Abstract:
It is an object of the present invention to provide a multi-channel radio frequency (RF) power delivery and control system for applying energy to multiple electrodes of an RF tissue heating device. 
     In a first embodiment of the invention an apparatus for controlling electrical cross-talk in an electro-surgical instrument is disclosed. The apparatus includes: a driver, a first electrode, a second electrode, a ground for delivery of power to a surgical site, a power measurement circuit and a waveform generator. The power measurement circuit computes differences between a target power and an actual power delivered to the first electrode and the second electrode to establish an amount by which to increase and to decrease the power emanating from the first electrode and the second electrode. The waveform generator modulates a driver signal generated by the driver to increase and to decrease an integer number of whole wavelengths of the driver signal to produce a first oscillating signal measured at the first electrode and a second oscillating signal measured at the second electrode. 
     In an alternate embodiment of the invention a method for power control in an electro-surgical instrument is disclosed.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of prior filed copending Provisional Application No. 60/061,193, filed on Oct. 6, 1997, entitled Linear Power Control With PSK Regulation, Provisional Application No. 60/062,458, filed on Oct. 6, 1997, entitled Linear Power Control With Digital Phase Lock, Provisional Application, Provisional Application No. 60/061,197, filed on Oct. 6, 1997, entitled Memory for Regulating Device Utilization and Behavior, Provisional Application No. 60/061,714, filed on Oct. 6, 1997, entitled Dual Processor Architecture For Electro Generator, Provisional Application No. 60/062,543, filed on Oct. 6, 1997, entitled Method And Apparatus For Power Measurement In Radio Frequency Electro-Surgical Generators, and Provisional Application No. 60/061,213, filed on Oct. 6, 1997, entitled Method And Apparatus for Impedance Measurement In A Multi-Channel Electro-Surgical Generator. 
     The present application is related to copending U.S. patent application Ser. No. 09/167,217, filed Oct. 6, 1998, entitled Linear Power Control With Digital Phase Lock, U.S. patent application Ser. No. 09/167,222, filed Oct. 6, 1998, entitled Memory for Regulating Device Utilization and Behavior, U.S. patent application Ser. No. 09/167,508, filed Oct. 6, 1998, entitled Dual Processor Architecture For Electro Generator, U.S. patent application Ser. No. 09/167,505, filed Oct. 6, 1998, entitled Method And Apparatus For Power Measurement In Radio Frequency Electro-Surgical Generators, U.S. patent application Ser. No. 09/167,215, filed Oct. 6, 1998, entitled Method And Apparatus for Impedance Measurement In A Multi-Channel Electro-Surgical Generator, International Application No. PCT/U.S. 98/21065, filed Oct. 6, 1998, entitled Linear Power Control With Digital Phase Lock, and International Application No. PCT/4598/21065, filed October 1998, entitled Dual Processor Architecture For Electro Generator. 
     Each of the above-cited applications is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to the field of electro-surgical medical devices. More particularly, this invention relates to devices that deliver energy in the form of radio-frequency electrical current to tissue in order to perform surgical functions. 
     2. Description of Related Art 
     Various medical procedures rely on high-frequency electrical currents to deposit energy and thus heat human and animal tissues. During such procedures, a high-frequency current is passed through the tissue between electrodes. One electrode is located at the tip of a surgical probe. Another electrode is located elsewhere, and may be a ground pad or another surgical probe tip. The tissue to be treated lies between the electrodes. 
     When the electrode circuit is energized, the electric potential of the electrodes at the probe tips oscillates at radio frequencies about a reference potential. If one is used, a ground pad remains at a floating reference potential. As the electric potential of the probe electrodes varies, a motive force on charged particles in the tissue is established that is proportional to the gradient of the electric potential. This electromotive force causes a net flow of electric charge, a current, to flow from one electrode, through the tissue, to any other electrode(s) at a lower potential. In the course of their flow, the charged particles collide with tissue molecules and atoms. This process acts to convert electrical energy to sensible heat in the tissue and is termed Joule heating. 
