clinician and the patient.
Conventional Electrosurgical Ablation of Tissue Volumes. Radiofrequency currents in tissue have been known for many years in the prior art for cutting a tissue mass, or for coagulating regions within a tissue mass. Conventional electrosurgical systems known in the art ablate tissue by applying an electrical field across the tissue to be treated. The actual energy-tissue interaction in Rf cutting is typically described in terms of a voltage differential that causes a spark or arc across a gap between an active electrode 2 a and the targeted site (e.g., coupled to a return electrode 2 b) as shown in
Electrosurgcal Ablation with the Coblator™ System. A recently commercialized invention in the field of electrosurgical ablation was invented by Eggers et al and is a called Coblator™ system (see. e.g., disclosures of Eggers et al in U.S. Pat. Nos. 5,873,855; 5,888,198; 5,891,095; 6,024,733; 6,032,674; 6,066,134 and the companion patents cited therein). The Coblator™ system relies on the creation of a voltage difference between a plurality of closely spaced rod-like electrode elements 2 a and a return electrode 2 b (see
The Coblator™ company promotional materials explain that at high voltage levels, the electrically conductive fluid 5 in the gaps 6 that intervene between the closely spaced active electrodes 2 a is converted into an ionized vapor or plasma. As evidence of the character of such a plasma, studies have shown that a typical plasma has an orange glow. Spectroscopic analysis of Coblator emissions show an emission peak of around 590 nm which is characteristic of the sodium ionization peak (NaCl normal solution used as conductive fluid), with negligible emissions above 600 nm. The company promotional material claims that conventional electrosurgical ablation yields a continuous spectrum from 490 nm to 900 nm, peaking at around 700 nm.
The supposition underlying the Coblator™ is that the actual energy-tissue interaction produced by the system relates to charged particles in the plasma having sufficient energy to cause dissociation of molecular bonds within tissue structures that come into contact with the plasma. Based on this hypothesis, the accelerated charged particles have a very short range of travel, and the energy-tissue interaction is confined largely to thin surface layers. Further, the supposition is that the energy-tissue interaction is a “cold” process that does not rely on the thermal vaporization of intra- and extracellular fluids to ablate tissue. In this respect, the Coblator™ system has been described as producing an ablation that compares to that of an excimer laser, both of which produce similar ablation by-products—if relying on comparison of spectrographic emission peaks. The energy required to cause molecular break-down of common molecular bonds in tissue is believed to be in the range of 3.0 eV to 5.0 eV or more. Considering the amount of energy utilized by the Coblator™ system to initially and thereafter continually vaporize NaCl from within a saline solution, it raises the question whether the plasma volumes can sustainably provide the energy levels required for true molecular dissociation of compositions in tissue surfaces, as with an excimer laser.
Another hypothesis that explains the Coblator™ ablation process in more mundane. Referring to
This hypothesis easily explains the two observations made by proponents of the Coblator™ that state (i) that spectroscopic analysis of energy delivery within a conductive fluid supports the theory of molecular dissociation, and (ii) that evidence of limited collateral thermal damage (so-called “cold” ablation) must be the result of a molecular dissociation process. First, the spectroscopic analysis of the Coblator™ ablation certainly would show emission peaks for vaporization of the sodium analytes in the fluid, which is a primary sink of the energy applied from the device—which is different from and predominates over any emission peaks from the tissue ablation. The tissue ablation, if the competing hypothesis of a spark-type interaction of
The types of ablation caused by conventional electrosurgical ablation (see
Another characteristic common to both conventional electrosurgical ablation (
While conventional electrosurgical ablation and Coblator™ type ablations are suitable for various procedures, the following characteristics common to both types of ablation prevent the possibility of more precise ablations with such prior art systems:
(i) the actual energy deposition site is randomly localized instead of precisely localized;
(ii) the energy density across the energy deposition site is random and uncontrolled;
(iii) the conductive path between and energy deposition site and the emission point on an electrode is random;
(iv) the dimensions of the energy deposition site are random and uncontrolled;
(v) the amount of energy applied per unit of surface at an energy deposition site is random;
(vi) the amount of energy applied per time interval at an energy deposition site is uncontrolled;
(vii) the duration of intervals between successive energy applications to a site are random;
(viii) the actual duration of an interval of energy application is random and uncontrolled;
(ix) the conductive or insulative characteristics of media between the active electrode and the targeted site, at a micro-scale, are uncontrolled,
(xi) the dimensions, localization, distribution and duration of insulative gas volumes or plasma volumes that facilitate energy delivery are random and uncontrolled; and
(xii) the prior art instrument working ends function dramatically differently depending on the axial distance between an electrode surface and the targeted tissue surface,
It would be highly desirable to have greater precision in thin layer ablations for microsurgeries, neurology, and precision skin resurfacing for burn debridement or cosmetic purposes. What is needed are systems and methods for selective volumetric removal of body surface layer portions (i) that are precisely controllable; (ii) that do not rely on thermal vaporization effects to ablate tissue; (iii) that can be activated in a controlled mode that ratably removes depths or volume of tissue in a given time interval to provide selective tissue decomposition; (iv) that allow for well-defined post treatment boundaries between layers of tissue decomposition and undamaged layers; (v) that removes surface structure that is exposed to a gas environment or immersed in a fluid environment; (vi) that provide for energy delivery to a patient's body structure via a working end that can be reduced in scale to less than 1 mm. to 3 mm. in diameter for micro-interventional use; and (vii) that utilize a working end that is simple to manufacture and therefore inexpensive and disposable.
