# Patent

## Title
System and method for monitoring ablation size

## Abstract

A system for monitoring ablation size is provided and includes a power source including a microprocessor for executing at least one control algorithm. A microwave antenna is configured to deliver microwave energy from the power source to tissue to form an ablation zone. An ablation zone control module is in operative communication with a memory associated with the power source. The memory includes one or more data look-up tables including data pertaining to a control curve varying over time and being representative of one or more electrical parameters associated with the microwave antenna. Points along the control curve correspond to a value of the electrical parameters and the ablation zone control module triggers a signal when a predetermined threshold value of the electrical parameter(s) is measured corresponding to the radius of the ablation zone.

## Background
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a divisional of and claims the benefit of and priority to U.S. patent application Ser. No. 12/607,268 filed by Brannan on Oct. 28, 2009, now U.S. Pat. No. 8,382,750, the entire contents of which is hereby incorporated by reference.


BACKGROUND
1. 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.
2. 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 sensor(s) 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 including a microprocessor for executing one or more control algorithms. A microwave antenna is configured to deliver microwave energy from the power source to tissue to form an ablation zone. An ablation zone control module is in operative communication with a memory associated with the power source. The memory includes one or more data look-up tables including data pertaining to a control curve varying over time and being representative of one or more electrical parameters associated with the microwave antenna. Points along the control curve correspond to a value of the electrical parameters and the ablation zone control module triggers a signal when a predetermined threshold value of the electrical parameter(s) is measured corresponding to the radius of the ablation zone.
The present disclosure also 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 a power source to tissue to form an ablation zone. An ablation zone control module in operative communication with a memory associated with the power source. The memory includes one or more data look-up tables including data pertaining to a control curve varying over time and being representative of one or more electrical parameter(s) associated with the microwave antenna. Points along the control curve correspond to a value of the electrical parameter(s) and the ablation zone control module triggers a signal when a predetermined threshold value of the at least one electrical parameter is measured corresponding to the radius of the ablation zone.
The present disclosure also provides a method for monitoring temperature of tissue undergoing ablation. The method includes an 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 reflected power associated with the microwave antenna as the tissue ablation zone forms. A step of the method includes communicating a control signal to the power source when a predetermined reflected power is reached at the microwave antenna. Adjusting the amount of microwave energy from the power source to the microwave antenna is another step of the method.



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 a functional block diagram of a power source for use with the system depicted in FIG. 1;
FIG. 3A is a schematic, plan view of the tip of a microwave antenna depicted in FIG. 1 illustrating radial ablation zones having a spherical configuration;
FIG. 3B is a schematic, plan view of the tip of a microwave antenna depicted in FIG. 1 illustrating radial ablation zones having an ellipsoidal configuration;
FIG. 4A is a graphical representation of a reflected power (Pr) versus time (t) curve;
FIG. 4B a graphical representation of a corresponding reflected power (Pr) versus ablation radii (Ar) curve;
FIG. 4C is a graphical representation of the derivative (dPr/dt) of the reflected power (Pr) versus time (t) curve; and
FIG. 5 is a flow chart illustrating a method for monitoring temperature of tissue undergoing ablation in accordance with the present disclosure.



