Patent Publication Number: US-10327845-B2

Title: System and method for monitoring ablation size

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application is a continuation application of U.S. patent application Ser. No. 14/306,865, filed on Jun. 17, 2014, now U.S. Pat. No. 9,820,813, which is a divisional application of U.S. patent application Ser. No. 12/692,856, filed on Jan. 25, 2010, now U.S. Pat. No. 8,764,744, the entire contents of all of the foregoing applications are incorporated by reference herein. 
    
    
     BACKGROUND 
     Technical Field 
     The present disclosure relates to systems and methods that may be used in tissue ablation procedures. More particularly, the present disclosure relates to systems and methods for monitoring ablation size during tissue ablation procedures in real-time. 
     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° C. while maintaining adjacent healthy cells at lower temperatures where irreversible cell destruction will not occur. Procedures utilizing electromagnetic radiation to heat tissue may include ablation of the tissue. 
     Microwave ablation procedures, e.g., such as those performed for menorrhagia, are typically done to ablate 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 microwave therapy is typically used in the treatment of tissue and organs such as the prostate, heart, and liver. 
     One non-invasive procedure generally involves the treatment of tissue (e.g., a tumor) underlying the skin via the use of microwave energy. The microwave energy is able to non-invasively penetrate the skin to reach the underlying tissue. However, this non-invasive procedure may result in the unwanted heating of healthy tissue. Thus, the non-invasive use of microwave energy requires a great deal of control. 
     Currently, there are several types of systems and methods for monitoring ablation zone size. In certain instances, one or more types of sensors (or other suitable devices) are operably associated with the microwave ablation device. For example, in a microwave ablation device that includes a monopole antenna configuration, an elongated microwave conductor may be in operative communication with a sensor exposed at an end of the microwave conductor. This type of sensor is sometimes surrounded by a dielectric sleeve. 
     Typically, the foregoing types of sensors are configured to function (e.g., provide feedback to a controller for controlling the power output of a power source) when the microwave ablation device is inactive, i.e., not radiating. That is, the foregoing sensors do not function in real-time. Typically, the power source is powered off or pulsed off when the sensors are providing feedback (e.g., tissue temperature) to the controller and/or other device(s) configured to control the power source. 
     SUMMARY 
     The present disclosure provides a system for monitoring ablation size in real-time. The system includes a power source. A microwave antenna is configured to deliver microwave energy from the power source to tissue to form an ablation zone. A plurality of spaced-apart electrodes is operably disposed along a length of the microwave antenna. The electrodes are disposed in electrical communication with one another. Each of the electrodes has a threshold impedance associated therewith corresponding to the radius of the ablation zone. 
     The present disclosure provides a microwave antenna adapted to connect to a power source configured for performing an ablation procedure. The microwave antenna includes a radiating section configured to deliver microwave energy from the power source to tissue to form an ablation zone. The microwave antenna includes a plurality of spaced-apart electrodes operably disposed along a length of the microwave antenna. The electrodes are disposed in electrical communication with one another. Each of the electrodes has a threshold impedance associated therewith corresponding to the radius of the ablation zone. 
     The present disclosure also provides a method for monitoring tissue undergoing ablation. The method includes the initial step of transmitting microwave energy from a power source to a microwave antenna to form a tissue ablation zone. A step of the method includes monitoring one or more electrodes impedance along the microwave antenna as the tissue ablation zone forms. Triggering a detection signal when a predetermined electrode impedance is reached at the at least one electrode along the microwave antenna is another step of the method. The method includes adjusting the amount of microwave energy from the power source to the microwave antenna. 