The principal objective of the present invention is to provide controlled and precise applications of energy to a thin surface layer of structure in a patient's body to cause volumetric removal of layer portions substantially without collateral thermal damage. More in particular, an instrument working end is adapted (i) to deliver high intensity electrical energy in a preferred modality to create a highly energetic and dynamic microplasmas for causing molecular volatilization of surface macromolecules to remove tissue with microscale precision, or (ii) to optionally deliver a spark-type energytissue interaction (herein termed a conventional electrosurgical ablation modality) but in a novel micro-scale modality to controllably ablate discrete tissue portions.
The novel energy delivery modalities and energy-tissue interactions are based on utilizing either of two novel principles (hereafter defined at times as control principles) in the application of electrical energy to body structures for ablation purposes. The principles may be applied to a system and instrument working surface independently, or preferably in tandem, and are designed to control the numerous factors that to date have been uncontrolled and random in tissue ablations (i.e., the 12 factors listed above at the end of the Section titled Description of the Related Art.
The first control principle relates to bifurcation of a discrete electrical energy delivery event into two components. (i) the provision of electrical potential at an electrode at an intensity level that can ablate tissue with the electrode spaced apart from the targeted site; and (ii) the ms or ns (microsecond or nanosecond) control of conductive the characteristics, or ionization, of selected media in the interface between the electrode and the targeted site. In other words, it is postulated that the selected media can be switched between non-conductive (or insulative) and conductive (or ionized) stated in a controlled and ultrafast manner to advantageously deliver energy to cause a novel energy-tissue interaction. In a subset of this principle, several ionization methods, as well as energy-enhancing means for energizing a plasma, are disclosed-with photoionization being a preferred means.
The second control principle relates to a tremendous reduction in scale, or micronization, of the features of the working surface for the purpose of controllably localizing a large number of discrete energy-tissue interactions, as well as controlling the physical dimensions of plasma volumes. As will be described below, the working surface may be fabricated by semiconductor processing techniques to provide features in the 5 μm to 10 μm range, by which is meant that widely dispersed micron-scale electrode elements may each apply electrical energy to a localized site to provide discrete spaced apart ablation events. Referring to
The system is coupled to a computer controller for controlling intervals of energy delivery-both the ionization means and the electrical energy means for the ablation-as well as for optional flows of the selected media. By controlling the duration of intervals of energy applications and the repetition rate in relation to the thermal relaxation time of the targeted surface, it is postulated that the disclosed plasma-assisted tissue removal method will be a substantially cold process, ie., there will be substantially no collateral thermal damage to tissue.
Each pulse, or burst, of energy application in the disclosed ablation process is assisted or enabled by a sequence of distinct plasma formation processes to achieve the operational objectives of the invention, followed by plasma decay intervals. In an initial portion of the process, an ionized gas (plasma) is formed to allow subsequent electrical potential in a spatially-controlled region at the surface layer. In a second portion of the process, an electrical discharge is created in the ionized gas and surface layer thus causing volatilization of macromolecules of the targeted surface layers to remove layer portions. To define the uses of plasmas herein, it is useful to provide the following background. A plasma is a quasineutral (partly ionized) gas having a significant proportion of charged particles relative to neutral particles which may, in an equilibrium state, exhibit collective behavior. Of interest to this invention are non-equilibrium or highly dynamic microplasmas formed proximate to structure in a patient's body to allow for energy transfer to, or deposition of energy within, thin surface layers of structure in a patient's body to remove (ablate) material without transfer of thermal energy. In the lexicon of physicists, a plasma may be defined simply as an ionized gas, to distinguish it from ordinary or neutral gases. In a neutral gas, each gas atom carries the same number of negatively charged electrons orbiting its nucleus as there are positively charged protons in that nucleus. While a neutral gas may carry the potential of substantial chemical activity together with dynamic effects (e.g., fluid turbulence), such a neutral gas exhibits little or no response to electric and magnetic fields—and such neutral gases are substantially unable to conduct electrical potential therethrough. A neutral gas, however, can be excited or energized to the plasma state when a sufficient proportion of its atoms become ionized by losing one or more electrons. Ionization can occur as a result of a number of processes, such as (i) a neutral gas becoming so hot as to cause the atoms to collide and jar loose electrons (thermal ionization or TI), (ii) a neutral gas being subjected to a high intensity light source that strikes the atoms with energetic photons that displace electrons from their orbits (photoionization or PI), (iii) a neutral gas being subjected to electric fields that are strong enough to strip away electrons from the atoms (field ionization of FI), and (iv) in the case of non-gaseous materials, a spark or electrical discharge pulse can ionize analytes in the surface of a solid.