     Upon heating, surgical functions such as cutting, cauterizing and tissue destruction can be accomplished. For example, tissues can be cut by heating and eventually vaporizing the tissue cell fluids. The vaporization causes the cell walls to rupture and the tissue to cleave. When it is beneficial to destroy tissue, comparatively higher rates of energy deposition can cause tissue ablation. 
     Ablation of cellular tissues in situ is used in the treatment of many diseases and medical conditions either alone or combined with surgical removal procedures. Surgical ablation is often less traumatic than surgical removal procedures and may be the only alternative where other procedures are unsafe. 
     Tissue ablation devices commonly utilize electromagnetic (microwave, radio frequency (RF), lasers) or mechanical (acoustic) energy. In the category of electro-surgical devices, microwave ablation systems utilize a microwave antenna which is inserted into a natural body opening through a duct to the zone of treatment. Electromagnetic energy then radiates from the antenna through the duct wall into the target tissue. However, there is often severe trauma to the duct wall in this procedure since there is a significant microwave energy flux in the vicinity of the intended target. The energy deposition is not sufficiently localized. To reduce this trauma, many microwave ablation devices use a cooling system. However, such a cooling system complicates the device and makes it bulky. Laser ablation devices also suffer the same drawback as microwave systems. The energy flux near the target site, while insufficient to ablate the tissue, is sufficient to cause trauma. 
     Application of RF electric currents emanating from electrode tips offers the advantage of greater localization of the energy deposition since the electrode tip is nearly a point source. However, these devices require consideration and accurate monitoring of the time rate of energy transfer to the tissue. Since the electric energy flux is localized, the electrical dissipation and storage characteristics of the tissue carrying the current may vary with time as a result of the current-induced heating. Thus, the power absorbed by the tissue could vary over the time of treatment due to changing values of the tissue&#39;s electrical properties. 
     The localization of energy flux in an RF electro-surgical device may require a number of electrodes to be included in the surgical probe to provide adequate area coverage. This may result in the electric power being delivered across several current paths. With multiple electrodes in a surgical probe, each probe electrode may or may not be at the same electric potential at each instant due to amplitude, frequency, or phase variations in their RF oscillations. If each probe electrode is at the same potential, then a current will flow between the probe electrode and the ground pad. This mode of operation is termed monopolar. If, however, each probe electrode is not an identical potential, current will flow between the probe electrodes. This mode of operation is termed multipolar. If there are potential differences between the probe electrodes and there is a ground pad, then there are currents between the probe electrodes as well as currents between the probe electrodes and the grounding pad. This model of operation is a combination of monopolar and multipolar modes. It is noteworthy that in the case of multipolar operation, the probe electrodes are electrically coupled by the currents flowing between them. The extent of the coupling is primarily determined by the difference in electric potential between the probe electrodes and the electrical properties of the tissue between the electrodes. This coupling can confuse monitoring of applied power and tissue response. 
     Power control is critical in an RF electro-surgical device since it is directly related to the intended medical effects. As described, the power absorbed by the tissue can vary over the time of treatment due to changing values of the tissue&#39;s electrical properties. This variation is due to a relation well-known to those skilled in the art in which the instantaneous power delivered to the tissue load is proportional to the square of the electrode voltage and inversely proportional to the tissue electrical impedance. Thus, to achieve equal power delivery, two surgical probe electrodes may have to be at different electric potentials (voltages) because of Joule heating effects on the tissue electrical impedance, or because of impedance gradients in the tissue. When the surgical probe electrodes are at different electric potentials, a current will flow between the electrodes. This electrode coupling is commonly referred to as cross-talk. Cross-talk confuses accurate power determination in most RF electro-surgical devices and thus there is a need for improved methods to control power delivery. 
     SUMMARY OF THE INVENTION 
     Methods and apparatus for power delivery and control in electro-surgery are fundamental to the intended medical benefits. During tissue heating, any electrode coupling confuses interpretation of power measurements by providing multiple current paths from electrodes. This coupling is commonly referred to as cross-talk or multi-pole behavior. It is an object of the present invention to provide a multi-channel radio frequency (RF) power delivery and control system for applying energy to multiple electrodes of an RF tissue heating device with independent control of the amplitude, frequency, inter-electrode phase and time duration of the energy applied to each electrode. This results in accurate, controlled heating of the target tissue. 