    
    
     
       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 perspective view of a system for monitoring ablation size according to an embodiment of the present disclosure; 
         FIG. 2  is partial, side view illustrating internal components of a distal tip of a microwave antenna depicted in  FIG. 1 ; 
         FIG. 3  is a functional block diagram showing a power source for use with the system depicted in  FIG. 1 ; 
         FIG. 4A  is a schematic, plan view of a tip of the microwave antenna depicted in  FIG. 2  illustrating radial ablation zones having a generally spherical configuration; 
         FIG. 4B  is a schematic, plan view of a tip of the microwave antenna depicted in  FIG. 2  illustrating radial ablation zones having a generally ellipsoidal configuration; 
         FIG. 5A  is a schematic, plan view of a portion of the microwave antenna depicted in  FIG. 1  showing a sequenced insertion of the microwave antenna into tissue; 
         FIG. 5B  is a graphical representation of corresponding impedances associated with respective electrodes of the microwave antenna depicted in  FIG. 5A ; 
         FIG. 6  is a flow chart illustrating a method for monitoring temperature of tissue undergoing ablation in accordance with the present disclosure; 
         FIG. 7  is partial, side view illustrating internal components of a distal tip of a microwave antenna according to an alternate embodiment of the present disclosure; and 
         FIG. 8  is a functional block diagram showing a power source for use with the microwave antenna depicted in  FIG. 7 . 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments of the presently disclosed system and method are described in detail with reference to the drawing figures wherein like reference numerals identify similar or identical elements. As used herein and as is traditional, the term “distal” refers to the portion which is furthest from the user and the term “proximal” refers to the portion that is closest to the user. In addition, terms such as “above”, “below”, “forward”, “rearward”, etc. refer to the orientation of the figures or the direction of components and are simply used for convenience of description. 
     Referring now to  FIGS. 1 and 2 , and initially with reference to  FIG. 1 , a system for monitoring ablation size in accordance with an embodiment of the present disclosure is designated  10 . A microwave antenna  12  operably couples to generator  100  and includes a controller  200  that connects to the generator  100  via a flexible coaxial cable  14 . In this instance, generator  100  is configured to provide microwave energy at an operational frequency from about 500 MHz to about 10 GHz. Microwave antenna  12  includes a radiating section or portion  16  ( FIGS. 1 and 2 ) that is connected by a feedline or shaft  18  to coaxial cable  14  and extends from the proximal end of the microwave antenna  12 . Cable  14  includes an inner conductor  13  that is operably disposed within the shaft  18  and in electrical communication with a radiating section  16  ( FIGS. 1 and 2 ). Microwave antenna  12  couples to the cable  14  through a connection hub  22 . The connection hub  22  includes an outlet fluid port  24  and an inlet fluid port  26  connected in fluid communication with a sheath or cannula  28  ( FIG. 2 ). Cannula  28  is configured to circulate coolant fluid  30  from ports  24  and  26  around the antenna assembly  12  via respective fluid lumens  32  and  34  ( FIG. 2 ). Ports  24  and  26 , in turn, couple to a supply pump  40 . For a more detailed description of the microwave antenna  12  and operative components associated therewith, reference is made to commonly-owned U.S. Pat. No. 8,118,808, filed on Mar. 10, 2009, the entire contents of which are incorporated by reference herein. 
     With continued reference to  FIGS. 1 and 2 , two or more spaced-apart electrodes  52  and  54  are operably disposed along a length of the shaft  18 . More particularly, the electrodes  52  and  54  are disposed in proximity to a distal end  19  of the shaft  18 . In the embodiment illustrated in  FIG. 2 , electrodes  52  include a series of proximal spaced-apart electrodes  52   a - 52   h  and a distal electrode  54 . As defined herein, a series of electrodes is meant to mean two or more electrodes. For purposes herein, the series of proximal spaced-apart electrodes  52   a - 52   h  referred to proximal electrodes  52 . The configuration of the electrodes  52  and  54  enables physical space sampling of an ablation site. More particularly, in one particular embodiment, during the delivery of microwave energy to the microwave antenna  12 , impedance between one or more of the proximal electrodes  52 , e.g., proximal electrode  52   a , and the distal electrode  54  is measured and compared with known impedance values associated with the microwave antenna  12  and/or proximal electrodes  52 , e.g., proximal electrode  52   a . The configuration of each proximal electrodes  52   a - 52   h  and distal electrode  54  provides a separate closed loop path for current to flow, i.e., an electrical circuit, when the microwave antenna  12  is inserted into tissue at a target tissue site. Impedance is measured between each proximal electrode  52   a - 52   h  and the distal electrode  54 , as described in greater detail below. 