A resulting plasma (ionized gas) consists of interpenetrating and interacting accumulations of freely roaming charges (both negative and positive). This gas state can shift from neutral to ionized by photoionization or field ionization by the means disclosed herein, it is believed, within the range of tens of femtoseconds to hundreds of picoseconds. For purposes of this disclosure, the method of forming the initial plasma (ionized gas) volume involves irradiating a captured neutral gas volume with a high intensity wavelength (λ) ranging between about 190 nm to 263 nm (or more broadly, within the UV spectrum defined as λ from 10 nm to 400 nm; or, frequency range of 1.0×1015 Hz to 6.0×1016 Hz). Thereafter, an intense application of energy is applied by to targeted surface layer by creating an ultrafast high-intensity electrical discharge within the ionized gas volume and the surface layer to cause an electrochemical volatilization of molecules of the surface layer, i.e., a plasma-mediated ablation. Each high intensity burst of electrical energy forms a critical density plasma at the surface layer thus removing surface material that is ionized. According to this plasma-mediated form of electrochemical material removal, each energy pulse applied to the surface layer when above a certain threshold level is channeled into to the formation of ejecta (gas plume and fragments) by the volatilization or decomposition of macromolecules of the surface layer. The plasma, or microplasma, decays rapidly as ejecta and the plasma transfers heat and energy away from the surface layer and also releases energy in the form of radiative emissions. It is postulated that such energy applications to the surface layer (dependent on ionization proportion of neutral gas and intensity of the electrical discharge), can be modeled to provide selective volumetric removal of material per pulse of energy. In sum, a controllable process for selective volumetric tissue removal is provided by a system including a computer controller for successively rapid sequencing of (ii) photoionization of a partially-captured neutral gas volume, and (ii) creation of a high-intensity electrical field for volatilization of a targeted thin surface layer.
In the present invention, at sufficiently high energy levels (or voltage V), charged particles in a plasma can possess sufficient energy to break down common molecular bonds in macromolecules of a tissue surface. For example, such bonds may be carbon-carbon bonds, carbon-nitrogen bonds, etc. It is believed that the energy required to disrupt such bonds is within the range of 3.0 eV to 4.0 eV. Preliminary calculations (to be refined for publication in a future disclosure) suggest that the estimated electron energy applied by the inventive system can far exceed 4.0 eV when operating at 300 to 1000 volts, making various assumptions concerning distances between the active electrode(s) and the targeted site; and the electron population of the media (gas or liquid) interface between the electrode(s) and the targeted site.
The invention provides a technique for biological material removal that allows precision of ablation depth by removing a discrete thin layer of material with optional modes of (i) pulsed applications of intense energy to a targeted site, or (ii) the creation of a sustainable high energy plasma at the interface between a working surface and the targeted site to cause molecular volatilization.
The invention advantageously provides a control technique for controlling material removal by causing a plurality of micron-dimensioned energy-tissue interactions over a grid of an instrument working surface.
The invention advantageously provides a control technique for controlling material removal by creating a plurality of micron-dimensioned gas volumes that can be switched between non-conductive and conductive states for controlling energy applications to the targeted tissue surface.
The invention provides a control technique for controlling the distribution of energy across a targeted site by creating a plurality of micron-dimensioned plasma volumes that evenly apply energy across tissue.
The invention provides a control technique for enhancing the energy of a plasma to facilitate a plasma-mediated ablation by utilizing electron-emissive coatings and UV irradiation of a microchannel plate.
The invention advantageously provides a technique for removal of surface layers of body structure substantially without collateral thermal damage.
The invention advantageously provides a technique for ablation (material removal) that is generally insensitive to tissue's linear absorption characteristics.
The invention advantageously provides a technique for ablation (material removal) that is generally insensitive to tissue hydration.
The invention advantageously provides a technique for ablation of surface layers of body structure that is exposed to a gas environment or immersed in a fluid environment.
I. Operational Principles of Plasma-Assisted Cold Ablation of Thin Surface Layers
The several principles of operation of an exemplary plasma-mediated ablation system 5 (see
A. Operational Principle: Ultrafast Pulsed Energy Application for Volatilization of Surface Layers.
A first objective of the present invention is to provide an ultrafast pulsed application of electrical energy to a targeted site on a surface layer of structure in a patient's body. In this invention, each discrete application of energy (pulse or burst) can be defined by quantity of energy (J) and a duration in which such energy is deposited within the surface layer. A first operational principal of the invention is that, for a given energy quantity in joules, the duration of energy application is less than a threshold electron-to-lattice energy transfer time for the surface layer of the targeted structure. It is postulated that for surface layers of structures here in question, this characteristic energy transfer time is in the range of about 10 ps to 100 ps. Thus, when a high-intensity pulse of energy is applied in a sub-threshold duration, the mechanism of layer removal can be characterized as a chemical alteration of surface molecules, or more specifically an electrochemical-tissue interaction since the energy is applied by means of an intense electrical field. In contrast, if the pulse duration of energy application is at an above-threshold level (e.g., in the ms range) with sufficient energy to cause damage, the mechanism and characteristics such of ablation would differ. That is, above-threshold energy delivery would cause damage that is largely thermal in nature and characterized by vaporization, melting, denaturation, fracture, etc. (In this above-threshold energy delivery modality, the electron kinetic energy transfer to the material's lattice structure is dependent on thermal diffusivity, a material property which expresses the ability of heat to diffuse and is equal to the square root of the ratio between heat conductivity and specific heat capacity). The present invention, however, is directed to ultrafast pulses (subthreshold duration) of energy application that create a critical density plasma from a thin surface layer. More specifically, the intensity of the electrical field (or discharge) and its absorption in the surface layer results in volatilization of macromolecules in the layer thus causing actual ionization of the thin layer thereby removing bulk. The energy burst is absorbed non-linearly and produces quasi-free electrons which, in turn, may act as seed electrons to cause an electron avalanche by various ionization processes. (Various ionization processes fall within the scope of the practice of the method, e.g., collisional ionization, multiphoton ionization, and field ionization). These ionization processes lead to irreversible alteration in the surface of the structure as ejecta (gas and bulk material) is ejected from the layer. It is believed that a very high fraction of the applied energy is removed by the high-velocity ejecta. Thus, any instantaneous high temperatures caused by the energy application are removed from the layer by the plasma formation process. It is further believed that mechanical shock waves to the bulk material will be insignificant compared to energy applications at above-threshold levels as defined above. At the subthreshold energy duration applications proposed herein in accordance with practice of the method, there will be insufficient time for lattice coupling, thus resulting in insignificant thermal diffusion-induced collateral damage to the structure.