     In a first embodiment of the invention an apparatus for controlling electrical cross-talk in an electro-surgical instrument is disclosed. The apparatus includes: a driver, a first electrode, a second electrode, a ground for delivery of power to a surgical site, a power measurement circuit and a waveform generator. The power measurement circuit computes differences between a target power and an actual power delivered to the first electrode and the second electrode to establish an amount by which to increase and to decrease the power emanating from the first electrode and the second electrode. The waveform generator modulates a driver signal generated by the driver to increase and to decrease an integer number of whole wavelengths of the driver signal to produce a first oscillating signal measured at the first electrode and a second oscillating signal measured at the second electrode. 
     In an alternate embodiment of the invention a method for power control in an electro-surgical instrument is disclosed. The electro-surgical instrument includes a driver, and at least one electrode and a ground for delivery of power in the form of a first oscillating signal to a surgical site. The method for power control comprises the acts of: 
     computing differences between a target power and an actual power delivered to the at least one electrode to establish an amount by which to increase and to decrease the power in the first oscillating signal; and 
     modulating a driver signal generated by the driver to increase and to decrease an integer number of whole wavelengths of the driver signal to produce the first oscillating signal, responsive to said computing act. 
     In another embodiment of the invention a method for power control in an electro-surgical instrument is disclosed. The method comprises the acts of: 
     computing differences between a target power and an actual power delivered to the first electrode and the second electrode to establish an amount by which to increase and to decrease the power emanating from the first electrode and the second electrode; and 
     modulating a driver signal generated by the driver to increase and to decrease an integer number of whole wavelengths of the driver signal to produce a first oscillating signal measured at the first electrode and a second oscillating signal measured at the second electrode, responsive to said computing act. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1A shows an illustrative embodiment of a RF treatment system. 
     FIG. 1B shows an exploded perspective view of the surgical handset. 
     FIG. 1C shows a cross-sectional view of a stylet of the RF heating device. 
     FIG. 2 shows a block diagram showing elements of the system architecture. 
     FIG. 3A shows electrode voltage versus time for an electrode at a lower than reference impedance, illustrating the use of null cycles for controlling multi-pole operation. 
     FIG. 3B shows electrode voltage versus time for an electrode at a reference impedance. 
     FIG. 4 illustrates the monopolar, multipolar and combined monopolar/multipolar modes of operation. 
     FIGS. 5A-B show process flow diagrams for alternate embodiments of the method for multi-electrode power delivery of the present invention. 
    
    
     DETAILED DESCRIPTION 
     This invention utilizes a novel method to control power delivery and inter-electrode coupling in a multi-electrode RF electro-surgical device. While the amplitude, frequency, and phase of the electrode voltages are adjustable in the system, the power delivery and inter-electrode cross-talk is controlled by introducing null intervals into the electrode&#39;s RF voltage wave train by means of switching. 
     FIG. 1A shows the apparatus for a typical embodiment of the RF electro-surgical device. The system comprises an RF power supply  100  with a user input and display panel  102 , a foot switch  104 , a surgical handset  106  with a surgical probe  108  and an electrical grounding pad  110 . 
     The RF power supply  100  converts the low frequency electrical energy supplied by a wall connection (not shown) into the high frequency or RF energy necessary for surgery. The user input and display panel  102  displays relevant parameters and provides buttons and switches for user input to the control systems. The foot switch  104  connected to the power supply provides means for switching off the unit should there be an emergency. The surgical handset  106  is also connected to the power supply and is the means for delivering the RF energy to the surgical probe  108 . The probe has one or more electrodes. The electrical grounding pad  110  is also connected to the power supply. Other embodiments have no grounding pad. 
     FIG. 1B is an exploded perspective view of the surgical handset  106  and surgical probe  108 . As shown, the surgical probe  108  includes a flexible catheter  111  which is attached to a control unit  106  by a connector  114 . The flexible catheter  111  includes a distal tip  116  having two stylets  118 ,  119 , which extend outward from stylet ports  120 ,  121 . Each stylet has a probe electrode  126 A-B. The surgical handset  106  includes an RF power connector  122  and a thermocouple connector  124 . 