     Proximal electrodes  52   a - 52   h  may be formed from any suitable conductive or partially conductive material. For example, proximal electrodes  52   a - 52   h  may be formed from copper, silver, gold, etc. Proximal electrodes  52   a - 52   h  are operably positioned along an outer peripheral surface  38  of the shaft  18  in a manner suitable for the intended purposes described herein. In embodiments, the proximal electrodes  52   a - 52   h  may extend circumferentially along the outer peripheral surface  38  or partially along a length of the shaft  18 . In the illustrative embodiment, proximal electrodes  52   a - 52   h  extend partially along the outer peripheral surface  38  in a linear manner forming a generally linear array along the outer peripheral surface  38  of the shaft  18 . Proximal electrodes  52   a - 52   h  may be secured to the outer peripheral surface  38  and/or the shaft  18  via any suitable method(s). In one particular embodiment, the proximal electrodes  52   a - 52   h  are secured to the outer peripheral surface  38  via an epoxy adhesive (or other suitable adhesive). Proximal electrodes  52   a - 52   h  are in operative communication with one or more modules, e.g., ablation zone control module  232  (AZCM), associated with the generator  100  and/or controller  200 . To this end, a portion of the proximal electrodes  52   a - 52   h  connects to one or more electrical leads (not explicitly shown) that provide an electrical interface for the proximal electrodes  52   a - 52   h  and the AZCM  232 . Moreover, the electrical leads provide an electrical interface that supplies current, i.e., from a current source (or other suitable device configured to generate current, voltage source, power source, etc.) to the proximal electrodes  52   a - 52   h.    
     In certain embodiments, one or more sensors, e.g., sensors  53   a - 53   h , may be in operative communication with a respective one or corresponding proximal electrode  52   a - 52   h  (as best seen in  FIG. 2 ). In this instance, the sensors  53   a - 53   h  may be configured to provide real-time information pertaining to the proximal electrodes  52   a - 52   h . More particularly, the sensors  53   a - 53   h  may be configured to provide real-time information pertaining to one or more electrical parameters (e.g., impedance, power, voltage, current, etc.) and/or other parameters associated with the proximal electrodes  52   a - 52   h . More particularly, the sensors  53   a - 53   h  may be in the form of one or more types of thermal sensors such as, for example, a thermocouple, a thermistor, an optical fiber, etc. In one particular embodiment, the sensors  53   a - 53   h  are thermocouples  53   a - 53   h.    
     Distal electrode  54  may be formed from any suitable conductive or partially conductive material, e.g., copper, silver, gold, etc. Distal electrode  54  may have any suitable configuration. For illustrative purposes, distal electrode  54  is shown operably disposed at a distal tip  21  of the shaft  18 . In the illustrated embodiment, distal electrode  54  defines a conductive tissue piercing tip. In this instance, the distal electrode  54  facilitates insertion of the microwave antenna  12  into tissue at a target tissue site. Alternatively, distal electrode  54  may have a relatively blunt configuration. An electrical lead (not explicitly shown) provides an electrical interface for returning current from the distal electrode  54  back to the current source. In certain embodiments, distal electrode  54  may be in operative communication with one or more modules, e.g., AZCM  232 , associated with the generator  100  and/or controller  200 . In this instance, the electrical lead may provide an electrical interface for the distal electrode  54  and the AZCM  232 . 
     In certain embodiments, one or more sensors, e.g., sensor  55 , may be in operative communication with the distal electrodes  54  (see  FIG. 2 , for example) and may provide information relevant to the proper operation of distal electrode  54  to the AZCM  232 . Sensor  55  may be any suitable type of sensor such as, for example, one or more types of thermal sensors previously described above, e.g., a thermocouple. 
     A dielectric sheath  60  having a suitable thickness and made from a suitable material is operably positioned along a length of the microwave antenna  12  and substantially encases the proximal electrodes  52   a - 52   h  and distal electrode  54  in a manner that allows current to flow from the proximal electrodes  52   a - 52   h  to the distal electrode  54 . Dielectric sheath  60  may be made from any suitable material and may be affixed to the microwave antenna  12  by any suitable affixing methods. In the illustrated embodiment, the dielectric sheath  60  is a vapor deposited dielectric material, such as, for example, parylene, that is applied to the microwave antenna  12 . Substantially encasing the microwave antenna  12  with dielectric sheath  60  results in capacitive impedance that can allow RF current flow to/from the electrodes  52   a - 52   h  and tissue. More particularly, during transmission of microwave energy from the generator  100  to the microwave antenna  12  and when the proximal electrodes  52   a - 52   h  and distal electrode  54  are positioned within tissue adjacent a target tissue site, current flows from proximal electrodes  52   a - 52   h  to the electrode  54 . Dielectric sheath  60  is configured to focus current densities “I” at the proximal electrodes  52   a - 52   h  and/or the distal electrode  54 , which, in turn, provides comprehensive and/or more accurate measurements of impedance at the proximal electrode  52   a - 52   h , as best seen in  FIG. 2 . In certain embodiments, the dielectric sheath  60  may fully encase the proximal electrodes  52   a - 52   h  and distal electrode  54 . In this instance, the dielectric sheath  60  includes a thickness that allows current to pass from the proximal electrodes  52   a - 52   h  through the dielectric sheath  60  and to the distal electrode  54 . In one particular embodiment, the thickness of the dielectric material of the dielectric sheath  60  ranges from about 0.0001 inches to about 0.001 inches. 