A further desirable consequence of using such ultrafast pulse of energy application for surface layer removal is that the method is relatively insensitive to hydration and density of the targeted surface layer, and is entirely chromophore independent. The drawback to the layer-by-layer plasma-mediated process disclosed herein is that the ablation rate, defined generally as the depth of layer removal per pulse, is small. That is, each pulsed energy application will only remove a layer having a thickness measured in μm's (microns). It is postulated that layer removal rate will range from about 1 μm to about 200 μm within the anticipated energy application parameters (from 0.1 J/cm.2 to 1000 J/cm.2; or 0.1 J to 10 J per pulse). In order to overcome the low rates of material removal per pulse of energy in the typical case that needs substantial volumetric removal, the repetition rate of pulsed energy application must be high. This operational principle of the invention will be described below in Section I(C).
B. Operational Principle: Photoionization of Neutral Gas and Confinement by Shape Structure and/or Fluid.
The previous Section I(A) described a means for plasma-mediated cold electrochemical ablation of layers of body structure, but to construct a system feasible for surgical use, it is necessary define operational principles for spatial control of the energy application at the targeted layer to cause molecular volatilization. The operational principles according to this aspect of the invention relate to (i) providing means for creating an electrically conductive (partially ionized) gas volume proximate to the layer that is targeted, since the plasma-mediated ablation process described in Section I(A) can only occur in such a condition; and (ii) providing means for at least partially confining the ionized gas volume in its non-equilibrium state for a sufficient time interval to create the intense electrical field therein to cause volumetric removal.
The present invention utilizes a probe with a distal working surface that in placed proximate to the targeted site for energy delivery. Thus, it is necessary to create the non-equilibrium ionized gas (an initial plasma) generally about this working surface. The invention utilizes a concept novel to biomedical applications for photoionizing a neutral gas volume introduced to the working surface. In such photoionization, a high-energy photon (e.g., from a UV source) irradiating the neutral gas may interact with an ionic electron leading to removal of that electron from the ion. To photoionize an ion requires a photon energy greater than the binding energy of the electron. As photons increase in energy (wavelength dependent), the cross-section for photoionization of a particular electronic state is zero until at a certain point the cross-section jumps to a finite value, which is termed a photoelectric edge. The method of the invention thus selects a radiation source characterized by photon/wavelength energies beyond the photoelectric edge as it relates to the selected neutral gas composition (for purposes of the disclosure, the gas may be any ionizable gas compatible with biomedical application, e.g., air, nitrogen, oxygen, CO2, etc.). In the case of such photoionization, the departing electrons must exchange virtual photons with the nuclei for the process to occur. Such photoionization, therefore, tends to preferentially remove inner-shell (i.e., K-shell) electrons since these can most readily interact with the nucleus. Due to this effect, this process general leaves highly excited ions. After photoionization by photons of energy, the liberated electrons possess a kinetic energy and successive electron-electron collisions distribute this energy throughout the electron population with such kinetic energy being thermalised into a surrounding plasma. The non-equilibrium plasma thus created at the working surface of the probe and at the surface layer is capable of interfacing an intense electrical field (electrical discharge) with the targeted layer to cause the electrochemical ablation described in Section I(A). It is postulated that the photoionization of the neutral gas volume occurs within fs or ps and the duration of UV light pulse for such photoioization is described below in Section I(C) relating to repetition rates
In accordance with practicing this method of creating a photoionized gas volume (non-equilibrium plasma), another aspect of the invention relates to confining or containing the ionized gas in its non-equilibrium state proximate to, or in contact with, the layer targeted for ablation. Essentially, two operational principles are employed to maintain the plasma's condition for sufficient duration to allow the high-intensity electrical discharge therethrough. First, the targeted surface layer is preferably immersed in a fluid environment (the fluid being any suitable water, non-conductive water-based solution or a saline solution, or a hydrogel) to prevent the ionized gas from rapid dispersion into the surrounding environment. In a surgery in the interior of a patient's body, such as an arthroscopic procedure, the standard use of saline immersion is compatible with the invention. By the term immersion, it is meant that the surface layer is covered in any suitable fluid to any particular depth, and the depth may be very slight and still accomplish the purpose of preventing dispersion of the ionized gas volume. For example, the fluid immersion may comprise a layer as thin as a film if it provides a seal between a perimeter of the working face and targeted layer to briefly contain the plasma. Thus, the method of the invention may be used on surface layers of body structure exposed to the atmosphere, such as a patient's skin, when covering the structure with a thin gel or water.