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 FIG. 1, a system for monitoring ablation size is designated 10. The system 10 includes a microwave antenna 100 that is adapted to connect to an electrosurgical power source, e.g., an RF and/or microwave (MW) generator 200 that includes or is in operative communication with one or more controllers 300 and, in some instances, a fluid supply pump 40. Briefly, microwave antenna 100 includes an introducer 116 having an elongated shaft 112 and a radiating or conductive section or tip 114 operably disposed within elongated shaft 112, a cooling assembly 120 having a cooling sheath 121, a handle 118, a cooling fluid supply 122 and a cooling fluid return 124, and an electrosurgical energy connector 126. Connector 126 is configured to connect the microwave antenna 100 to the electrosurgical power source 200, e.g., a generator or source of radio frequency energy and/or microwave energy, and supplies electrosurgical energy to the distal portion of the microwave antenna 100. Conductive tip 114 and elongated shaft 112 are in electrical communication with connector 126 via an internal coaxial cable 126a (see FIG. 3A, for example) that extends from the proximal end of the microwave antenna 100 and includes an inner conductor tip that is operatively coupled to a radiating section 138 operably disposed within the shaft 112 and adjacent the conductive or radiating tip 114 (see FIG. 3A, for example). As is common in the art, internal coaxial cable 126a is includes a dielectric material and an outer conductor surrounding each of the inner conductor tip and dielectric material. A connection hub (not shown) disposed at a proximal end of the microwave antenna 100 operably couples connector 126 to internal coaxial cable 126a, and cooling fluid supply 122 and a cooling fluid return 124 to a cooling assembly 120. Radiating section 138 by way of conductive tip 114 (or in certain instances without conductive tip 114) is configured to deliver radio frequency energy (in either a bipolar or monopolar mode) or microwave energy (having a frequency from about 500 MHz to about 10 GHz) to a target tissue site. Elongated shaft 112 and conductive tip 114 may be formed of suitable conductive material including, but not limited to copper, gold, silver or other conductive metals having similar conductivity values. Alternatively, elongated shaft 112 and/or conductive tip 114 may be constructed from stainless steel or may be plated with other materials, e.g., other conductive materials, such as gold or silver, to improve certain properties, e.g., to improve conductivity, decrease energy loss, etc. In an embodiment, the conductive tip may be deployable from the elongated shaft 112. In one particular embodiment, microwave antenna 100 may include an introducer 116 having an elongated shaft 112 and a tip 114 that is non-conductive. In this instance, the tip 114 may be made from a non-conductive material such as, for example, ceramic, plastic, etc.
With reference to FIG. 2, a schematic block diagram of the generator 200 is illustrated. The generator 200 includes a controller 300 having one or more modules (e.g., an ablation zone control module 332 (AZCM 332), a power supply 237 and a microwave output stage 238). In this instance, generator 200 is described with respect to the delivery of microwave energy. The power supply 237 provides DC power to the microwave output stage 238 which then converts the DC power into microwave energy and delivers the microwave energy to the radiating section 138 of the microwave antenna 100. The controller 300 may include analog and/or logic circuitry for processing sensed values provided by the AZCM 332 and determining the control signals that are sent to the generator 200 and/or supply pump 40 via a microprocessor 335. The controller 300 (or component operably associated therewith) accepts one or more measured signals indicative of reflected power Pr associated with the microwave antenna 100 when the microwave antenna is radiating energy.
One or more modules e.g., AZCM 332, of the controller 300 analyzes the measured signals and determines if a threshold reflected power Pr, e.g., Pr1 has been met. If the threshold reflected power Pr1 has been met, then the AZCM 332, a microprocessor 335 and/or the controller instructs the generator 200 to adjust the microwave output stage 238 and/or the power supply 237 accordingly. Additionally, the controller 300 may also signal the supply pump to adjust the amount of cooling fluid to the microwave antenna 100 and/or the surrounding tissue. The controller 200 includes microprocessor 335 having memory 336 which may be volatile type memory (e.g., RAM) and/or non-volitile type memory (e.g., flash media, disk media, etc.). In the illustrated embodiment, the microprocessor 335 is in operative communication with the power supply 237 and/or microwave output stage 238 allowing the microprocessor 335 to control the output of the generator 300 according to either open and/or closed control loop schemes. The microprocessor 335 is capable of executing software instructions for processing data received by the AZCM 332, and for outputting control signals to the generator 300 and/or supply pump 40, accordingly. The software instructions, which are executable by the controller 300, are stored in the memory 336.
One or more electrical properties (e.g., voltage, current, power, impedance, etc.) associated with a signal (or pulse) generated by the generator 200 may be monitored and measured. More particularly, electrical properties associated with a forward and reflected portion of the signal generated by the generator 200 is monitored and measured. For example, in one particular embodiment, forward and reflected power, Pf and Pr, respectively, of a signal for ablating tissue is measured by the AZCM 332, controller 300, microprocessor 337 or other suitable module associated with the generator 200 and/or controller 200.