     As mentioned above, proximal electrodes  52   a - 52   h  (and in some instances distal electrode  54 ) is in operative communication with the generator  100  including AZCM  232  and/or controller  200 . More particularly, the proximal electrodes  52   a - 52   h  and distal electrode  54  couple to the generator  100  and/or controller  200  via one or more suitable conductive mediums (e.g., a wire or cable  56 ) that extends from proximal electrodes  52   a - 52   h  and distal electrode  54  to the proximal end of the microwave antenna  12  and connects to the generator  100  (see  FIG. 2 , for example). In the illustrated embodiment, wire  56  is operably disposed within cable  14 . Wire  56  electrically connects to the proximal electrodes  52   a - 52   h  and distal electrode  54  via the one or more leads previously described. The configuration of wire  56 , proximal electrodes  52   a - 52   h  and distal electrode  54  forms a closed loop current path when the electrodes  52  and distal electrode  54  are positioned within tissue adjacent a target tissue site. In the illustrated embodiment, the wire  56  extends along the outer peripheral surface  38  of the shaft  18  and is encased by the dielectric sheath  60 . Alternatively, the wire  56  may extend within and along a length of the shaft  18 . 
     With reference to  FIG. 3 , a schematic block diagram of the generator  100  is illustrated. The generator  100  includes a controller  200  including one or more modules (e.g., an AZCM  232 ), a power supply  137 , a microwave output stage  138 . In this instance, generator  100  is described with respect to the delivery of microwave energy. The power supply  137  provides DC power to the microwave output stage  138  which then converts the DC power into microwave energy and delivers the microwave energy to the radiating section  16  of the microwave antenna  12  (see  FIG. 2 ). In the illustrated embodiment, a portion of the DC power is directed to the AZCM  232 , described in greater detail below. The controller  200  may include analog and/or logic circuitry for processing sensed analog responses, e.g., impedance response, generated by the proximal electrodes  52   a - 52   h  and determining the control signals that are sent to the generator  100  and/or supply pump  40  via the microprocessor  235 . More particularly, the controller  200  accepts one or more signals indicative of impedance associated with proximal electrodes  52   a - 52   h  adjacent an ablation zone and/or the microwave antenna  12 , namely, the signals generated by the AZCM  232  as a result of the impedance measured and/or produced by proximal electrodes  52   a - 52   h . One or more modules e.g., AZCM  232 , of the controller  200  monitors and/or analyzes the impedance produced by the proximal electrodes  52   a - 52   h  and determines if a threshold impedance has been met. If the threshold impedance has been met, then the AZCM  232 , microprocessor  235  and/or the controller  200  instructs the generator  100  to adjust the microwave output stage  138  and/or the power supply  137  accordingly. Additionally, the controller  200  may also signal the supply pump to adjust the amount of cooling fluid to the microwave antenna  12  and/or the surrounding tissue. 
     The controller  200  includes microprocessor  235  having memory  236  which may be volatile type memory (e.g., RAM) and/or non-volatile type memory (e.g., flash media, disk media, etc.). In the illustrated embodiment, the microprocessor  235  is in operative communication with the power supply  137  and/or microwave output stage  138  allowing the microprocessor  235  to control the output of the generator  100  according to either open and/or closed control loop schemes. The microprocessor  235  is capable of executing software instructions for processing data received by the AZCM  232 , and for outputting control signals to the generator  100  and/or supply pump  40 , accordingly. The software instructions, which are executable by the controller  200 , are stored in the memory  236 . 
     In accordance with the present disclosure, the microwave antenna  12  is configured to create an ablation zone “A” having any suitable configuration, such as, for example, spherical ( FIG. 4A ), hemispherical, ellipsoidal ( FIG. 4B  where the ablation zone is designated “A- 2 ”), and so forth. In one particular embodiment, microwave antenna  12  is configured to create an ablation zone “A” that is spherical ( FIG. 4A ). To facilitate understanding of the present disclosure, ablation zone “A” is being defined having a plurality of concentric ablation zones having radii r 1 -r 8  when measured from the center of the ablation zone “A,” collectively referred to as radii r. 
     With reference to  FIGS. 5A and 5B , proximal electrodes  52   a - 52   h  in combination with distal electrode  54  are configured to provide comprehensive monitoring of an ablation zone “A” ( FIGS. 4A and 5A  at microwave position J). More particularly, the concept of the integration of impedance Z associated with proximal electrodes  52  over time may be used to indicate tissue damage, e.g., death or necrosis. For a given microwave antenna  12 , each of the proximal electrodes  52   a - 52   h  has a predetermined threshold impedance Z associated therewith. The predetermined threshold impedance Z associated with a corresponding electrode  52   a - 52   h , e.g., electrode  52   a , and a corresponding radius “r,” e.g., r 1 , may be determined via any suitable methods. For example, predetermined threshold impedances Z may be determined via known experimental test data, model equations, functions and graphs, or combination thereof. 
     In one particular embodiment, a control algorithm of the present disclosure uses known (or in certain instances predicted) threshold impedances Z at specific radii to create an ablation zone “A” having a radius “r.” That is, impedances Z associated with proximal electrodes  52   a - 52   h  that correspond to specific radii are compiled into one or more look-up tables “D” and are stored in memory, e.g., memory  236 , accessible by the microprocessor  235  and/or the AZCM  232  ( FIG. 3 ). The AZCM  232  includes control circuitry that receives information from the proximal electrodes  52   a - 52   h , and provides the information and the source of the information (e.g., the particular proximal electrode  52  providing the information) to the controller  200  and/or microprocessor  235 . More particularly, AZCM  232  monitors the impedance Z at the proximal electrodes  52 , e.g., proximal electrode  52   a , and triggers a command signal in response to the proximal electrode  52   a  reaching a predetermined impedance Z such that the electrosurgical output power from the generator  100  may be adjusted (see  FIG. 5A  at microwave antenna  12  position F). 
     AZCM  232  may be configured to monitor impedance Z at the proximal electrodes  52   a - 52   h  by any known method(s). For example, in one particular embodiment, the AZCM  232  utilizes one or more equations (V=I×Z) to calculate the impedance at a particular electrode  52   a - 52   h . In this instance, the voltage V and current I is known and the AZCM  232  calculates the impedance Z. Alternatively, or in combination therewith, the sensor(s)  53   a - 53   h  may provide thermal measurements at a respective electrode  52   a - 52   h . With the impedance of a respective proximal electrode  52   a - 52   h  calculated and/or determined, AZCM  232 , microprocessor  235  and/or controller  200  may access the one or more look-up tables “D” and confirm that the threshold impedance Z has been met and, subsequently, instruct the generator  100  to adjust the amount of microwave energy being delivered to the microwave antenna  12 , see  FIG. 5B  at corresponding graphical representation F. This combination of events will provide an ablation zone “A” with a radius approximately equal to r 3 , i.e., an ablation zone approximately equal to 3 cm by 1 cm. It should be noted, that in this instance, the ablation zone “A” is more ellipsoidal than spherical. In embodiments, one or more control algorithms may utilize interpolation between the radii associated with the electrodes  52   a - 52   h  to calculate impedance between discreetly measured radii, e.g., impedance measured between electrode  52   a  and electrode  52   b . More particularly, various (and commonly known) interpolation techniques may be utilized via curve fitting along the electrodes  52   a - 52   h.    
     In certain instances, the one or more data look-up tables may be stored into memory during the manufacture process of the generator  100  and/or controller  200  or downloaded during programming; this is particularly useful in the instance where the generator  100  is configured for use with a single type of microwave antenna. Alternatively, the one or more data look-up tables may be downloaded into memory  236  at a time prior to use of the system  10 ; this is particularly useful in the instance where the generator  100  is configured for use with multiple microwave antennas that are configured to perform various ablation procedures. 
     In one particular embodiment, data look-up table “D” may be stored in a memory storage device  73  associated with the microwave antenna  12 . More particularly, a data look-up table “D” may be stored in a memory storage device  73  operatively associated with the microwave antenna  12  and may be downloaded, read and stored into microprocessor  235  and/or memory  236  and, subsequently, accessed and utilized in a manner described above; this would dispose of the step of reprogramming the generator  100  and/or controller  200  for a specific microwave antenna. More particularly, the memory storage device  73  may be operably disposed on the microwave antenna  12 , such as, for example, on or adjacent the hub  22  ( FIG. 1 ). In this instance, when a user connects the microwave antenna  12  to the generator  100 , the information contained in the memory storage device may be automatically read, downloaded and stored into the generator  100  and accessed for future use. The memory storage device  73  may also include information pertaining to the microwave antenna  12 . Information, such as, for example, the type of microwave antenna, the type of tissue that the microwave antenna is configured to treat, the type of ablation zone desired, etc., may be stored into the storage device  73  associated with the microwave antenna  12 . 
     In the embodiment illustrated in  FIG. 1 , the generator  100  is shown operably coupled to fluid supply pump  40 . The supply pump  40  is, in turn, operably coupled to a supply tank  44 . In embodiments, the microprocessor  235  is in operative communication with the supply pump  40  via one or more suitable types of interfaces, e.g., a port  140  operatively disposed on the generator  100 , that allows the microprocessor  235  to control the output of a cooling fluid  30  from the supply pump  40  to the microwave antenna  12  according to either open and/or closed control loop schemes. The controller  200  may signal the supply pump  40  to control the output of cooling fluid  30  from the supply tank  44  to the microwave antenna  12 . In this way, cooling fluid  30  is automatically circulated to the microwave antenna  12  and back to the supply pump  40 . In certain embodiments, a clinician may manually control the supply pump  40  to cause cooling fluid  30  to be expelled from the microwave antenna  12  into and/or proximate the surrounding tissue. 
     Operation of system  10  is now described. For illustrative purposes, proximal electrodes  52   a - 52   h  may be considered as individual anodes and distal electrode  54  may be considered as a cathode. Each of the proximal electrodes  52   a - 52   h  includes a predetermined threshold impedance Z that has been previously determined by any of the aforementioned methods, e.g., experimental test data. Initially, microwave antenna  12  is connected to generator  100 . In one particular embodiment, one or more modules, e.g., AZCM  232 , associated with the generator  100  and/or controller  200  reads and/or downloads data, e.g., the type of microwave antenna, the type of tissue that is to be treated, data look-up tables, etc., from storage device  73  associated with the antenna  12 . In the present example, the AZCM module  232  recognizes the microwave antenna  12  as having 8 proximal electrodes  52   a - 52   h  each with a predetermined threshold impedance Z that corresponds to a specific ablation zone “A.” In one particular embodiment, the generator  100  prompts a user to enter the desired ablation zone size, e.g., ablation zone equal to 5 cm by 5 cm having a generally spherical configuration, see  FIGS. 4A and 5A  at microwave antenna  12  position J. After a user inputs the desired ablation zone size information, the AZCM  232  matches the desired ablation zone size with the particular electrode  52   a - 52   h , e.g., proximal electrode  52   h . The AZCM  232  sets the threshold impedance Z, e.g., Z ablated, for that particular proximal electrode. Thereafter, the generator  100  may be activated supplying microwave energy to the radiating section  16  of the microwave antenna  12  such that the tissue may be ablated. 
     AZCM  232  transmits DC current (or in some instances an RF signal, e.g., in the KHz or low MHz frequency spectrum) to each of the proximal electrodes  52   a - 52   h . Prior to insertion of microwave antenna  12  into tissue “T”, impedance Z associated with each of the plurality of proximal electrodes  52   a - 52   h  is relatively high, e.g., infinite; this is because an open circuit exists between the proximal electrodes  52   a - 52   h  and the distal electrode  54 . Microwave antenna  12  including proximal electrodes  52   a - 52   h  may then be positioned within tissue (see  FIGS. 5A and 5B  at microwave antenna  12  position E and corresponding graph at position E, respectively) adjacent a target tissue site. Impedance Z associated with each of the plurality of proximal electrodes  52   a - 52   h  is relatively low, e.g., non-zero; this is because uncooked tissue has a finite or infinitesimal impedance. During tissue ablation, the AZCM  232  monitors impedance Z of the proximal electrodes  52   a - 52   h . During tissue ablation, when a predetermined threshold impedance is reached (such as the impedance Z that corresponds to radius r 8 ) at the particular proximal electrode  52   a - 52   h , e.g., electrode  52   h , and is detected by the AZCM  232 , the AZCM  232  instructs the generator  100  to adjust the microwave energy accordingly. In the foregoing sequence of events, the proximal electrodes  52   a - 52   h , distal electrode  54  and AZCM  232  function in real-time controlling the amount of microwave energy to the ablation zone such that a uniform ablation zone of suitable proportion is formed with minimal or no damage to adjacent tissue. 
     It should be noted that at any time during the ablation procedure, a user may adjust the previously inputted ablation zone size information. More particularly, if a user determines that during the course of the microwave ablation procedure the original ablation zone size needs to be adjusted, e.g., original ablation zone size is too big or too small, a user may simply input the new ablation zone size, and the AZCM  232  will adjust automatically. For example, if during the above example a user decides to adjust the ablation zone size to 4 cm by 4 cm (see  FIGS. 5A and 5B  at microwave antenna position I) the AZCM  232  monitors proximal electrode  52   e  until proximal electrode  52   e  reaches the predetermined threshold impedance Z. 
     With reference to  FIG. 5  a method  400  for monitoring tissue undergoing ablation is illustrated. At step  402 , microwave energy from a generator  100  is transmitted to a microwave antenna  12  adjacent a tissue ablation site. At step,  404 , one or more electrodes&#39; impedance at the ablation site is monitored. At step  406 , a detection signal is triggered when a predetermined electrode impedance is reached at the one or more electrodes along the microwave antenna. At step  408 , the amount of microwave energy from the generator  200  to the microwave antenna may be adjusted. 
     From the foregoing and with reference to the various figure drawings, those skilled in the art will appreciate that certain modifications can also be made to the present disclosure without departing from the scope of the same. For example, the system  10  may be adapted to connect to an RF electrosurgical power source, e.g., an RF generator that includes or is in operative communication with one or more controllers  200  including an AZCM  232 . 
     While the electrodes  52  have been described herein as including a series of proximal electrodes  52   a - 52   h  and a distal electrode  54  that is positioned at a distal tip  21  of the shaft  18 , it is within the purview of the present disclosure that the distal electrode  54  may be positioned anywhere along the shaft  18 , e.g., positioned adjacent the series of proximal electrodes  52   a - 52   h . Or, in another embodiment, a distal electrode  54  may not be utilized. In this instance, one of the series of proximal electrodes  52   a - 52   h  may be configured to function in a manner as described above with respect to distal electrode  54 . 
     In certain embodiments, it may prove useful not to utilize a lead wire  56  that couples to distal electrode  54 . In this instance, inner conductor  13  takes the place of the lead wire  56  and operably couples to the distal electrode  54 , see  FIG. 7 , for example. More particularly, one or more types of DC blocks  58  are operatively associated with the microwave antenna  12 . More particularly, DC block  58  is operably disposed within the generator  100  and in electrical communication with the inner conductor  13 , shown schematically in  FIG. 8 . The DC block  58  prevents and/or limits direct current (DC) frequencies present at the distal electrode  54  from interfering with the microwave signals produced by the radiating section  16 . DC block  58  may be configured in a manner that is conventional in the art. More particularly, DC block  58  may include one or more capacitors “C” configured in series with inner conductor  13  of the coaxial conductor  14 , in series with an outer conductor (not explicitly shown) of the coaxial conductor  14 , or in series with both the inner conductor  13  and outer conductor of the coaxial conductor  14 . DC block  58  may be configured to function as a notch filter and designed to allow impedance measurement signals, i.e., impedance measurement signals that are in the KHz frequency range. In the embodiment illustrated in  FIG. 7 , lead wire  56  couples to the plurality of electrodes  52   a - 52   h  in a manner described above. Lead wire  56  is dimensioned to accommodate a respective RF signal that is transmitted to the distal electrode  54  and/or plurality of electrodes  52   a - 52   h  from the AZCM  232 . 
     AZCM  232  is configured to transmit an RF impedance measurement signal to the proximal electrodes  52   a - 52   h  and/or the distal electrode  54 . In the embodiment illustrated in  FIG. 8 , AZCM is configured to transmit an RF impedance measurement signal to the proximal electrodes  52   a - 52   h  and/or the distal electrode  54  that ranges from about 3 KHz to about 300 MHz. 
     Operation of system  10  that includes a generator  100  with a DC block  58  that is in operative communication with a microwave antenna  12  is substantially similar to that of a generator  100  without a DC block  58  and, as a result thereof, is not described herein. 
     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.