A second (optional) principle that is utilized to confine the plasma is the provision of a shape structure at the working surface of the probe, such as a concavity to at least partially confine the photoionized gas. This shape structure also is useful for controlling the volume of ionized gas which will play a role in the plasma-mediated transfer of energy to the targeted surface layer. This shape structure, as well as particular dimensions of an exemplary embodiment, will be described in Section II below.
C. Operational Principle: Controlled High Repetition Rate of Pulsed Energy Applications.
The above Sections described a novel pulsed plasma-mediated ablation process that occurs, per pulse, in a time interval that may range from picoseconds to microseconds. In the sequential energy deliveries and plasma formations, only a thin layer of tissue is removed by the electrochemical-tissue interaction. In order to provide functional volumetric removal, the ultrafast pulsed energy event must be repeated at a high rate, and this Section describes operational principles relating to the selection of such a repetition rate. At a theoretical high repetition rate, there will be a high rate of volumetric removal. However, if the repetition rate is too high there may be collateral damage from thermal effects conducted to the surface layer by the plasma, which would defeat the objective of the invention in providing a substantially cold volumetric removal process. While it is postulated that the electrochemical-tissue interaction will occur without substantial thermal effects, a succession of ultrafast plasma creation and decay events may build up thermal effects in the plasma that would be conducted to the surface layer thus being capable of causing collateral thermal damage.
The operational principle in determining a maximum theoretical repetition rate for pulsed energy applications concerns the relationship between duration of energy absorption effects by the surface layer and the layer's confinement of heat, which thus may result in unwanted collateral thermal damage. The operative construct is the so-called thermal relaxation time, and is defined as the time required for significant cooling of a defined volume of a body structure that has been elevated in temperature. Thermal relaxation is often defined as the time required for an elevated tissue temperature to be reduced by 50 percent. Many processes are involved in such cooling, such as conduction, convection and radiation. Macroscale conduction cooling in body structures probably dominates, but microscale radiational cooling at a very small target site proximate to a plasma may be important. For targeted sites on structure in a patient's body in laser applications, the rule of thumb is that the thermal relaxation time in seconds approximately equals the squared dimension of the targeted site in mm. (For example, a 0.5 μm size structure (5×10.−4 mm.) will substantially cool in about 250 ns (25×10.−8 seconds)). In the case of the inventive method disclosed herein, the fluid immersion aspect of the method is advantageous and will help provide rapid thermal relaxation. The fluid further may be pre-cooled which is beyond the scope of this disclosure.
In accordance with practicing the principles of the method, it is postulated that ultrafast pulses of energy will be absorbed in the range of 100's of ps to 100's of ns. It is further postulated that repetition rates in the range of from about 10 Hz to about 500 Hz (or even 1000 Hz) are possible without exceeding the relaxation time of the surface layer. At this time, the repetition rates are theoretical and are to be explored with bench tests. As a practical matter, the timing of controlled flows of a neutral gas to the working surface and irradiation of the gas volume may prevent extremely high repetition rates. It is believed that with repetition rates in range of 2 Hz to 100 Hz, layer removal rates could be as high a 1 mm./s while maintaining the desired minimal collateral damage characteristics of the plasma-mediated ablation.
D. Operational Principle: Plasma Luminescence Feedback System for Tissue Layer Differentiation.
A final operational principle underlying the practice of plasma-mediated ablation relates to the potential need for diagnostic means in the selective layer removal process to prevent the removal of layers of structures that must be protected. In many procedures relating to tissue removal by a surgeon, the ablation process will be initiated and terminated on the basis of the surgeon's visual observation of the targeted surface layer and evaluation of the ablation process. However, when ablating surface layers in the interior of a patient's body, particularly in a fluid environment, endoscopic observation of the ablation process may be poor. A principal reason for the author's investigation of plasma-mediated ablation in a fluid environment is that the inventive method advantageously will allow spectroscopic analysis of luminescence of the plasma, and control signals may be derived therefrom to terminate the ablation process. In other words, as each successive plasma is formed by the pulsed high-intensity electrical discharge, an opportunity is provided to use tissue layer differentiation diagnostic procedures based on spectroscopic plasma emission signatures. Such a spectroscopy system would utilize an optic fiber for collecting emitted light from the plasma generated by the volatilization of the surface layer and ejecta in the plasma. The light would be dispersed and analyzed by an analyzer system, such as an intensified, gated, optical multi-channel analyzer/spectrograph. Emission peaks characteristic of different tissue types would be used as reference data within the analyzer system. Thus, when the system detected emission characteristics of material that is not targeted, a signal would be provided to the controller to terminate the ablation process. It is believed this type of feedback system is novel to any electrosurgical ablation modality, and is made possible only because the practice of the method includes the creation of successive microplasmas at the targeted site. Further, this type feedback signal system is novel in that it can be utilized in a fluid operating environment. (Optical feedback systems based on spectroscopy have been tested with forms of laser energy delivery, but such laser systems cannot operate in a fluid environment. See, e.g., B. M. Kim, M. D. Feit, A. M. Rubenchik, B. M. Mammini & L. B. Da Silva, Optical feedback signal for ultra short pulse ablation of tissue, Appl. Surface Sci. 127-129(1998) pp. 857-862.)