     The flexible catheter  111  preferably has a stiffness gradient for easier insertion through a natural body opening into a body duct. For example, the flexible catheter  111  can be relatively stiff near the surgical handset  106  and more flexible near the distal tip  116 . The flexible catheter  111  can be constructed of an inner slotted stainless steel tube with an outer flexible sleeve, such as the catheter described in detail in copending application Ser. No. 08/126,431 filed Sep. 24, 1993, now U.S. Pat. No. 5,322,064, the entire contents of which are incorporated herein by reference. The catheter may also be constructed of a coiled or braided wire having a bonded outer sleeve. 
     FIG. 1C is a cross-sectional view of a stylet such as  118  or  119 . Each stylet includes a probe electrode  126 A-B enclosed within a retractable insulating sleeve  128 . The stylets are described in detail in the copending application Ser. No. 08/012,370 filed Feb. 2, 1993, now U.S. Pat. No. 5,370,675. As shown in FIG. 1C, the insulating sleeve  128  has a tapered tip  130 . The probe electrode  126  is disposed in the center portion  138  of the insulating sleeve  128  such that it can slide within the sleeve. A thermocouple  142  is mounted near the tapered tip  130  of the insulating sleeve  128  for measuring the temperature of a target tissue as it is heated. 
     FIG. 2 shows a block diagram showing elements of the system hardware architecture of an exemplary embodiment. FIG. 2 shows a block diagram of the RF power supply  100 , surgical probe  108  and grounding pad  110 . Within the power supply, the user input and display panel  102 , micro-controller  202 , first and second electrode channels  204  and  206 , tissue temperature measurement system  208 , memory unit  210 , memory files  212 , control parameter schedule  214 , and RF oscillator  203  are indicated. Electrode channels  204  and  206  are identical, each comprising a control system  220 A-B, waveform generator  222 A-B, an isolation switch  224 A-B, a power drive  226 A-B, a transformer  228 A-B, a filter  230 A-B, current and voltage sensors  232 A-B, and power measurement system  234 A-B. 
     In FIG. 2, the user input and display panel  102  is connected to the micro-controller  202  which is connected to the memory unit  210  including memory files  212 , including a control parameter schedule  214 . Exemplary control parameters are power and tissue temperature at the surgical site. Other control parameters are apparent to persons skilled in the art. The micro-controller is connected with the identical electrode channels  204  and  206  and also to the tissue temperature measurement system  208  and the RF oscillator  203 . Within each electrode channel, the control systems  220 A-B are connected to the micro-controller as well as to the RF oscillator and the tissue temperature measurement system. The control system also connects to the waveform generators  222 A-B. The waveform generators are connected to the power drive  226 A-B through the isolation switch  224 A-B. It is obvious to those skilled in the art that the isolation switch  224 A-B may be located elsewhere than shown in FIG.  2 . It is equally obvious that the function of the isolation switch can be performed by other elements in FIG. 2 such as the microcontroller, control system, waveform generator, power drive, filter or electrode. The RF signals from the transformer  228 A-B feed into filter  230 A-B. The current and voltage sensors  232 A-B connect to the filter, grounding pad  110 , surgical probe  108  and the power measurement systems  234 A-B. 
     The micro-controller  202  implements control programs and logic contained in memory files  212 , providing the principal intelligence of the control system including the selection of values for time scales and power levels. In other embodiments, control is accomplished by analog hardware in control system  220 A-B. In this alternate embodiment the micro-controller downloads new target values to the control system  220 A-B. The control system  220 A-B uses these new values as well as power measurements performed by the power measurement system  234 A-B to adjust the drive level to the waveform generators  222 A-B so as to minimize the error between target power and actual power delivered by the channels. To act as a means for control, the micro-controller is in two way communication with the user through user input and display panel  102  as well as receives input from the RF oscillator  203 , and power and tissue temperature measurement systems  234 A-B,  208 A-B. Control variables are passed to control systems  220 A-B and filters  230 A-B to achieve the desired amplitude, frequency, and phase of the electrode potentials. The RF oscillator and waveform generator  222 A-B generate RF oscillations, termed a driver signal. The driver signal, or a modulated driver signal incorporating on-off switching with the driver signal, drives the output of the power drive  226 A-B. The modulation of the driver signal is determined by the micro-controller. It is obvious to persons skilled in the art that switch  224 A-B may be located in a variety of positions other than shown in the embodiment of FIG.  2 . Power is coupled through transformer  228 A-B by the principle of induction, isolating the patient from direct current (DC). Further frequency filtering is accomplished by filter  230 A-B. Current and voltage sensors  232 A-B provide required signals for the power measurement systems  234 A-B to determine the actual, aka true power or nonreactive, power transferred to the tissue by the current passing between the surgical probe  108  to grounding pad  110 . 
     Micro-controller  202  can differentially control the voltage waveforms of each electrode in the surgical probe  108 . By altering the amplitude or frequency, as well as by introducing null intervals to the voltage applied to each electrode tip, the electric power transferred to the tissue in the face of changing tissue electrical impedance can be controlled over the time of the surgical procedure. In other embodiments, this is accomplished by analog hardware in control system  220 A-B. In this alternate embodiment the micro-controller downloads new target values to the control system  220 A-B. The control system  220 A-B uses these new values as well as power measurements performed by the power measurement system  234 A-B to adjust the power so as to minimize the error between target power and actual power delivered by the channels. 
     FIG.  3 A and FIG. 3B show the electrode voltage (monopolar mode) versus time for two electrodes illustrating the novel use of null intervals to control power delivery and multi-pole operation. The electrode of FIG. 3B delivers power across tissue with a given reference electrical impedance. The electrode of FIG. 3A delivers power across tissue with a comparatively smaller impedance. RF voltage wavetrain  300 , and voltage wavelets  304 A-E oscillate about null crossing  302 A-B. By removing wave cycles by means of switch  224 A-B (see FIG. 2) at the lower impedance electrode, the electric power delivered to the tissues (equal to the electrode voltage squared divided by the impedance) at both sites can be matched over tissue heating time scales. 
     In the preferred embodiment, only full waves are nulled. Arbitrary nulling results in both high and low frequency Fourier signal components that affect the other medical electronics and the patient, respectively. Half-wave nulling could cause the patient to accumulate charge due to the current-voltage lag caused by tissue capacitance and inductance. 
     In another embodiment, a constant voltage source can be used. Since the voltage is constant in this embodiment, the use of a voltage sensor is not required 
     During a null period, an inter-electrode current driven by their potential difference (bipolar mode) is prevented in the preferred embodiment by connecting the nulled electrode to an extremely high impedance. If a strictly monopolar delivery is desired after the null period, the RF cycles are resumed in phase with those of the other electrode thereby avoiding any bipolar effects. In an alternate embodiment, the RF cycles may be resumed out of phase and the resulting electrode potential difference will drive an interelectrode current. 
     FIG. 4 illustrates the monopolar, multipolar and combined monopolar/multipolar modes of operation. Two probe electrodes  126 A-B are shown. In monopolar operation, current  450  flows due differences in electric potential between the electrodes  126 A-B and the floating ground provided by either the grounding pad or other electrode (not shown). There is no inter-electrode current. It is obvious to persons skilled in the art that elements other than the grounding pad can be substituted equivalently, for example an electrode connected to a floating ground. In the bipolar mode of operation, the ground pad is removed from the circuit either physically or effectively by connecting it to an extremely large impedance. In bipolar operation, current  460  flows between the probe electrodes  126 A-B due to the differences in electric potential. There is no current flow to the ground pad. Multipolar operation is an extension of bipolar operation with more than two probe electrodes. Combined monopolar and bipolar operation occurs when potential differences drive currents both to the floating ground  480  and between the electrodes  470 . Note that the extent of the treatment zone is affected by the mode of operation. 
     A significant advantage of the present invention is the availability of a combined monopolar and bipolar mode of operation in addition to a purely monopolar mode. In the monopolar mode, the same RF voltage signal is applied to each probe electrode  126 A-B (see FIG. 1B) and current flows from the electrodes to an indifferent ground pad electrode placed in contact with the patient. Since the control system maintains the two probe electrodes  126 A-B at the same potential, no current flows between them. Current only flows between each electrode and the electrical ground pad. However, by changing the amplitude, frequency or phase of one of the RF signals, a potential difference is created across the probe electrodes  126 A-B and current flows between them. This provides combined monopolar and bipolar operation. Combined monopolar and bipolar operation allows a larger tissue volume to be heated as shown in FIG.  4 . 
     FIG.  5 A and FIG. 5B show flowcharts for the RF wave cycles adjustment process for a two electrode embodiment. The processes shown in FIGS. 5A-B is implemented by the micro controller  202  (see FIG.  2 ). In FIG. 5A, the process begins in block  502  where an initial value for the target power is obtained from either the user input and display panel  102  or memory unit  210 . Control then passes to block  504  where electric power is applied to the tissue through the electrodes. Control then passes to decision block  510  where a determination is made whether the tissue temperature exceeds a predetermined maximum. If the maximum tissue temperature is exceeded, RF power is cut off in process block  514 . If the tissue temperature is not exceeded, control passes to process block  512  where the power delivered to the tissue is determined by power measurement system  234 A-B (see FIG.  2 ). Control then passes to process block  516  where an updated target value for the power corresponding to the surgical time is obtained from memory unit  210 . Control then passes to control sequence  520 A where the null periods are determined, thereby altering the applied power. In the preferred embodiment, the null intervals are applied at every other wave cycle when nulling is required. The thermal lag of the tissue response integrates the effect of this quantized (full on-half nulled-full off) range. 
     In control sequence  520 , a determination is made at decision block  522  whether the target value for the power exceeds the power actually delivered to the surgical site, as determined by the power measurement system  234 A-B (see FIG.  2 ). In the case that the actual power delivered to the surgical site is less than the target value, null periods are removed to increase the applied power. In the case that the actual power delivered to the surgical site is greater than the target value, null intervals are added to decrease the applied power in process block  524 . If necessary, null intervals are removed in process block  526 . Control then passes to decision block  550  where a determination is made whether the surgical time is expired. If the surgery continues, control passes to decision block  510  where the measured tissue temperature is again compared to a predetermined value. If the surgery is finished, process block  552  stops the operation. 
     FIG. 5B shows detail of control sequence  520 B where the null intervals are implemented. The process flow is as described in FIG. 5A up to block  516 . In process block  530  micro controller  202  (see FIG.2) computes the difference between the target value of power and that actually delivered relative to the actually delivered power. Control then passes to process block  532  where the relative error calculated in block  530  is used to correct the number of voltage wavelets  304 A-E per unit time that are passed to the tissue. This integer value is stored in a wavelet down counter in micro controller  202  (see FIG.  2 ). Control then passes to block  534  that begins a control time interval over which RF wavelets are counted. The preferred interval is an integer multiple of the inverse RF frequency and is approximately one second in duration. Control then passes to block  536  where the wavelet down counter decrements after each time period corresponding to the inverse RF frequency. Control then passes to decision block  538  where a determination is made whether the down counter has reached zero, indicating a null interval is to commence. If a null interval  306 A-D (see FIG. 3A) is to commence, the electrode to be nulled has voltage removed and is switched to a extremely large impedance circuit element to prevent any current flow in process block  540 . If the down counter indicates by a nonzero value that a null interval is not desired, the electrode remains active in an active interval. Control then passes to decision block  542  where a determination is made whether the control time interval is finished. Control then passes to decision block  550  where a determination is made whether the surgical time is expired. If the surgery continues, control passes to decision block  510  where the measured tissue temperature is again compared to a predetermined value. If the surgury is finished, process block  552  stops the operation. 
     From the foregoing, it will be appreciated that the present invention represents a significant advance in the field of RF electro-surgical devices. Although several preferred embodiments of the invention have been shown and described, it will be apparent that other adaptations and modifications can be made without departing from the spirit and scope of invention. Accordingly, the invention is not to be limited, except as by the following claims. 
     The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in this art.