One or more control algorithms for predicting tissue ablation size is implemented by the controller 300. More particularly, the concept of correlating reflected power Pr associated with a particular microwave antenna, e.g., the microwave antenna 100, with an ablation zone “A” having a radius “r” may be used to indicate tissue death or necrosis. More particularly, reflected power Pr associated with the microwave antenna 100 varies over the course of an ablation cycle due to tissue complex permittivity changes caused by temperature increase (see FIGS. 4A and 4B, for example). A relationship of reflected power Pr as a function of time is represented by a control curve illustrated in FIG. 4A. Likewise, a relationship of reflected power Pr as a function of ablation size is represented by a control curve illustrated in FIG. 4B. The control curves represented in FIGS. 4A and 4B are based on model functions f(t) and known measured values of reflected power Pr that have been taken during an ablation procedure performed with the microwave antenna 100, controller 300 and/or generator 200. In accordance with the present disclosure, the control curves depicted in FIGS. 4A and 4B (and/or equations mathematically associated therewith) may be utilized to calculate and/or verify when a specified threshold reflected power Pr (e.g., reflected powers Pr1-ss) within a specified time range (e.g., t1-tss) not exceeding tss, i.e., time when the microwave antenna 100 and ablated tissue is at a steady-state condition, see FIG. 4A or FIG. 4B, for example. The significance of when the microwave antenna 100 and ablated tissue is at a steady-state condition is described in greater detail below.
With reference now to FIGS. 4A and 4B, initially, an impedance mismatch between the microwave antenna 100 and tissue is present when the microwave antenna 100 is inserted into uncooked tissue. This impedance mismatch is due to the 50 ohm impedance associated with the internal cable 126a not matching the impedance of the radiating section 118 and/or conductive tip 114. The impedance mismatch results in a non-zero reflected power Pr1, at the beginning of the ablation procedure, see FIGS. 4A and 4B, for example. During the course of the ablation procedure, tissue in a “near field” heats up resulting in a decrease in reflected power Pr (in a non-linear rate) (see FIG. 4A for example) until an optimal impedance match between the microwave antenna 100 and tissue is reached (see FIG. 4A at a time equal to time t2 in combination with FIG. 4B at an ablation size having a radius “r” equal to 2 cm). That is, the total impedance Zt of the microwave antenna 100 and tissue in the “near field” is approximately equal to 50 ohms. The microwave antenna 100 and tissue in the “near field” remain at this optimal impedance match for a brief period of time. At a time after time t2, the microwave antenna 100 and tissue in the near field diverge from the optimal impedance match (in a non-linear rate). Ultimately, when the microwave antenna 100 has heated tissue to a maximum attainable temperature, an ablation zone “A” having a corresponding radius “r” (e.g., rss) is formed (see FIG. 3A in combination with FIGS. 4A and 4B, for example). At this maximum temperature, a dielectric constant and conductivity associated with the ablated tissue reach a steady-state condition (this steady-state condition occurs at time tss) that corresponds to a steady-state reflected power Prss (hereinafter referred to simply as Prss) associated with the microwave antenna 100. That is, because the ablated tissue is in a “near field” of the microwave antenna 100, the ablated tissue essentially becomes part of the microwave antenna 100. Accordingly, when a dielectric constant and conductivity associated with the ablated tissue reaches a steady-state condition, the reflected power Pr at the microwave antenna 100 also reaches a steady-state condition, e.g., Prss, FIG. 4A.
As noted above, the foregoing control algorithm includes one or more model functions f(t) that are representative of the model curves illustrated in FIGS. 4A and 4B. The model functions f(t), model curves depicted in FIGS. 4A and 4B, and/or known measured values of reflected power Pr, are utilized to obtain information relevant to the reflected power Pr such that real-time monitoring of an ablation zone may be achieved. More particularly, a measurement of a slope of a tangent line at a point along either of the control curves (e.g., curve illustrated in FIG. 4A) is equal to a derivative (dPr/dt) of the curve at that point. The calculation of the derivative at a particular point along the curve(s) provides information pertinent to the reflected power Pr. More particularly, a rate of change of reflected power Pr with respect to time and, more particularly, to a vector quantity of the rate of change of the reflected power Pr (i.e., direction (positive or negative) and magnitude of the rate of change) is calculated from the control curve(s) depicted in FIGS. 4A-4C. This rate of change associated with reflected power Pr with respect to time may be utilized, for example, to distinguish between a rise and fall of the reflected power Pr. More particularly, points taken along the control curves depicted in FIGS. 4A and 4B correspond to values of reflected power Pr, e.g., reflected powers Pr1 and Pr3, which correspond to ablation zones “A” having radii “r,” e.g., radii r1 and r3, at corresponding times t, e.g., times t1 and t3. It should be noted that a value of reflected power, e.g., Pr1, corresponds to more than one radius, e.g., r1 and r3 of an ablation zone.