II. Construction of Exemplary Type “A” System for Plasma-Assisted Cold Ablation
The exemplary probe 10 of the system comprises an elongate extension member 20 having proximal (handle) end 21 and a distal working surface end indicated at 15 with a cross-section or diameter of body 22 being about from 1.0 mm. to 5.0 mm. (not limiting), the extension member defining longitudinal axis 27. The extension member may be fabricated in a variety of configurations to mechanically support at least one active electrode about in the working surface 15 and to allow the operator to position the working surface 15 in relation to a targeted site on a surface layer by manual or robotic manipulation of the proximal end 21 of the extension member. Typically, the extension member 20 comprises a small cross-section member or tube being dimensioned for introduction into the interior of a patient's body through a trocar (cannula) in a minimally invasive procedure, for example in arthroscopy, laparoscopy, or another thoracoscopic or endoscopic procedure. The elongate member may have any suitable length. The elongate member also may be rigid or flexible. A flexible probe may be provided with pull wires, shape memory actuators, and other mechanisms known in the art for articulating a distal end for positioning the working face proximate to the targeted site.
A computer controller 45 is operatively connected to the high-intensity light source 40, and controls the activation of the source, as well as its pulse repetition rate, in response to control signals that are provided by the system operator. Any sort of on-off switch (foot pedal or hand switch not shown) is connected to controller 45 and provides activation signals in response to the actuation of the switch. The pulse repetition rate may be set by the operator by a form of rheostat control connected to controller 45 which increases or decreases the repetition rate in response to the operator selection. It should be appreciated that any suitable high-intensity UV source 40 may be suitable for delivering energy along the light-channeling means 35, and a conventional discharge lamp known in the art is preferred together with optional filters, combiners, and associated optics may be suitable to perform the method of photoionization aspect of the method described above (i.e, a discharge lamp passes an electric current through a rare gas or metal vapor, wherein electrons collide with gas atoms exciting them to higher energy levels which then decay to lower levels by emitting light; mercury, hydrogen, deuterium, Ne, Ar, Kr, and Xe discharge lamps all may useful for the photoionization aspect of the method). The light source is preferably remote and carried to probe 10 by a flexible fiber (not shown) but the light source also could be carried in a handle portion of the probe. The light source 40 also may be an excimer laser, such as an OPTex system by Lambda Physik, Inc., 3201 West Commercial Blvd., Ft. Lauderdale, Fla.
The UV energy radiated from source 40 is pulsed as is known in the art by a computer controller 45 capable of controlling the delivery of light pulses having a pulse duration ranging from about 100 ps to tens of ms with a repetition rate ranging from about 1 Hz to 1000 Hz. In the exemplary embodiment of
In the block diagram portion of
The computer controller 45 of the invention depicted in
III. Method of Use of Type “A” System for Plasma-Assisted Cold Ablation of Surface Layers.
In practicing the method of the invention in a laminectomy/disketomy procedure to treat a herniated disc by removing surface layers of tissue, the patient is be prepared in any conventional manner with anesthetics and
Now referring to
Now turning to
The method of the invention may further include the step of analyzing the emission spectra by means of the fiber optic 35 which is coupled to a remote plasma luminescence spectroscopy system and controller 45. For example, the plume following a ultrafast plasma creation event in removing disc material in the exemplary procedure will provide emissions that can be compared with reference data for a known material in a disc, such as calcium If the emissions from the plasma exhibit characteristics that compare with non-targeted disc material, such as a nerve, the spectroscopy system can signal the controller 45 to terminate energy delivery.
IV. Construction of Type “B” System for Plasma-Assisted Cold Ablation.
A Type “B” embodiment (not shown) is very similar to the Type “A” embodiment except that the central core of the probe carries an optic fiber that is utilized to collect emitted light emissions from the plasma P2 indicated in
The Type “B” system also includes impedance-measuring circuitry for triggering the delivery of electrical energy to the (photoionized gas) plasma P1 as indicated in
Another aspect of the Type “B” embodiment of the invention (not shown) is the provision for aspiration of ejecta and fluid from the workspace. For example, if the targeted layer is a portion of malignant tissue, it may be useful to sequence aspiration of the ablated tissue fragments (along with gas and fluid) from the workspace. The controller would be adapted to provide such pulsed aspiration between pulsed applications of energy. The passageway for receiving such removed material particles would preferably be a concentric lumen around the extension member 20 of
V. Construction and Method of Use of Exemplary Type “C” System for Plasma-Assisted Material Removal.
In a previous part of this disclosure (see “Summary and Objects of the Invention”), a number of processes were described to cause, induce or develop ionization of a captured neutral gas volume (or bubble), and thereafter terminate ionization of the gas, in an interface between a probe working end and a targeted site ts. Such an instantaneously and independently ionized gas volume can then cause an intense application or arc of electrical energy from an electrode to the targeted site ts across the gas volume to volumetrically remove surface layers of an anatomic structure. Several ionization processes were described above, viz., photoionization (PI), field ionization (FI), thermal ionization (TI) and the ionization of surface analytes from a solid material. In this Type “C” embodiment 205, the use of independent field ionization (FI) means is described as an option in Section V(C) below and at times referred to as the first energy source. In addition, a working end carrying a microchannel plate (MCP) is disclosed in Section V(B) below which comprises an electron-emissive coating on a microchannel structure that, it is postulated, can enhance the energy of a plasma for causing volumetric material removal. Both the Type “A” and “C” systems utilize the same gas source 50 for introducing the neutral gas volume to working surface. The Type “C” system also is coupled to controller 45 and electrical source 60 (second energy source) as described above that allows for the time-controlled and volume-metered introduction of the neutral gas volume (media m2) to the working end. All parameters of electrical energy delivery may be programmed to generate trains of energy applications to the targeted site ts at selected repetition rates, or in a continuous mode, as described in the Type “A” system above.
A. Intense Pulsed Electric Discharge Mode of Operation of Type “C” Working End
More in particular, the Type “C” system of
In this Type “C” system 205, an exemplary working end 210 of a probe member 212 (see
As can be seen in
In a first operational mode of the working end 210,
Again referring to
Of particular interest,
The next step of the method of the invention (not shown) is (i) the termination of photoionizing UV radiation to the gas media in the microchannel structure thereby returning such gases to a non-conductive nature (see
Following the above sequence of steps which occur in a micro-second time frame, the steps of the method are repeated after a very brief selected interval that exceeds the thermal relaxation of the targeted site, as described above. The time intervals for utilizing the photoionization means for switching the gas media from non-conductive to conductive may be repeated at a pulse rate between about 200 ns and 500 ms. Stated another way, the interval between repetitions of the complete steps of photoionization and electrical energy applications, may range from about 500 ns to 500 ms to insure thermal relaxation of the targeted site.
It can be seen in
The working end 215 of the device of
B. Alternative Energetic Plasma Mode of Operation of Type “C” Working End
The previous section described the application of high intensity electrical energy in pulses to a targeted site ts to cause volumetric material removal wherein the ablation will cause substantially little collateral thermal damage to tissue due to (i) the micron dimension of the discrete energy-tissue interactions that ablate only very thin cell layers, and (ii) the thermal relaxation of the targeted site ts between timed applications of energy at the 100's or 1000's of discrete, localized sites. It is believed that an alternative mode of operation will prove possible with the Type “C” working end in which electrical energy is delivered continuously to the microchannel structure, along with optional means of enhancing such energy delivery, to cause sustainable highly energetic microplasma volumes about the open microchannel terminations 222 a-222 c (see
More in particular, referring to
C. Alternative Field Ionization (FI) Mode of Operation of Type “C” Working End
The previous sections have described the method of using a Type “C” working end wherein the ionization means comprise a light-carrying channel 35 and a UV source to deliver energy to the working end. Other ionization means independent of the ablation electrode 225 are possible have been disclosed (see, e.g., co-pending U.S. Ser. No. 09/317,768 filed May 24, 1999 (Docket No. S-QP-002) titled Photoionized Gas Enabled Electrical Discharge Technique for Plasma-Mediated Cold Tissue Ablation) and are listed again in the first paragraph of Section V above. There may be requirements for certain instruments that make it difficult or expensive to rely on light-channeling means 35 to deliver UV energy to a working surface. For example, it may be difficult to fabricate a light channel in an elongate catheter having a very small diameter. Also, it may be expensive to fabricate a light channel in an elongate introducer member in which the axis of the microchannel structure is transverse to the axis of the introducer.
For this reason, a working surface 215 with microchannel structure substantially the same as depicted in
In performing the method of the invention with the microchannel structure of
VI. Construction of Exemplary Type De “D” System for Plasma-Assisted Material Removal.
The previous sections described application of high intensity electrical energy to cause volumetric removal of tissue layers (i) in a pulsed high intensity mode for creating micron dimensioned discrete energy-tissue interactions to ablate tissue, or (ii) in the creation of a continuously sustained layer of high energy microplasmas about the working surface of the instrument to cause true plasma-mediated molecular volatilization of surface macromolecules to remove tissue volume. The Type “D” system described next functions almost identically to plasma-mediated system described previously, with the exception that the electrode arrangement of the working surface is adapted to insure that maximal average voltage is applied to the ionized gas volumes v continuously (cf
A portion of the Type “D” working surface 315 and microchannel structure is shown in
In general, it is postulated that such a system with spaced apart electrode layers 325 a and 325 b will cooperate with a return electrode 85 to maintain high voltage energy delivery to microplasma volumes v formed about the open distal terminations 322 of microchannels to insure a continuous high energy plasma for causing molecular volatilization of the surface molecules at the targeted site ts. In the operation of the previous Type “C” embodiment, the electric field between the electrode 225 and return electrode 85 would diminish in intensity depending of the voltage waveform as voltage difference between the active electrode 225 and the return electrode 85 varied. In this embodiment, an additional voltage differential would occur at certain intervals of the selected frequency between the out-of-phase voltages at the electrode layers 325 a and 325 b, in addition to the field created between these electrode layers and the return electrode 85. Thus, a method of the invention relating to the Type “D” system comprises (i) providing at least two closely spaced electrodes that have overlapping or out-of-phase voltage waveforms in an instrument working surface, in addition to a return electrode that cooperates with the working surface in contact with the targeted site ts or on the spaced apart portion of the instrument working end, (ii) creating and transiently capturing (by any suitable bubble creation means) a neutral gas volume that engages the at least two out-of-phase electrodes and further engages the targeted site ts; (iii) delivering intense energy to the electrode arrangement (e.g., in frequency range of about 250 kHz to 3.5 MHz) to sustainably ionized the transient gas volumes at high energy levels sufficient to cause volumetric removal of the surface layers of the targeted site ts.
VII. Construction of Exemplary Type “E” System for Energy Application
More in particular, it has been found the application of vibrational energy to, and about, the working surface can increase the efficiency of the electrical energy application across the transient gas volume or bubbles that are formed between the working surface and the targeted tissue. In one embodiment, referring to
The embodiment of
When the above described ultrasonic assembly is energized, a vibratory motion in the form of a standing wave is generated throughout the length of the instrument body 410. The propagation of such vibratory motion at particular points along the length of the instrument body 410 depends on the exact longitudinal location at which the vibratory motion is measured. A minimum in the vibratory motion or standing wave is commonly referred to as a node, wherein motion is at minimal level. The location at which the vibratory motion reaches a peak in the standing wave is referred to as an anti-node, and the length of the instrument body 410 is selected to provide an anti-node characteristics generally at the working surface 415 to deliver a maximum amount of energy thereto. Any suitable electrical source and controller can be coupled to the piezoelectric elements to drive or excite the ultrasonic assembly at any suitable resonant frequency of the tuned acoustic assembly.
In operation, the piezoelectric elements are energized in response to an electrical signal provided by source to thereby produce an acoustic standing wave in the instrument body contemporaneous with the introduction of a selected gas to the working surface (see
The vibrational or acoustic energy as described above can be applied in a continuous manner to the working end, or it can be applied in a pulsed manner. The scope of the invention includes the use of such vibrational energy in any electrosurgical working end that is adapted to apply energy to tissue, and in particular to such instruments that are adapted to apply energy for purposes of ablation or volumetric removal as described above. The types of instrument working ends that fall within the scope of the invention includes, but is not limited to, probe working ends for applying energy to tissue in procedures in orthopedics, endovascular interventions, neurointerventions, ENT, urology and general surgery. The invention also may be used in the jaws of grasping-type instruments. The vibrational energy can be combined with the instruments described above that use photoionization of the gas volume, or the vibrational energy can be used independently thereof—still in combination with the electrical energy application components described above.
In another embodiment shown in
The scope of the invention includes the application of acoustic energy to create cavitation bubbles in a fluid media about the working surface of the probe. Thereafter, the invention includes the application of electrical energy across the environment of expanding and collapsing cavitation bubbles to ablate tissue. This type of probe does not need a gas inflow source of the previous embodiments, since the acoustic cavitation system provides the desired interface between the working surface 515 and the targeted tissue. This type of working end can be used with or without the photoionization method and apparatus described in previous embodiments.
While the plasma-assisted energy delivery methods above have been described in connection with several surgical procedures for volumetric tissue removal, such as skin resurfacing, the ablation of pre-cancerous or malignant cell layers and in spine surgery, it will be clear to those having skill in the art that the system has operational characteristics that may be suitable for a wide range of volumetric tissue removal procedures in, or on, structure of a patient's body. The system and method of the invention are suitable for other arthroscopic surgeries, including partial meniscectomies, synovectomies, chondroplasties, tendon and cartilage removals, and in general resurfacing and texturing of cartilage, tendon and bone surfaces. In addition, in the ENT and GI fields, there are a variety of procedures that require volumetric tissue removal either at a tissue surface or at the end of a probe inserted percutaneously in treating nose and throat disorders, for example, soft palate volume reduction surgery, turbinate reduction surgery and jaw bone surgery. These procedures require that the surgeon be provided with means to remove tissue in close proximity to delicate structures and nerves that cannot be damaged, which procedures lend themselves to the methods disclosed herein. In many fields, the selective removal of malignant tissue or other tumors may be accomplished by the present invention. For example, a form of stereotactic-directed probe may be used to ablate breast lesions. The method of the invention also may have use in interventional cardiology to remove vascular occlusions. The method of the invention also may be useful for drilling holes in tissue such as in TMR procedures (transmyocardial revascularization). The material removal methods described above apply to all body structures, which include non-anatomic structures such as accretions, calculi and the like.
Those skilled in the art will appreciate that the exemplary embodiments and descriptions thereof are merely illustrative of the invention as a whole, and that variations in controlling the duration of intervals of energy delivery, in controlling the repetition rate, and in controlling the voltage applied to create the interval of intense electric fields may be made within the spirit and scope of the invention. Accordingly, the present invention is not limited to the specific embodiments described herein, but includes the features disclosed in the author's co-pending applications listed in the Section above titled “CROSS-REFERENCE TO RELATED APPLICATIONS” and the invention is defined by the scope of the appended claims. Specific features of the invention may be shown in some figures and not in others, and this is for convenience only and any feature may be combined with another in accordance with the invention. While the principles of the invention have been made clear in the exemplary embodiments, it will be obvious to those skilled in the art that modifications of the structure, arrangement, proportions, elements, and materials may be utilized in the practice of the invention, and otherwise, which are particularly adapted to specific environments and operative requirements without departing from the principles of the invention. The appended claims are intended to cover and embrace any and all such modifications, with the limits only of the true purview, spirit and scope of the invention.