Patent Publication Number: US-2017348051-A1

Title: Re-hydration antenna for ablation

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation application of U.S. patent application Ser. No. 14/954,980, filed on Nov. 30, 2015, which is a continuation application of U.S. patent application Ser. No. 12/413,023, filed on Mar. 27, 2009, now U.S. Pat. No. 9,198,723, which claims priority to U.S. Provisional Application No. 61/041,072 filed on Mar. 31, 2008, the entire contents of each of which are incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates generally to devices that may be used in tissue ablation procedures. More particularly, the present disclosure relates to devices and methods for maintaining ablation temperatures surrounding microwave antennas radiofrequency probes during ablation procedures. 
     Background of Related Art 
     In the treatment of diseases such as cancer, certain types of cancer cells have been found to denature at elevated temperatures which are slightly lower than temperatures normally injurious to healthy cells. These types of treatments, known generally as hyperthermia therapy, typically utilize electromagnetic radiation to heat diseased cells to temperatures above 41° Celsius while maintaining adjacent healthy cells at lower temperatures where irreversible cell destruction will not occur. Other procedures utilizing electromagnetic radiation to heat tissue also include ablation and coagulation of the tissue. Such ablation procedures, e.g., such as those performed for menorrhagia, are typically done to ablate and coagulate the targeted tissue to denature or kill the tissue. Many procedures and types of devices utilizing electromagnetic radiation therapy are known in the art. Such therapy is typically used in the treatment of tissue and organs such as the prostate, heart, kidney, lung, brain, and liver. 
     Presently, there are several types of microwave probes in use, e.g., monopole, dipole, and helical, which may be inserted into a patient for the treatment of tumors by heating the tissue for a period of time sufficient to cause cell death and necrosis in the tissue region of interest. Such microwave probes may be advanced into the patient, e.g., laparoscopically or percutaneously, and into or adjacent to the tumor to be treated. The probe is sometimes surrounded by a dielectric sleeve. 
     However, in transmitting the microwave energy into the tissue, the outer surface of the microwave antenna typically may heat up and unnecessarily desiccate, or even necrose, healthy tissue immediately adjacent the antenna outer surface. This creates a water or tissue phase transition (steam) that allows the creation of a significant additional heat transfer mechanism as the steam escapes from the local/active heating area and re-condenses further from the antenna. The condensation back to water deposits significant energy further from the antenna/active treatment site. This local tissue desiccation occurs rapidly resulting in an antenna impedance mismatch, which both limits power delivery to the antenna and effectively eliminates steam production/phase transition as a heat transfer mechanism for tissue ablation. 
     To prevent the charring of adjacent tissue, several different cooling methodologies are conventionally employed. For instance, some microwave antennas utilize balloons which are inflatable around selective portions of the antenna to cool the surrounding tissue. Thus, the complications associated with tissue damaged by the application of microwave radiation to the region are minimized. Typically, the cooling system and the tissue are maintained in contact to ensure adequate cooling of the tissue. 
     Other devices attempt to limit the heating of tissue adjacent the antenna by selectively blocking the propagation of the microwave field generated by the antenna. These cooling systems also protect surrounding healthy tissues by selectively absorbing microwave radiation and minimizing thermal damage to the tissue by absorbing heat energy. 
     SUMMARY 
     The present disclosure provides a system for use with a microwave antenna including a microwave antenna configured to deliver microwave energy from a power source to tissue and a sensor module in operative communication with the power source and configured to detect a reflectance parameter. The system further includes a jacket adapted to at least partially surround the microwave antenna to define a fluid channel between the jacket and the microwave antenna. A plurality of fluid distribution ports are defined through the jacket and are in fluid communication with the fluid channel to permit the flow of fluid through the jacket. The system further includes a fluid pumping system operably coupled to the power source and configured to selectively provide cooling fluid to the fluid channel for distribution through the fluid distribution ports based on the reflectance parameter. 
     In another embodiment, a system for use with a microwave antenna includes a microwave antenna configured to deliver microwave energy from a power source to tissue and a temperature sensor operably coupled to the microwave antenna and configured to detect at least one of a tissue temperature and an antenna temperature. The system further includes a jacket adapted to at least partially surround the microwave antenna to define a fluid channel between the jacket and the microwave antenna. A plurality of fluid distribution ports are defined through the jacket and are in fluid communication with the fluid channel to permit the flow of fluid through the jacket. The system further includes a fluid pumping system operably coupled to the power source and configured to selectively provide cooling fluid to the fluid channel for distribution through the fluid distribution ports based on a comparison between the detected temperature and a predetermined temperature. 
     The present disclosure also provides for a method for impedance matching during an ablation procedure. The method includes the initial steps of applying microwave energy from an antenna to tissue and detecting a reflectance parameter. The method also includes the steps of analyzing the reflectance parameter to determine an impedance mismatch and selectively expelling an amount of fluid from the antenna into the tissue based on the mismatch. The method further includes the step of repeating the step of analyzing the reflectance parameter. 
     In another embodiment of the present disclosure, a method for regulating temperature of tissue undergoing ablation includes the initial steps of applying microwave energy from an antenna to tissue and providing a temperature sensor to detect at least one of a tissue temperature and an antenna temperature. The method also includes the steps of comparing the detected temperature with a predetermined temperature and selectively expelling an amount of fluid from the antenna into the tissue based on the comparison between the detected temperature and the predetermined temperature. The method further includes the step of repeating the step of comparing the detected temperature with a predetermined temperature. 
     In another embodiment of the present disclosure, a method for regulating temperature of tissue undergoing ablation includes the initial steps of applying microwave energy from an antenna to tissue and detecting at least one of a tissue temperature and an antenna temperature. The method also includes the steps of comparing the detected temperature with a predetermined temperature and selectively expelling an amount of fluid from the antenna into the tissue based on the comparison between the detected temperature and the predetermined temperature. The method also includes the step of repeating the step of comparing the detected temperature with a predetermined temperature. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a microwave antenna assembly according to an embodiment of the present disclosure; 
         FIG. 2  is a perspective view of the microwave antenna assembly of  FIG. 1  having a conduit defined therein; 
         FIG. 3  is a cross-sectional view of a microwave antenna according to one embodiment of the present disclosure; 
         FIGS. 4A and 4B  are enlarged views of the areas of detail of the microwave antenna of  FIG. 3 ; 
         FIGS. 4C and 4D  are alternative embodiments of the area of detail of the microwave antenna shown in  FIG. 4B ; 
         FIG. 5  is a schematic block diagram of a generator control system according to one embodiment of the present disclosure; 
         FIG. 6  is a flowchart diagram showing one method for hydrating tissue undergoing treatment according to the present disclosure; and 
         FIG. 7  is a flowchart diagram showing another method for hydrating tissue undergoing treatment according to the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     In the drawings and in the description that follows, the term “proximal”, as is traditional, will refer to the end of the apparatus that is closest to the clinician, while the term “distal” will refer to the end that is furthest from the clinician. 
     Microwave or radiofrequency ablation is capable of causing significant temperature elevations and desiccation of tissue surrounding the applicator. This elevation of temperature creates a water or tissue phase transition by which steam escapes from the active heating area and recondenses further from the applicator. In this way, the tissue phase transition effectively serves as a heat transfer mechanism. As well as adding a new heat transfer mechanism, the movement of water, and, specifically, the loss of water in some volumes of tissue are expected to affect other tissue properties, such as impedance. Changes in tissue thermal properties directly affects the heat conduction within tissue and changes tissue dielectric properties that lead to changes in the location of energy deposition within the targeted, as well as the surrounding tissues. That is, the condensation back to water deposits significant energy further from the active heating area. However, the desiccation of tissue surrounding the applicator effectively eliminates steam production as a heat transfer mechanism and as a result, the temperature of the active heating area significantly elevates to cause an impedance mismatch. 
     The present disclosure provides for a system and method to re-hydrate tissue undergoing treatment through use of various ablation apparatuses (e.g., a microwave antenna, radiofrequency probe, pump, etc.), which compensates for the power imbalance and/or impedance mismatch that are inherent with dynamic tissue changes. In particular, hydration of tissue may be achieved utilizing cooling systems in which cooling fluid is circulated through and expelled from a microwave antenna or radiofrequency probe. The following disclosure is directed towards a microwave antenna application; however, teachings of the present disclosure may be applied to other types of ablation devices, such as radiofrequency probes, or even ultrasonic and laser tissue treatment devices. 
       FIG. 1  shows a diagram of an ablation antenna assembly  10  that may be any type of probe suitable for delivering microwave energy and may be used with a cooling system as described herein. The antenna assembly  10  generally includes a radiating portion  12  that may be coupled by feedline  14  (or shaft) via conduit  16  to connector  18 , which may further connect the assembly  10  to a power generating source  30  (e.g., a generator) and a supply pump  40 . 
     Assembly  10  includes a dipole ablation probe assembly. Other antenna assemblies, e.g., monopole or leaky wave antenna assemblies, may also be utilized. Distal portion  22  of radiating portion  12  may include a tapered end  26  that terminates at a tip  28  to allow for insertion into tissue with minimal resistance. In those cases where the radiating portion  12  is inserted into a pre-existing opening, tip  28  may be rounded or flat. 
     Junction member  20  is located between proximal portion  24  and distal portion  22  such that a compressive force may be applied by distal and proximal portions  22 ,  24  upon junction member  20 . Placing distal and proximal portions  22 ,  24  in a pre-stressed condition prior to insertion into tissue enables assembly  10  to maintain a stiffness that is sufficient to allow for unaided insertion into the tissue while maintaining a minimal antenna diameter, as described in detail below. 
     Feedline  14  electrically connects antenna assembly  10  via conduit  16  to generator  30  and typically includes a coaxial cable (not explicitly shown) made of a conductive metal, which may be semi-rigid or flexible. Feedline  14  may also have a variable length from a proximal end of radiating portion  12  to a distal end of conduit  16  ranging between about 1 to 15 inches. The feedline  14  may be constructed of copper, gold, stainless steel or other conductive metals with similar conductivity values. The metals may also be plated with other materials, e.g., other conductive materials, to improve conductivity or decrease energy loss, or for other purposes known in the art. 
     As shown in  FIG. 2 , conduit  16  includes a flexible coaxial cable  17  and one or more flexible tubes, namely, inflow tubing  19  and outflow tubing  21  for supplying and withdrawing cooling liquid  31  into and out of radiating portion  12 , respectively. Cable  17  includes an inner conductor  23  (e.g., wire) surrounded by an insulating spacer  25 , which is concentrically disposed within an outer conductor  27  (e.g., cylindrical conducting sheath). Cable  17  may also include an outer insulating sheath  29  surrounding the outer conductor  27 . Connector  18  couples the inflow tubing  19  and outflow tubing  21  to the supply pump  40  and the cable  17  to the generator  30 . The supply pump  40  is coupled to a supply tank  41  ( FIG. 1 ) that stores cooling liquid  31  and maintains the liquid at a predetermined temperature (e.g., ambient room temperature). In one embodiment, the supply tank  41  may include a cooling unit that cools the returning cooling liquid  31  from the outflow tubing  19 . 
     The cooling fluid  31  may be pumped using positive pressure through inflow tubing  19 . Alternatively, negative pressure may also be used to draw the cooling fluid  31  out of the region through outflow tubing  21 . Negative pressure through outflow tubing  21  may be utilized either alone or in conjunction with positive pressure through inflow tubing  19 . Alternatively, positive pressure through inflow tubing  19  may be utilized either alone or in conjunction with negative pressure through outflow tubing  21 . In pumping the cooling fluid  31 , the cooling fluid  31  may be passed at a constant flow rate. In another variation, the flow may be intermittent such that a volume of cooling fluid  31  may be pumped into the radiating portion  12  and allowed to warm up by absorbing heat from the antenna. Once the temperature of the cooling fluid  31  reaches a predetermined level below temperatures where thermal damage to tissue occurs, the warmed fluid may be removed and displaced by additional cooling fluids. 
     The cooling fluid  31  used may vary depending upon desired cooling rates and the desired tissue impedance matching properties. Biocompatible fluids may be included that have sufficient specific heat values for absorbing heat generated by radio frequency ablation probes, e.g., liquids including, but not limited to, water, saline, liquid chlorodifluoromethane, etc. In another variation, gases (such as nitrous oxide, nitrogen, carbon dioxide, etc.) may also be utilized as the cooling fluid  31 . For example, an aperture defined within the radiating portion  12  may be configured to take advantage of the cooling effects from the Joule-Thompson effect, in which case a gas, e.g., nitrous oxide, may be passed through the aperture to expand and cool the radiating portion  12 . In yet another variation, a combination of liquids and/or gases, as mentioned above, may be utilized as the cooling medium. 
       FIG. 3  show a cross-sectional side view and an end view, respectively, of one variation of the antenna assembly  10  (e.g., cooling assembly  100 ) that may be utilized with any number of conventional ablation probes (or the ablation probes described herein), particularly the straight probe configuration as shown in  FIG. 1 . Although this variation illustrates the cooling of a straight probe antenna, a curved or looped ablation probe may also utilize much of the same or similar principles, as further described below. 
     Cooling assembly  100  includes a cooling handle assembly  102  and an elongated outer jacket  108  extending from handle assembly  102 . As will be described in further detail below, a plurality of fluid distribution ports  114  ( FIG. 4B ) are defined through the thickness of outer jacket  108  to facilitate the introduction of cooling fluid  31  from the cooling assembly  100  into surrounding tissue. Outer jacket  108  extends and terminates at tip  110 , which may be tapered to a sharpened point to facilitate insertion into and manipulation within tissue, if necessary. Antenna  104  is positioned within handle assembly  102  such that the radiating portion  106  of antenna  104  extends distally into outer jacket  108  towards tip  110 . Inflow tubing  19  extends into a proximal end of handle body  112  and distally into a portion of outer jacket  108 . Outflow tubing  21  extends from within handle body  112  such that the distal ends of inflow tubing  19  and outflow tubing  21  are in fluid communication with one another, as described in further detail below. 
       FIG. 4A  shows handle assembly detail  118  from  FIG. 3 . As shown, handle body  112  includes proximal handle hub  122 , which encloses a proximal end of antenna  104 , and distal handle hub  124 , which may extend distally to engage outer jacket  108 . Proximal handle hub  122  and distal handle hub  124  are configured to physically interfit with one another at hub interface  130  to form a fluid tight seal. Accordingly, proximal handle hub  122  may be configured to be received and secured within a correspondingly configured distal handle hub  124  (seen in  FIG. 3  as a male-female connection). A slide button  116  is disposed on handle body  112  and operably coupled to a tube  140  disposed coaxially through at least a portion of outer jacket  108  (see  FIGS. 4B and 4C ). Movement of the slide button  116  relative to handle body  112 , as depicted in FIG.  4 A by bidirectional arrow A, translates corresponding movement of the tube  140  relative to an inner surface of outer jacket  108 , as depicted in  FIG. 4B  by bidirectional arrow B, to facilitate the placement of cooling fluid and/or steam into surrounding tissue, as will be discussed in further detail below. 
     The distal ends of inflow tubing  19  and outflow tubing  21  may be positioned within the handle body  112  such that fluid is pumped into handle body  112  via the supply pump  40  through the inflow tubing  19 . Cooling fluid  31  entering the handle body  112  comes into direct contact with at least a portion of the shaft of the antenna  104  to allow for convective cooling of the antenna shaft to occur. The cooling fluid  31  may be allowed to exit the handle body  112  via the outflow tubing  21 . An additional inlet tube  126  is positioned within the antenna cooling assembly  100  to extend between the handle body  112  and the radiating portion  106  ( FIG. 4B ) of the antenna  104  and a corresponding outlet tube  128  may also extend between the handle body  112  and the radiating portion  106 . The proximal end of the inlet tube  126  is in fluid communication with the inflow tubing  19  to allow the cooling fluid  31  to flow distally within the outer jacket  108  towards antenna radiating portion  106  ( FIG. 4B ). Alternatively, the inlet tube  126  and the outlet tube  128  may be omitted from the cooling assembly  100  and the outer jacket  108  may remain in direct fluid communication with the inflow tubing  19  and the outflow tubing  21  such that cooling fluid  31  contacts the antenna  104  directly along a portion of the length, or a majority of the length, or the entire length of the antenna  104 . Thus, the cooling assembly  100  is effective in cooling the antenna  104  directly. 
       FIG. 4B  shows outer jacket detail embodiment  120 , from  FIG. 3 . The illustrated embodiment shows the distal end  132  of inlet tube  126 , which extends distally through outer jacket  108 . The opening at distal end  132  is positioned within outer jacket  108  near or at the distal end of outer jacket  108  such that distal end  132  opens to fluid channel  134 . The cooling fluid  31  enters fluid channel  134  and fills the volume surrounding the radiating portion  106  and surrounding at least a portion of the antenna  104 . As cooling fluid  31  enters fluid channel  134 , the cooling fluid  31  is withdrawn through a distal opening in outlet tube  128 , which is located proximally of distal end  132  to allow for increased convective cooling between the cooling fluid  31  and the antenna  104 . 
     The cooling fluid  31  is pumped using positive pressure through inlet tube  126 . Alternatively, negative pressure may also be used to draw the fluid out of the region through outlet tube  128 . Negative pressure through outlet tube  128  may be utilized either alone or in conjunction with positive pressure through inlet tube  126 . Alternatively, positive pressure through inlet tube  126  may be utilized either alone or in conjunction with negative pressure through outlet tube  128 . 
     The cooling fluid  31  used may vary depending upon desired cooling rates and the desired tissue impedance matching properties. Biocompatible fluids having sufficient specific heat values for absorbing heat generated by microwave ablation antennas may be utilized, e.g., liquids including, but not limited to, water, saline, Fluorinert®, liquid chlorodifluoromethane, etc. (As is well-known, the material sold under the trademark Fluorinert is a perfluorocarbon fluid distributed commercially by Minnesota Mining and Manufacturing Company (3M), St. Paul, Minn., USA.) 
     The illustrated embodiment in  FIG. 4B  shows tube  140  and a plurality of fluid distribution ports  114  defined through the thickness of the outer jacket  108 . The fluid distribution ports  114  enable cooling fluid  31  to be expelled from the fluid channel  134  into and/or proximate the target tissue. Tube  140  is disposed coaxially through at least a portion of outer jacket  108  such that fluid communication between one or more fluid distribution ports  114  and fluid channel  134  is selectively interrupted. More specifically, as tube  140  is moved from a distal most position (see  FIGS. 4B and 4C ) proximally relative to outer jacket  108  by corresponding proximal movement of slide button  116 , an increasing number of fluid distribution ports  114  are exposed to fluid channel  134  from a distal end of fluid channel  134  toward a proximal end of fluid channel  134 , to permit cooling fluid  31  to be expelled via the exposed fluid distribution ports  114  into and/or proximate the target tissue. Similarly, distal movement of slide button  116  relative to handle body  112  causes distal movement of tube  140  to interrupt fluid communication between fluid distribution ports  114  and fluid channel  134  from a proximal end thereof toward a distal end thereof. In this manner, a user may manipulate the slide button  116  relative to the handle body  112  to control the placement of cooling fluid and/or steam as desired or depending on the size of the ablation. In some embodiments, the fluid distribution ports  114  may be microporous, macroporous, or any combination thereof. The higher the porosity, the more freely the cooling fluid  31  will flow through the outer jacket  108 . The fluid distribution ports  114  may be defined through the outer jacket  108  along the entire length thereof. Alternatively, the fluid distribution ports  114  may only be defined through the portion of the outer jacket  108  that will be adjacent the ablation region (e.g., a distal end of the radiating portion  106 ). The cooling fluid  31  flows outwardly through the fluid distribution ports  114  as shown by the arrows extending outwardly therefrom. Alternatively, one or more of the fluid distribution ports  114  may be defined at an angle with respect to the surface of the outer jacket  108  (not explicitly illustrated) such that the cooling fluid  31  may flow outwardly in various radial directions (e.g., proximal, distal, etc.). 
     In some embodiments, cooling assembly  100  may include passive-type plugs or seals (not explicitly shown) to passively seal each fluid distribution port  114 . The seals may be expanded outward by positive fluid pressure communicated through the fluid distribution ports  114  to allow cooling fluid  31  to be expelled from the cooling assembly  100 . In this way, cooling fluid  31  may remain circulated within the fluid channel  134  until the supply pump  40  creates additional positive fluid pressure to expand the seals outward, thereby permitting cooling fluid  31  to exit the fluid channel  134  via the fluid distribution ports  114 . 
     In some embodiments, the cooling assembly  100  may be configured to selectively inject cooling fluid  31  into the surrounding tissue through any one or more specific fluid distribution ports  114 . That is, cooling fluid  31  may be injected into the surrounding tissue from any port or group of ports positioned about the circumference of the outer jacket  108 . In this configuration, the cooling assembly  100  may include one or more additional inflow tubes (not explicitly shown) in direct fluid communication with a specific port or specific group of ports. As such, the controller  34  may cause the supply pump  40  to pump cooling fluid  31  through specific inflow tubes and/or specific groups of inflow tubes into and/or proximate the surrounding tissue via specific ports or specific groups of ports. In this way, cooling fluid  31  may be targeted proximally, distally, or in a specific radial direction. 
       FIG. 4C  shows an alternative embodiment of inlet tube  126  shown as a helical shape extending distally through outer jacket  108 . In this configuration, inlet tube  126  is in contact with the radiating portion  106  to facilitate faster heating of the cooling fluid within inlet tube  126  such that steam may be expelled from a plurality of ports  127  disposed through inlet tube  126 . 
     In some embodiments, as shown in  FIG. 4D , one or more infusion inlet tubes  150  may be disposed coaxially through outer jacket  108  to provide infusion fluid (not shown) directly from the supply pump  40 , as opposed to cooling fluid  31  supplied via inlet tube  126 , such that infusion fluid and cooling fluid circulate separately within the antenna assembly  10 . In this scenario, additional inflow tubing (not shown) is disposed in fluid communication between the supply pump  40  and infusion inflow tubes  150  and supplies infusion fluid to the infusion inflow tubes  150  using positive pressure from the supply pump  40 . Infusion inlet tubes  150  are in fluid communication with one or more fluid distribution ports  114  such that positive pressure from the supply pump  40  causes the infusion fluid in the infusion inflow tubes  150  to be expelled from one or more fluid distribution ports  114  and into and/or proximate the target tissue. The embodiment in  FIG. 4D  may be particularly suitable for radiofrequency ablation. 
       FIG. 5  shows a schematic block diagram of the generator  30  operably coupled to the supply pump  40 . The supply pump  40  is, in turn, operably coupled to the supply tank  41 . The generator  30  includes a controller  34 , a power supply  37 , a microwave output stage  38 , and a sensor module  32 . The power supply  37  provides DC power to the microwave output stage  38  which then converts the DC power into microwave energy and delivers the microwave energy to the radiating portion  106 . The controller  34  includes a microprocessor  35  having a memory  36  which may be volatile type memory (e.g., RAM) and/or non-volatile type memory (e.g., flash media, disk media, etc.). The microprocessor  35  includes an output port connected to the supply pump  40 , which allows the microprocessor  35  to control the output of cooling fluid  31  from the supply pump  40  to the cooling assembly  100  according to either open and/or closed control loop schemes. In the illustrated embodiment, the microprocessor  35  also includes an output port connected to the power supply  37  and/or microwave output stage  38  that allows the microprocessor  35  to control the output of the generator  30  according to either open and/or closed control loop schemes. Further, the cooling assembly  100  may include suitable input controls (e.g., buttons, activators, switches, etc.) for manually controlling the output of the supply pump  40 . Specifically, the input controls may be provided with leads (or wireless) for transmitting activation signals to the controller  34 . The controller  34  then signals the supply pump  40  to control the output of cooling fluid  31  from the supply tank  41  to the cooling assembly  100 . In this way, clinicians may manually control the supply pump  40  to cause cooling fluid  31  to be expelled from the cooling assembly  100  into and/or proximate the surrounding tissue. 
     A closed loop control scheme generally includes a feedback control loop wherein the sensor module  32  provides feedback to the controller  34  (i.e., information obtained from one or more sensing mechanisms for sensing various tissue and/or antenna parameters, such as tissue impedance, antenna impedance, tissue temperature, antenna temperature, output current and/or voltage, etc.). The controller  34  then signals the supply pump  40  to control the output thereof (e.g., the volume of cooling fluid  31  pumped from the supply tank  41  to the cooling assembly  100 ). The controller  34  also receives input signals from the input controls of the generator  30  and/or antenna assembly  10 . The controller  34  utilizes the input signals to adjust the cooling fluid  31  output of the supply pump  40  and/or the power output of the generator  30 . 
     The microprocessor  35  is capable of executing software instructions for processing data received by the sensor module  32 , and for outputting control signals to the generator  30  and/or supply pump  40 , accordingly. The software instructions, which are executable by the controller  34 , are stored in the memory  36  of the controller  34 . 
     The controller  34  may include analog and/or logic circuitry for processing the sensed values and determining the control signals that are sent to the generator  30  and/or supply pump  40 , rather than, or in combination with, the microprocessor  35 . The sensor module  32  may include a plurality of sensors (not explicitly shown) strategically located for sensing various properties or conditions, e.g., tissue impedance, antenna impedance, voltage at the tissue site, current at the tissue site, tissue temperature, antenna temperature, etc. The sensors are provided with leads (or wireless) for transmitting information to the controller  34 . The sensor module  32  may include control circuitry that receives information from multiple sensors, and provides the information and the source of the information (e.g., the particular sensor providing the information) to the controller  34 . 
     When coupling electromagnetic radiation such as microwaves from a source to an applicator, in order to maximize the amount of energy transferred from the source (microwave generator) to the load (surgical implement), the line and load impedances should match. If the line and load impedances do not match (e.g., an impedance mismatch) a reflected wave may be created that can generate a standing wave, which contributes to a power loss associated with the impedance mismatch. As used herein, “load impedance” is understood to mean the impedance of the radiating portion  12  and “line impedance” is understood to mean the impedance of the feedline  14 . 
     In some embodiments, the controller  34  is configured to control the cooling fluid  31  output from the supply pump  40  to the antenna assembly  10  based on a reflectance parameter, such as a mismatch detected between the load impedance and the line impedance. Such an impedance mismatch may cause a portion of the power, so called “reflected power,” from the generator  30  to not reach the tissue site and cause the power delivered, the so called “forward power,” to vary in an irregular or inconsistent manner. It is possible to determine the impedance mismatch by measuring and analyzing the reflected and forward power. In particular, the generator  30  measures energy delivery properties, namely the forward power, and dynamically adjusts the cooling fluid  31  output of the supply pump  40  to compensate for a detected mismatch between the line impedance and the load impedance. That is, upon detection of an impedance mismatch, additional cooling fluid  31  is pumped through inflow tubing  19  and into the fluid channel  134  using positive pressure from the supply pump  40 . This positive pressure causes additional fluid pressure in the fluid channel  134 , which in turn, causes cooling fluid  31  to flow through the fluid distribution ports  114  (e.g., by expanding the seals outward) into and/or proximate the surrounding tissue. In this manner, the cooling fluid  31  effectively re-hydrates surrounding tissue to generate additional steam. This generation of additional steam allows for the transfer of heat away from the target tissue site for the duration of the procedure. The resulting drop in tissue temperature (or more specifically, a change in a dielectric constant el of the tissue surrounding the antenna) effectively lowers the load impedance to match the line impedance, thereby optimizing energy delivery to the target tissue site. Other reflectance parameters include reflectance coefficient, standing wave ratio (SWR), and reflectance loss. 
     In operation, the sensor module  32  is coupled to the microwave output stage  37  and is configured to measure a reflectance parameter. The sensor module  32  may include one or more directional couplers or other voltage and current sensors that may be used to determine voltage and current measurements as well as the phase of the voltage and current waveforms. The voltage and current measurements are then used by the sensor module  32  to determine the reflectance parameter. The sensor module  32  converts the measured parameter into corresponding low level measurement signals (e.g., less than 5 V) which are transmitted to the controller  34 . 
     The controller  34  accepts one or more measurements signals indicative of power delivery, namely, the signals indicative of the reflectance parameter. The controller  34  analyzes the measurement signals and determines an impedance mismatch based on the reflectance parameter. The controller  34  thereafter determines whether any adjustments to the output of the supply pump  40  have to be made to adjust (e.g., re-hydrate) the surrounding tissue to compensate for the mismatch in impedance based on the reflectance parameter. Additionally, the controller  34  may also signal the microwave output stage  38  and/or the power supply  37  to adjust output power based on the reflectance parameter. 
       FIG. 6 , in conjunction with  FIGS. 3, 4A, 4B, and 5 , illustrates a method  200  for selectively re-hydrating tissue undergoing treatment according to one embodiment. In step  210 , energy from the generator  30  is applied to tissue via the antenna  104  to heat a target treatment area. In step  235 , one or more reflectance parameters are detected by the sensor module  32  (e.g., using sensors) and communicated to the controller  34  for storage in the memory  36 . In the illustrated embodiment, the reflectance parameters detected in step  235  include a load impedance (detected in step  220 ) and a line impedance (detected in step  230 ). In step  240 , the microprocessor  35  compares the load impedance to the line impedance. If the load impedance and the line impedance are not at least substantially equivalent in step  250 , the microprocessor  35  outputs a control signal to the supply pump  40  in step  260  to cause cooling fluid  31  to be expelled from the cooling assembly  100  into and/or proximate the surrounding tissue. If the load impedance and line impedance are substantially equivalent in step  250 , step  240  is repeated. The method  200  may loop continuously throughout the duration of the procedure to re-hydrate the target tissue and generate additional steam as a heat transfer mechanism. The resulting drop in tissue temperature (or change in dielectric constant el of the tissue surrounding the antenna) acts to improve energy delivery to the target tissue by facilitating an impedance match between the line and the load. 
       FIG. 7 , in conjunction with  FIGS. 3, 4A, 4B, and 5 , illustrates a method  300  for selectively re-hydrating tissue undergoing treatment according to another embodiment. In step  310 , energy from the generator  30  is applied to tissue via the antenna  104  to heat a target treatment area. In step  320 , a tissue temperature and/or an antenna temperature is detected by the sensor module  32  (e.g., using an optical temperature sensor) and communicated to the controller  34  for storage in the memory  36 . In step  330 , the microprocessor  35  compares the detected temperature to a predetermined temperature (e.g., about 104° C.). If the detected temperature is greater than or equal to the predetermined temperature in step  340 , the microprocessor  35  outputs a control signal to the supply pump  40  in step  350  to cause cooling fluid to be expelled from the cooling assembly  100  into and/or proximate the surrounding tissue. If the detected temperature is less than the predetermined temperature in step  340 , step  330  is repeated. The method  300  may loop continuously throughout the duration of the procedure to re-hydrate the target tissue and generate additional steam as a heat transfer mechanism. The resulting drop in tissue temperature acts to improve energy delivery by maintaining the target tissue site at a temperature below a temperature at which significant tissue dehydration may occur. 
     In some embodiments, the disclosed methods may be extended to other tissue effects and energy-based modalities including, but not limited to, ultrasonic and laser tissue treatments. The methods  200  and  300  are based on impedance measurement and monitoring and temperature measurement and monitoring, respectively, but other tissue and energy properties may be used to determine state of the tissue, such as current, voltage, power, energy, phase of voltage and current. In some embodiments, the method may be carried out using a feedback system incorporated into an electrosurgical system or may be a stand-alone modular embodiment (e.g., removable modular circuit configured to be electrically coupled to various components, such as a generator, of the electrosurgical system). 
     While several embodiments of the disclosure have been shown in the drawings and/or discussed herein, it is not intended that the disclosure be limited thereto, as it is intended that the disclosure be as broad in scope as the art will allow and that the specification be read likewise. Therefore, the above description should not be construed as limiting, but merely as exemplifications of particular embodiments. Those skilled in the art will envision other modifications within the scope and spirit of the claims appended hereto.