More particularly, the representative control curve of reflected power Pr depicted in FIGS. 4A and 4B illustrates reflected power Pr having an initial value, e.g., Pi, at the beginning of an ablation procedure. The reflected power Pr decreases until the reflected power Pr is approximately equal to zero, i.e., when the total impedance of microwave antenna 100 and tissue in a “near field” is approximately equal to 50 ohms. The reflected power Pr increases at a time after time t2 when the total impedance of microwave antenna 100 and tissue in the “near field” is not equal to 50 ohms. Accordingly, a measure of reflected power Pr taken along the control curve provides one or more numerical values of the reflected power Pr that is indicative of one or more ablation zones “A” having corresponding radii “r.” For example, a measure of the reflected power Pr, e.g., Pr1, at time t1 corresponds to an ablation zone “A” having a radius r1 that is approximately equal to 1 cm, see FIGS. 4A and 4B collectively. Likewise, a measure of the reflected power Pr, e.g., Pr1, at time t3 also corresponds to an ablation zone “A” having a radius r3 that is approximately equal to 2.2 cm, see FIGS. 4A and 4B collectively.
In accordance with the present disclosure, samples of a derivative taken at selective points along the control curve (e.g., points corresponding to radii r1 and r3 and/or points corresponding to times t1 and t3) provide information pertaining to the precise location (e.g., rise or fall portions of the control curve) of the reflected power Pr with respect to the control curve.
More particularly, and for example, a derivative taken at a point along the control curve at time t1 when the reflected power Pr is approximately equal to Pr1 is negative because the slope of the reflected power Pr is declining, as best seen in FIG. 4C. In this instance, reflected power Pr1 may be thought of as having and is assigned a negative value indicating to one or more modules, e.g., AZCN 332, associated with the controller 300 and /or generator 200 that this value of the reflected power Pr1 is indicative of and corresponds to an ablation zone “A” having a radius r1. Similarly, samples of a derivative taken at a point along the control curve at time t3 when the reflected power Pr is approximately equal to Pr1 is positive because the slope of the reflected power Pr is increasing, as best seen in FIG. 4C. In this instance, reflected power Pr1 may be thought of as having and is assigned a positive value indicating to one or more modules, e.g., AZCN 332, associated with the controller 300 and /or generator 200 that this value of the reflected power Pr1 is indicative of and corresponds to an ablation zone “A” having a radius r3.
Implementing a control algorithm that utilizes a calculation of a derivative taken at a point on the control curve facilitates in determining the precise size of the ablation zone “A.” That is, one or more modules, e.g., AZCM 332, associated with the controller 300 and generator 200 is capable of distinguishing between which ablation zone radius “r,” e.g., radius r1 or r2, corresponds to the reflected power Pr, e.g., measured reflected power Pr1. Moreover, in the instance where multiple ablation zones “A” are located adjacent to one another, tissue impedance of uncooked tissue at a near field of an ablation zone “A” may effect a reflected power Pr measurement. More particularly, tissue impedance of uncooked tissue at the near field may be slightly higher or lower (depending on a specific adjacent ablation zone “A”), which, in turn, may cause the reflected power Pr to be higher or lower at the beginning of an ablation procedure then is expected. Thus, in the instance where the initial reflected power Pi is approximately equal to Pr4, a calculation of the derivative taken at a point on the control curve indicates that the microwave antenna 100 is positioned adjacent cooked or ablated tissue. That is, the initial positive value of Pr4 indicates that the reflected power Pr is increasing, and, thus, a steady-state condition is approaching, i.e., a calculation of the derivative indicates that the measured reflected power Pr is in the rising portion of the control curve and the reflected power Pr will not approach /zero, i.e., a point on the control curve where the total impedance associated with the microwave antenna 100 and tissue adjacent the near field is approximately equal to 50 ohms.
The microwave antenna 100 of the present disclosure may be configured to create an ablation zone “A” having any suitable configuration (e.g., a width “w” and a length “1”), such as, for example, spherical (FIG. 3A), hemispherical, ellipsoidal (FIG. 3B where the ablation zone is designated “A-2”), and so forth. In one particular embodiment, microwave antenna 100 is configured to create an ablation zone “A” that is spherical (FIG. 3A). As noted above, when the microwave antenna 100 has heated tissue in the “near field” to a maximum temperature, a dielectric constant and conductivity associated with the ablated tissue reaches a steady-state that corresponds to a steady-state reflected power Prss associated with the microwave antenna 100. Correlating the Prss associated with the microwave antenna 100 with the ablated tissue (i.e., ablated tissue, where the dielectric constant and conductivity are in a steady-state condition), indicates a specific size (e.g., radius rss) and shape (e.g., spherical) of the ablation zone “A.” Thus, a measure of Prss associated with the microwave antenna 100 corresponds to an ablation zone “A” having a radius r, e.g., rss. The control algorithm of the present disclosure uses known steady-state reflected powers associated with specific microwave antennas at specific radii to predict an ablation size. That is, reflected powers Pr, e.g., Prss, associated with a specific microwave antenna, e.g., microwave antenna 100, and corresponding radius, e.g., rss, are compiled into one or more look-up tables “D” and are stored in memory, e.g., memory 336, accessible by the microprocessor 335 and/or the AZCM 332. Thus, when a measured reflected power for a specific microwave antenna, e.g., microwave antenna 100, reaches Prss one or more modules, e.g. AZCM 332, associated with the controller 300, commands the controller 200 to adjust the power output to the microwave antenna 100 accordingly. This combination of events will provide an ablation zone “A” with a radius approximately equal to rss.
In an embodiment, for a given microwave antenna, e.g., microwave antenna 100, reflected power measurements may be taken at times prior to tss, e.g., times t1-t4. In this instance, reflected powers, e.g., Pr1-Pr4, associated with the microwave antenna 100 may be correlated with an ablation zone “A” defined by a plurality of concentric ablation zones having radii r1-r4 (collectively referred to as radii “r”) when measured from the center of the ablation zone “A.” More particularly, the reflected powers Pr1- Pr4 and corresponding radii “r” may be correlated with each other in a manner as described above with respect to Prss and rss (see FIG. 3A in combination with FIGS. 4A and 4B, for example). In this instance, when specific reflected power, e.g., P3, is met one or more modules, e.g. AZCM 332, associated with the controller 300, commands the controller 200 to adjust the power output to the microwave antenna 100 accordingly.
It should be noted, that a reflected power Pr associated with a microwave antenna 100 may vary for a given microwave antenna. Factors that may contribute to a specific reflected power Pr for a given microwave antenna include but are not limited to: dimensions associated with the microwave antenna (e.g., length, width, etc.); type of material used to manufacture the microwave antenna (or portion associated therewith, e.g., a radiating section) such as copper, silver, etc; and the configuration of the radiating section (e.g., dipole, monopole, etc.) and/or a conductive tip (e.g., sharp, blunt, curved, etc) associated with the microwave antenna. Other factors that may contribute to a specific reflected power Pr for a given microwave antenna may include, for example, type of microwave antenna (e.g., microwave antenna configured for use in treating lung, kidney, liver, etc.), type of tissue being treated (e.g., lung, kidney, liver, heart etc.), tumor size, and so on.
AZCM 332 may be a separate module from the microprocessor 335, or AZCM 332 may be included with the microprocessor 335. In an embodiment, the AZCM 332 may be operably disposed on the microwave antenna 100. The AZCM 332 may include control circuitry that receives information from one or more control modules and/or one or more impedance sensors (not shown), and provides the information to the controller 300 and/or microprocessor 335. In this instance, the AZCM 332, microprocessor 335 and/or controller 300 may access look-up table “D” and confirm that a paraticular reflected power (e.g., Prss) associated with microwave assembly 100 corresponds to a specific ablation zone (e.g., specific ablation zone having a radius rss) has been met and, subsequently, instruct the generator 200 to adjust the amount of microwave energy being delivered to the microwave antenna. In one particular embodiment, look-up table “D” may be stored in a memory storage device (not shown) associated with the microwave antenna 100. More particularly, a look-up table “D” may be stored in a memory storage device operatively associated with handle 118 and/or connector 126 of the microwave antenna 100 and may be downloaded, read and stored into microprocessor 335 and/or memory 336 and, subsequently, accessed and utilized in a manner described above; this would do away with reprogramming the generator 200 and/or controller 300 for a specific microwave antenna. The memory storage device may also be configured to include information pertaining to the microwave antenna 100. 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 associated with the microwave antenna. In this instance, for example, generator 200 and/or controller 300 of system 10 may be adapted for use with a microwave antenna configured to create an ablation zone, e.g. ablation zone “A-2,” different from that of microwave antenna 100 that is configured to create an ablation zone “A.”
In the embodiment illustrated in FIGS. 1-4, the generator is shown operably coupled to fluid supply pump 40. The supply pump 40 is, in turn, operably coupled to the supply tank 44. In embodiments, the microprocessor 335 is in operative communication with the supply pump 40 via one or more suitable types of interfaces, e.g., a port 240 operatively disposed on the generator 200, which allows the microprocessor 335 to control the output of a cooling fluid from the supply pump 40 to the microwave antenna 100 according to either open and/or closed control loop schemes. The controller 300 may signal the supply pump 40 to control the output of the cooling fluid from the supply tank 44 to the microwave antenna 100. In this way, cooling fluid 42 is automatically circulated to the microwave antenna 100 and back to the supply pump 40. In certain embodiments, a clinician may manually control the supply pump 40 to cause cooling fluid 42 to be expelled from the microwave antenna 100 into and/or proximate the surrounding tissue.
Operation of system 10 is now described. In the description that follows, it is assumed that losses associated with the connector 126 and/or cable 162a are negligible and, thus, are not needed in calculating and/or determining a reflected power of the microwave antenna 100 adjacent the ablation zone during the ablation procedure. Initially, microwave antenna 100 is connected to generator 200. In one particular embodiment, one or more modules, e.g., AZCM 332, associated with the generator 200 and/or controller 300 reads and/or downloads data from a storage device associated with the antenna 100, e.g., the type of microwave antenna, the type of tissue that is to be treated, etc. Microwave antenna 100 may then be positioned adjacent tissue (FIG. 3A). Thereafter, generator 200 may be activated supplying microwave energy to radiating section 138 of the microwave antenna 100 such that the tissue may be ablated. During tissue ablation, when a predetermined reflected power, e.g., Prss, at the microwave antenna 100 is reached, the AZCM 332 instructs the generator 200 to adjust the microwave energy accordingly. In the foregoing sequence of events the AZCM 332 functions in real-time controlling the amount of microwave energy to the ablation zone such that a uniform ablation zone of suitable proportion (e.g., ablation zone “A” having a radius rss) is formed with minimal or no damage to adjacent tissue.
With reference to FIG. 5 a method 400 for monitoring temperature of tissue undergoing ablation is illustrated. At step 402, microwave energy from generator 200 is transmitted to a microwave antenna 100 adjacent a tissue ablation site. At step, 404 reflected power Pr associated with the microwave antenna is monitored. At step 406, a detection signal is communicated to the generator 200 when a predetermined reflected power Pr is reached at the microwave antenna 100. At step 408, the amount of microwave energy from the generator 200 to the microwave antenna 100 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, one or more directional couplers (not shown) may be operatively associated with the generator 200, controller 300 and/or AZCM 332, and configured to direct the forward, reflected, and/or load power portions of a sampled output signal (or pulse) to the AZCM 332. More particularly, the directional coupler provides samples of the forward and reflected signal (or pulse) generated by the generator 200. The power, magnitude and phase of the generated output signal may be obtained or calculated from the measured forward and reflected signals by conventional algorithms that employ one or more suitable equations.
It should be noted that energy values or parameters (e.g., power, voltage, current, impedance, magnitude and phase) of an output pulse are valid at the output of generator 200. That is, and as alluded to above, the connector 126 and/or internal cable 126a may include transmission line losses. Accordingly, in order to get a more accurate reading and/or measurement of the energy values or parameters that are delivered to the microwave antenna 100 and/or reflected back to the generator 200, one would have to know the actual transmission line losses associated with connector 126 and/or internal cable 126a. