Patent Publication Number: US-2010130976-A1

Title: Reducing cross-talk effects in an rf electrosurgical device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority to U.S. Provisional Application Ser. No. 61/116,933, filed on Nov. 21, 2008. 
    
    
     TECHNICAL FIELD 
     This description is related to reducing the effects of cross-talk in a radiofrequency (RF) electrosurgical device. 
     BACKGROUND 
     Radiofrequency (RF) ablation or lesioning is a technique that uses RF energy to produce heat to destroy tissue. This technique is used in a number of procedures, such as the lesioning of heart tissue to correct abnormal heartbeats and the destruction of tumors. RF lesioning is also used in procedure known as rhizotomy to treat pain, such as back pain, by stunning or destroying problematic spinal nerves. This procedure may be performed, for example, to treat pain caused by a herniated disc or from facet joint syndrome. The RF energy is transmitted through a probe placed adjacent to a sensory nerve. The RF energy produces heat to destroy the sensory nerve(s) carrying the pain. 
     SUMMARY 
     In one aspect, an electrosurgical system includes a source of electrical energy, a grounding pad, a first probe, a second probe, a first switch, a second switch, and a control system. The grounding pad is coupled to the source of electrical energy and configured to be coupled to a body of a patient. The first probe is coupled to the source of electrical energy and configured to be inserted into tissue of the patient. In addition, the first probe is configured to create a lesion when the first probe is inserted into tissue and electrical energy is applied to the first probe from the source of electrical energy. Similarly, t second probe is coupled to the source of electrical energy and configured to be inserted into tissue of the patient. Also, the second dprobe is configured to create a lesion when the second probe is inserted into tissue and electrical energy is applied to the second probe from the source of electrical energy. The first switch is coupled to the first probe such that the first switch couples the first probe to ground when in a closed state and the second switch is coupled to the second probe such that the second switch couples the second probe to ground when in a closed state. The control system is configured to apply electrical energy from the source of electrical energy to the first probe in a manner that causes the first probe to create a lesion when the first probe is inserted into tissue and to apply electrical energy from the source of electrical energy to the second probe in a manner that causes the second probe to create a lesion when the second probe is inserted into tissue. Furthermore, the control system is configured to receive an indication of a first parameter associated with the first probe, control the first switch based on the first parameter, receive an indication of a second parameter associated with the second probe, and control the second switch based on the second parameter. 
     Implementations of any aspect may include one or more of the following features. For example, the first parameter may include a first temperature at the first probe such that the control system is configured to control the first switch based on the first temperature and the second parameter may include a second temperature at the second probe such that the control system is configured to control the second switch based on the second temperature. To control the first switch based on the first temperature, the control system may be configured to close the first switch when the first temperature is above a first value. To control the second switch based on the second temperature, the control system may be configured to close the second switch when the second temperature is above the first value. The control system may be configured to open the first switch when the first temperature is below the first value and to open the second switch when the second temperature is below the first value. 
     To apply electrical energy from the source of electrical energy to the first probe in a manner that causes the second probe to create a lesion when the first probe is inserted into tissue, the control system may be configured to apply electrical energy to the first probe when the first temperature is below a second value and remove the applied electrical energy from the first probe when the first temperature is above the second value. Tto apply electrical energy from the source of electrical energy to the second probe in a manner that causes the second probe to create a lesion when the second probe is inserted into tissue, the control system may be configured to apply electrical energy to the second probe when the second temperature is below the second value and remove the applied electrical energy from the second probe when the second temperature is above the second value. 
     The system may include a third switch and a fourth switch. The third switch may be coupled between the first probe and the source of electrical energy such that the first probe is disconnected from the source of electrical energy when the third switch is in an open state and connected to the source of electrical energy when the third switch is in a closed state The fourth switch may be coupled between the second probe and the source of electrical energy such that the second probe is disconnected from the source of electrical energy when the fourth switch is in an open state and connected to the source of electrical energy when the fourth switch is in a closed state. To apply electrical energy to the first probe, the control system may be configured to close the third switch and, to remove the applied electrical energy from the first probe, the control system is configured to open the third switch. To apply electrical energy to the second probe, the control system is configured to close the fourth switch and, to remove the applied electrical energy from the second probe, the control system is configured to open the fourth switch. 
     The control system may be configured to pulse width modulate the electrical energy applied to the first probe by opening and closing the third switch; and to pulse width modulate the electrical energy applied to the second probe by opening and closing the fourth switch. 
     To apply electrical energy to the first probe, the control system may be configured to cause the source of electrical energy to output a voltage with a non-zero magnitude and, to remove the applied electrical energy from the first probe, the control system may be configured to cause the source of electrical energy to output a voltage with a zero magnitude. To apply electrical energy to the second probe, the control system may be configured to cause the source of electrical energy to output a voltage with a non-zero magnitude and, to remove the applied electrical energy from the second probe, the control system may be configured to cause the source of electrical energy to output a voltage with a zero magnitude. 
     The first parameter may include a first current through the first probe such that the control system is configured to control the first switch based on the first current and the second parameter may include a second current through the second probe such that the control system is configured to control the second switch based on the second current. To control the first switch based on the first current, the control system may be configured to open the first switch when the first current is below a first value and close the first switch when the first current is above the first value. To control the second switch based on the second current, the control system may be configured to open the second switch when the second current is below the first value and close the second switch when the second current is above the first value. 
     The control system may be configured to close the third switch when the first parameter is below a first value, open the third switch when the first parameter is above the first value, close the fourth switch when the second parameter is below the first value, and open the fourth switch when the second parameter is above the first value. The control system may be configured to control an amount of power applied to the first probe or the second probe by controlling a magnitude of a voltage output by the source of electrical energy. 
     The first probe may include a first probe tip and the second probe may include a second probe tip. The first probe and first switch may be configured such that current flows from the first probe to ground without passing through the first probe tip when the first switch is closed. The second probe and second switch may be configured such that current flows from the second probe to ground without passing through the second probe tip when the first switch is closed. 
     The first probe and first switch may be configured such that an impedance between the first probe and ground is less than an impedance between the first probe and the grounding pad when the first probe is inserted in the tissue of the patient and the first switch is closed. The second probe and second switch may be configured such that an impedance between the second probe and ground is less than an impedance between the second probe and the grounding pad when the second probe is inserted in the tissue of the patient and the second switch is closed. 
     In another aspect, a method of performing electrosurgery may include coupling a grounding pad to a body of a patient, where the grounding pad is also coupled to a source of electrical energy. The method includes inserting a first probe into tissue of the patient and a second probe into tissue of the patient. The first probe and second probes are each coupled to the source of electrical energy and configured to create a lesion when inserted into tissue and electrical energy is applied from the source of electrical energy. The method further includes applying electrical energy from the source of electrical energy to the first probe in a manner that causes the first probe to create a lesion in the tissue into which the first probe is inserted and applying electrical energy from the source of electrical energy to the second probe in a manner that causes the second probe to create a lesion in the tissue into which the second probe is inserted. The method further includes receiving an indication of a first parameter associated with the first probe; controlling a first switch based on the first parameter, wherein the first switch is coupled to the first probe such that the first switch couples the first probe to ground when in a closed state; receiving an indication of a second parameter associated with the second probe; and controlling a second switch based on the second parameter, wherein the second switch is coupled to the second probe such that the second switch couples the second probe to ground when in a closed state; 
     Implementations of any aspect may include one or more of the following features. For example, the first parameter may include a first temperature at the first probe such that controlling the first switch comprises controlling the first switch based on the first temperature and the second parameter may include a second temperature at the second probe such that controlling the second switch comprises controlling the second switch based on the second temperature. Controlling the first switch based on the first temperature may include closing the first switch when the first temperature is above a first value and controlling the second switch based on the second temperature may include closing the second switch when the second temperature is above the first value. Controlling the first switch based on the first temperature may include opening the first switch when the first temperature is below the first value and controlling the second switch based on the second temperature may include opening the second switch when the second temperature is below the first value. 
     Applying electrical energy from the source of electrical energy to the first probe in a manner that causes the first probe to create a lesion in the tissue into which the first probe is inserted may include applying electrical energy to the first probe when the first temperature is below a second value and removing the applied electrical energy from the first probe when the first temperature is above the second value Applying electrical energy from the source of electrical energy to the second probe in a manner that causes the second probe to create a lesion in the tissue into which the second probe is inserted may include applying electrical energy to the second probe when the second temperature is below the second value and removing the applied electrical energy from the second probe when the second temperature is above the second value. 
     Applying electrical energy to the first probe may include closing a third switch, with the third switch being coupled between the first probe and the source of electrical energy such that the first probe is disconnected from the source of electrical energy when the third switch is in an open state and connected to the source of electrical energy when the third switch is in a closed state. Removing the applied electrical energy from the first probe may include opening the third switch. Applying electrical energy to the second probe may include closing a fourth switch, with the fourth switch being coupled between the second probe and the source of electrical energy such that the second probe is disconnected from the source of electrical energy when the fourth switch is in an open state and connected to the source of electrical energy when the fourth switch is in a closed state. Removing the applied electrical energy from the second probe may include opening the fourth switch. 
     The electrical energy applied to the first probe may be pulse width modulated by opening and closing the third switch. The electrical energy applied to the second probe may be pulse width modulated by opening and closing the fourth switch. 
     An amount of power applied to the first probe or the second probe may be controlled by controlling a magnitude of a voltage output by the source of electrical energy. Applying electrical energy to the first probe may include causing the source of electrical energy to output a voltage with a non-zero magnitude and removing the applied electrical energy from the first probe may include causing the source of electrical energy to output a voltage with a zero magnitude. Similarly, applying electrical energy to the second probe may include causing the source of electrical energy to output a voltage with a non-zero magnitude and removing the applied electrical energy from the second probe may include causing the source of electrical energy to output a voltage with a zero magnitude. 
     Applying electrical energy from the source of electrical energy to the first probe in a manner that causes the first probe to create a lesion in the tissue into which the first probe is inserted may include closing a third switch when the first parameter is below a first value and opening the third switch when the first parameter is above the first value. The third switch may be coupled between the first probe and the source of electrical energy such that the first probe is disconnected from the source of electrical energy when the third switch is in an open state and connected to the source of electrical energy when the third switch is in a closed state. Likewise, applying electrical energy from the source of electrical energy to the second probe in a manner that causes the second probe to create a lesion in the tissue into which the second probe is inserted may include closing a fourth switch when the second parameter is below the first value and opening the fourth switch when the second parameter is above the first value. The fourth switch may be coupled between the second probe and the source of electrical energy such that the second probe is disconnected from the source of electrical energy when the fourth switch is in an open state and connected to the source of electrical energy when the fourth switch is in a closed state. 
     In one aspect, an electrosurgical system includes a source of electrical energy, a first probe coupled to the source of electrical energy, and a second probe coupled to the source of electrical energy. A first switch is coupled to the first probe and couples the first probe to ground when in a closed state. A second switch is coupled to the second probe and couples the second probe to ground when in a closed state. A control system is configured to receive an indication of a first temperature at the first probe and control the state of the first switch based on the first temperature. The control system is also configured to receive an indication of a second temperature at the second probe and control the state of the second switch based on the second temperature. 
     The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features, objects, and advantages will be apparent from the description and drawings. 
    
    
     
       DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic of an RF electrosurgical system. 
         FIG. 2  is a graph of the voltages applied to the probes of the RF electrosurgical system. 
         FIG. 3  is a graph of temperature versus time at the probes of the RF electrosurgical system. 
         FIG. 4  is a schematic showing an alternative RF generation system for the RF electrosurgical system. 
         FIG. 5  is a schematic showing another alternative RF generation system for the RF electrosurgical system. 
         FIG. 6  is an illustration depicting the use of RF lesioning to treat back pain caused by facet joint syndrome. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to  FIG. 1 , an electrosurgical system  100 , such as an RF lesioning system, includes an RF generation system  102 , a first RF probe  104   a , a second RF probe  104   b , and a ground pad  114 . Probes  104   a  and  104   b  include temperature sensors  106   a  and  106   b  (for example, T-type thermocouples), and are coupled to RF generation system  102  through leads  116   a  and  116   b , respectively. The ground pad  114  is coupled to RF generation system  102  through a lead  118 . The RF generation system  102  includes an RF generator  102   a , which may be regulated to maintain a constant RF voltage waveform. The RF generation system  102  also includes source AC switches  102   b - 1  and  102   b - 2  and ground AC switches  102   c - 1  and  102   c - 2 . As described more fully below, the ground AC switches  102   c - 1  and  102   c - 2  can provide an alternate path to ground for cross-talk currents, which can reduce or eliminate the effects of the cross-talk currents on the temperatures at the tips of the probes  104   a  and  104   b.    
     The RF generation system  102   d  includes a control system  102   d  to control the state of the source AC switches  102   b - 1  and  102   b - 2  and the ground AC switches  102   c - 1  and  102   c - 2 . The control system  102   d  may be implemented, for example, using a microprocessor or microcontroller. The control system  102   d  receives temperature readings from temperature sensors  106   a  and  106   b . As described in more detail below, based on those temperature readings, the control system controls the operation of the source AC switches  102   b - 1  and  102   b - 2  and ground AC switches  102   c - 1  and  102   c - 2  to maintain the temperatures at the probes  104   a  and  104   b  at or near a target temperature. 
     To perform RF lesioning, the RF probes  104   a  and  104   b  are inserted into human tissue  116  and each probe is situated in or near the tissue to be lesioned. For example, if the procedure being performed is rhizotomy, the probes  104   a  and  104   b  are each positioned near a nerve to be lesioned (not shown) using, for example, fluoroscopy. The probes  104   a  and  104   b  may be placed, for example, within 5 mm of the nerve for a 10 mm diameter lesion size. More generally, the probes  104   a  and  104   b  are positioned so that the distance to the nerve is within the lesion size. The ground pad  114  is also attached to the patient&#39;s body. 
     Depending on the procedure, the physician can first place the RF generation system in a diagnostic mode to insure proper placement of the probes  104   a  and  104   b . For example, if the device  100  is used to lesion nerves, a diagnostic mode (described in more detail with respect to  FIG. 6 ) can be used to insure that the probes  104   a  and  104   b  are placed near the proper nerves. Once the probes  104   a  and  104   b  are properly positioned, the physician places the RF generation system  102  into a RF Lesion mode. At this point, the control system  102   d  closes or maintains closed both source AC switches  102   b - 1  and  102   b - 2 , and opens or maintains open the ground AC switches  102   c - 1  and  102   c - 2 . The control system  102   d  then causes the regulated RF generator to apply, for example, a continuous RF voltage to each probe  104   a  and  104   b  through the closed source AC switches  102   b - 12  and  102   b - 2 . 
     Referring to  FIG. 2 , as shown by the graph  200 , the RF generator  102   a  applies the same RF voltage signal to each probe  104   a  and  104   b . For instance, a continuous RF voltage with a frequency of 460 KHz and a peak voltage of 65 Vrms can be applied to the probes  104   a  and  104   b . Other frequencies and voltages may equally be used. Because the same RF signal is applied to each probe, the voltages at the probes  104   a  and  104   b  are substantially phase synchronous. This results in the voltage at each probe  104   a  and  104   b  being substantially the same at any given moment. 
     Referring again to  FIG. 1 , the application of the RF voltage to the probes  104   a  and  104   b  results in current flow  110   a  and  110   b  from the tips of probes  104   a  and  104   b , respectively, to ground pad  114 . Because the voltages at each probe  104   a  and  104   b  are substantially the same, a substantially zero potential difference exists between the probes  104   a  and  104   b  and substantially all of the current flows from the probes  104   a  and  104   b  to the ground pad  114 . The current flow is generally related to the impedance between the ground pad  114  and the probes  104   a  and  104   b , which is typically on the order of about 200 to about 500 Ohms. When a voltage with a peak magnitude of about 65 Vrms is used, this can result in peak currents between 200 and 700 mA. The current flow  110   a  and  110   b  causes heating of the tissue near the tips of probes  104   a  and  104   b , which forms lesions  108   a  and  108   b , respectively. 
     To properly create the lesions  108   a  and  108   b  without collateral damage to surrounding tissue, the temperature at the tips of the probes  108   a  and  108   b  is raised to and maintained within a threshold amount of a particular target temperature for a certain duration. The target temperature is generally between about 75 degrees Celsius and about 90 degrees Celsius, and the duration between about 30 to about 120 seconds, although longer durations can be used. In a particular embodiment, the target temperature is 80 degrees Celsius and the duration is 120 seconds. The threshold amount is, for example, plus or minus two degrees Celsius. 
     The control system  102   d  receives temperature readings from the temperature sensors  106   a  and  106   b  and when the temperature at a probe  104   a  or  104   b  raises to within the threshold amount of the target temperature, the control system  102   d  opens the corresponding source AC switch  102   b - 1  or  102   b - 2  to cut-off the supply of RF energy to that probe. 
     Referring to  FIG. 3 , as shown by graph  300 , the temperature T at one of the probes may reach the lower threshold T 1  around the target temperature T 1  faster than the other probe. In the example shown, the temperature at probe  104   a  reaches the lower threshold T 1  at time t 1 , while the temperature at probe  104   b  at time t 1  is still below the lower threshold T 1 . This difference can be caused, for example, by the differences in impedances between the probe  104   a  and the ground pad  114  and the probe  104   b  and the ground pad  114 , which can result in a greater current flow through the probe with the least impedance between it and the ground pad. 
     Following the example illustrated in  FIG. 3 , when the temperature at probe  104   a  reaches the lower threshold T 1  at t 1 , the control system  102   d  opens source AC switch  102   b - 1 , while maintaining source AC switch  102   b - 2  closed and ground AC switches  102   c - 1  and  102   c - 2  open. Opening the source AC switch  102   b - 1  disconnects probe  104   a  from the RF generator  102   a.    
     Referring again to  FIG. 1 , when the source AC switch  102   b - 1  is opened and no voltage is applied to the probe  104   a , a potential difference exists between the probe  104   a  and the probe  104   b . As a result of the potential difference, a cross-talk current  112  flows from the probe  104   b  to the probe  104   a . With the ground AC switch  102   c - 1  open, the cross-talk current  112  flows through the probe  104   a  to the ground pad  114 . In that case, the cross-talk current  112  causes the temperature at the tip of the probe  104   a  to continue increasing above the target temperature T t , which, if uncorrected, can result in collateral tissue damage. 
     To reduce or eliminate the temperature increase at the probe  104   a  as a result of cross-talk currents, the control system  102   d  closes the ground AC switch  102   c - 1  when the temperature at the probe  104   a  exceeds the upper threshold amount. The system  100  is designed so that the impedance between the probe  104   a  through the ground AC switch  102   c - 1  is less than the impedance between the probe  104   a  and the ground pad  114 . As a result, the cross-talk current  112  flows from the probe  104   a  through the switch  102   c - 1  into ground, instead of flowing from the probe  104   a  through the tissue  116  to the ground pad  114 . This can reduce or eliminate the increase in temperature caused by cross-talk currents. 
     If the temperature at the probe  104   a  then decreases below the upper threshold amount, the ground AC switch  102   c - 1  is opened. If the temperature at the probe  104   a  continues to drop below the lower threshold amount, then the control system  102   d  closes the source AC switch  102   b - 1  to reconnect the RF source to the probe  104   a . This results in an increase of the temperature at the probe  104   a . Once the temperature at the probe  104   a  raises to within the lower threshold amount, the source AC switch  102   b - 1  is opened again. The control system  102   d  continues to control the source AC switch  102   b - 1  and the ground AC switch  102   c - 1  in the same fashion until the end of the procedure. 
     The control system  102   d  also controls the source AC switch  102   b - 2  and ground AC switch  102   c - 2  in the same fashion. In particular, when the temperature at the probe  104   b  is within the lower threshold amount, the control system  102   d  opens the source AC switch  102   b - 2  and keeps the ground AC switch  102   c - 2  opened until the temperature at the probe  104   b  exceeds the upper threshold, at which point the ground AC switch  102   c - 2  is closed. As a result, temperature increases due to cross-talk between the probes  104   a  and  104   b  can be controlled by providing an alternate path for that current, namely, from the probes  104   a  and  104   b  to ground through the ground AC switches  102   c - 1  and  102   c - 2 , respectively, rather than through the tissue  116  to the ground pad  114 . 
     Referring to  FIG. 4 , in another embodiment, an RF generation system  402  also includes a voltage and current measurement network  402   e - 1  coupled to the probe  404   a  and a voltage and current measurement network  402   e - 2  coupled to the probe  404   b . These networks  402   e - 1  and  402   e - 2  are used to the measure the voltage and current provided to a given one of the probes  404   a  and  404   b . The control system  102   d  uses the temperature readings from the sensors on probes  404   a  and  404   b , the voltage measurements, and the current measurements to control the operation of the source AC switches  402   b - 1  and  402   b - 2  so as to control the power delivered to a given probe  404   a  and  404   b.    
     In particular, as with system  102 , when the temperature of a probe needs to be increased, the control system  402   d  closes the associated source switch  402   b - 1  or  402   b - 2 . However, rather than applying constant power to the probes  404   a  and  404   b  by maintaining the source AC switch closed, the amount of power applied to a given probe  404   a  or  404   b  is controlled by rapidly opening and closing the source AC switch  402   b - 1  or  402   b - 2 , effectively pulse width modulating (PWM) the RF signal applied to the probes  404   a  and  404   b . The control system  402   d  implements a controller, such as a proportional-integral-derivative (PID) controller, that controls the PWM of a given one of the source AC switches  402   b - 1  and  402   b - 2 , so as to control the power delivered, based on the lower threshold amount, and the temperature, voltage, and current measurement for that probe. 
     To measure the voltage and current for a given probe, the other probe may be isolated by opening the associated source AC switch  402   b - 1  or  402   b - 2  so that the RF voltage from the generator  402   a  is applied only to one of the probes, and the current returning to the RF generator is only the current flowing through that probe. When the other probes are isolated, the voltage and current measurement networks  402   e - 1  or  402   e - 2  for the non-isolated probe can detect the voltage and current being applied to that probe (which can also be used to obtain the power applied to that probe). The control system  402   d  can cycle through the probes to detect the voltage and current a certain number of times per second, such as five times per second. The total duration for one cycle can be, as an example, from 5 to 10 milliseconds. 
     The measured voltage and current for a given probe can also be used to determine the impedance between that probe and the ground pad. An impedance drop below a certain amount (for example, about 100 Ohms) may indicate a problem with the procedure. The control system  402   d  monitors this impedance for each probe, and if the impedance drops below a certain level, shuts-down the system  402  as a safety precaution. 
     Once the temperature of a probe is within the lower and upper threshold amounts, the control system  402   d  controls the source AC switches  402   b - 1  and  402   b - 2  and the ground AC switches  402   c - 1  and  402   c - 2  in the same fashion as described with respect to system  100 . 
       FIG. 5  is a schematic illustrating another embodiment of an RF generation system  502  in which the amount of power supplied to a probe is controlled through a controller. In system  502 , independent RF sources  502   a - 1  and  502   a - 2  are used to provide RF voltages to probes  504   a  and  504   b , respectively. 
     The independent RF sources  502   a - 1  and  502   a - 2  are unregulated RF sources and the magnitude of the RF voltages supplied by the sources  502   a - 1  and  502   a - 2  can be controlled by one or more control signals from the control system  502   d . Because the RF sources  504   a  and  504   b  are unregulated, active or passive voltage, current, and power limiting networks  502   f - 1  and  502   f - 2  are included. These networks  502   f - 1  and  502   f - 2  limit the amount of voltage and current (and, hence, power) that can be transmitted through a given probe to help insure the safety of the patient. 
     System  502  includes a voltage and current measurement networks  502   e - 1  coupled to the probe  504   a  and a voltage and current measurement networks  402   e - 2  coupled to the probe  504   b . Ground AC switches  502   c - 1  and  502   c - 2  are included in system  502 , but source AC switches are not. To measure the voltage and current for a given probe, the other probe may be isolated by setting the magnitude of the voltage applied to the other probe to zero or switching off the corresponding RF source  502   a - 1  or  502   a - 2 . 
     System  502  operates in a similar fashion as system  402 . However, instead of controlling the amount of power supplied to a given probe by using source AC switches, the amount of power provided to a given probe is controlled by controlling the magnitude of the voltage supplied from the associated RF source  502   a - 1  or  502   a - 2 . Similar to the system  402 , the control system  502   d  implements a controller, such as a PID controller, that controls power supplied to a given probe. However, instead of controlling the PWM of a source AC switch, the controller changes the magnitude of the voltage supplied from the associated RF source based on the lower threshold amount, and the temperature, voltage, and current measurement for that probe. 
     Also, once the temperature of a probe is above the lower threshold amount, the control system  502   d  sets the magnitude of the associated RF source  502   a - 1  or  502   a - 2  to zero to cut off the supply of energy to that probe, rather than opening a source AC switch. The control system  502   d  controls the ground AC switches  502   c - 1  and  502   c - 2  in the same fashion as described with respect to systems  102  and  402 . 
       FIG. 6  is an illustration depicting the use of the electrosurgical device  100  to treat back pain caused by facet joint syndrome. A given vertebra  620  of the spinal column includes a pair of joints  622   a  and  622   b , referred to as facet joints. These joints connect a given level of the spinal column to the levels above and below that level. On a given level, one or both of the facet joints  622   a  and  622   b  can become inflamed due to injury and/or arthritis, resulting in potentially severe back pain. 
     To treat this pain, the probe  104   a  is inserted through the skin and muscle  616  of the back and placed near the medial branch nerve  624   a  that supplies the facet joint  622   a . While not shown, the probe  104   a  may be inserted and placed near the medial branch nerve  624   a  using an introducer cannula. The physician may use fluoroscopy to aid in the placement of the cannula or probe  104   a . The ground pad  114  may be placed on the patient&#39;s body. Typically, with facet joint syndrome, both of the facet joints of a given level are inflamed and causing pain. If this is the case, the second probe  104   b  is also inserted through the skin and muscle  616  and placed near the medial branch nerve  624   b  that supplies the other facet joint  622   b . Using both probes  104   a  and  104   b  simultaneously to lesion both nerves  624   a  and  624   b  can reduce the amount of time taken to perform the procedure, which can be desirable because the lesioning process can be painful for the patient. Also, reduction of procedure time may provide significant cost advantages. 
     After the initial placement of the probes  104   a  and  104   b , the physician places the RF generation system  102  in a diagnostic mode to insure proper placement of the probes  104   a  and  104   b . In the diagnostic mode, a low level of RF energy is separately applied to each probe  104   a  and  104   b  to cause sensory stimulation and motor stimulation. For example, the physician can use the RF generation system  102  to separately apply a pulsed RF voltage to each probe  104   a  and  104   b  with a peak magnitude of 0-1 Vrms, a base frequency of 460 KHz, a pulse frequency of 50 Hz, and a pulse duration of 0.1-3 ms to perform sensory stimulation. After sensory stimulation is complete, the physician can use the RF generation system to separately apply a pulsed RF voltage to each probe  104   a  and  104   b  with a peak magnitude of 0-10V, a base frequency of 460 KHz, a pulse frequency of 2 Hz, and a pulse duration of 0.1-3 ms to perform motor stimulation. 
     If the results of the sensory and motor stimulations indicate to the physician that the probes  104   a  and  104   b  are properly positioned, the physician then places the RF generation system  102  in the destructive mode with the RF generation system  102  operating as described above to control the temperatures at the probes  104   a  and  104   b  to effect lesioning, while reducing the effects of cross-talk between the probes  104   a  and  104   b . If either of the RF generation systems  402  or  502  is used, then the RF generation system  402  or  502  controls the RF power provided to the probes  104   a  and  104   b , in addition to reducing the effects of cross-talk. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. For example, while control systems  102   d ,  402   d , and  502   d  are described as being implemented with a microprocessor or microcontroller, these control systems can alternatively be implemented using analog circuitry or other digital circuitry, such as an FPGA or ASIC. Also, while the control systems  102   d ,  204   d , and  502   d  are described as implementing a PID controller, other control schemes can be used, such as a proportional-integral (PI) controller. 
     Furthermore, the above described implementations control the power supplied to the probes and the ground AC switches based on the temperature at the probes. Other implementations can, alternatively or additionally, control the power and/or ground AC switch based on different parameters. For example, in one implementation, a source AC switch and a ground AC switch for each probe are controlled based on the current through that probe. Generally, as a lesion forms, the impedance in the probe decreases and the current increases. 
     This implementation includes an RF generation system configured similar to the system  400  of  FIG. 4 , except that temperature sensors are not included on the probes  404   a  and  404   b  or are included but not utilized. When the procedure starts in this implementation, the source AC switches are closed and the ground AC switches are opened. Power is applied to each probe, and the current through each probe is measured by opening the source AC switch for the other probe to isolate the probe to be measured, as described above with respect to the implementation of  FIG. 4 . When the current through a probe exceeds a threshold current (for example, a current in the range of 100-150 mA), the source AC switch for that probe is opened to stop the supply of current to that probe. Once the source AC switch is opened, cross-talk current may flow through the probe. If this cross-talk current exceeds the threshold current, the ground AC switch is closed to divert the cross-talk current to ground without passing through the tip of the probe. 
     In an alternative implementation, rather than using a source AC switch, the voltage of the RF source is controlled to keep the current below the threshold current when power is applied to the probe, and the ground AC switch is closed when the cross-talk current exceeds the threshold current. This implementation includes an RF generation system configured similar to the system  500  of  FIG. 5 . When the procedure starts, the ground AC switches are opened and the same voltage is applied to each probe. The current through each probe is measured by switching off the RF source or setting the magnitude of the voltage to zero for the other probe to isolate the probe to be measured. When the current through a probe exceeds a threshold current (for example, a current in the range of 100-150 mA), the magnitude of the voltage applied to the probe is reduced to maintain the current below the threshold current. If the magnitude is reduced to zero, but the current still exceeds the current threshold, then the ground AC switch is closed to divert any cross-talk current to ground without passing through the tip of the probe. 
     Other implementations may use, for example, the voltage or impedance at each probe to control the power and/or ground AC switches. 
     In addition, while two probes have been described, the methodology for reducing the effects of cross-talk can be extended to more than two probes. For example, often facet joint syndrome includes not only the inflammation of the facet joints of a given level of the spine, but also the inflammation of the facet joints above or below that level. In this situation, three, four, five, or six probes can be used as appropriate to treat the inflamed facet joints simultaneously, while ground AC switches are used to direct cross-talk current into ground without passing through the tissue to the ground pad. 
     Also, various features of the described embodiments of the RF generation systems can be used together. For instance, voltage and current limiting networks can be used with a regulated RF generator. Also, source AC switches can be used to control power delivery even if controllable, unregulated RF sources are used. While RF generation system  502  uses multiple unregulated RF sources, a single unregulated RF source can be used. Similarly, while RF generation systems  102  and  402  use a single regulated RF generator, multiple regulated RF generators can be used instead. 
     While the ground AC and source AC switches have been illustrated as being housed with the RF generator, any combination of these switches can be placed at other locations in the system. For instance, the ground AC switch for a probe can be included in a handle associated with the probe, rather than being housed in the RF generation system. 
     Furthermore, while specific procedures have been describe, the electrosurgical devices described above may be used for other procedures. 
     One or more, of the implementations may provide certain advantages. For example, one or more implementations may allow the RF energy to be applied to a probe more continuously than in other system designs; Providing a more continuous application of RF energy may be desirable because doing so may have a better therapeutic effect during certain procedures, such as denervation. 
     Some systems with multiple probes may be designed to multiplex the RF energy to each probe. In this case, RF energy is applied consecutively to each probe for a period of time, until the last probe is reached, at which point the cycle is started again with the first probe. In a system with four probes, for instance, the RF energy may be applied consecutively to each probe for about 1 millisecond, resulting in each probe receiving RF energy every 5 milliseconds. Once the temperature at a probe is at or near the target temperature, the probe is included, for example, only once every two to three cycles, so that RF energy is applied every 10-15 milliseconds to maintain the temperature near the target temperature. 
     Because multiplexed systems continuously cycle through applying RF energy to each probe, some or all of the implementations described above (or other implementations) may provide a more continuous application of RF energy than a multiplexed system. For instance, system  400  provides continuous RF energy until the temperature at the probe nears the target temperature, at which time the corresponding source AC switch is switched on and off to control the power delivered until the lower threshold is reach and the source AC switch is maintained open. Even though the application of RF energy is not continuous until the lower threshold is reached, the RF energy is applied more continuously than in a multiplexed system. As another example, system  500  provides continuous RF energy to each probe until the lower threshold of the target temperature is reached. 
     Other system designs may employ pulsed RF energy, in which the RF energy is periodically applied to each probe for a certain duration. For example, the RF energy may be applied to each probe for 1 millisecond every 1 second. The “on” pulses may be applied to each probe at the same time or at different times. Some or all of the implementations may provide a more continuous application of RF energy than pulsed RF systems. 
     A number of implementations have been described. Nevertheless, it will be understood that various modifications may be made. Accordingly, other implementations are within the scope of the following claims.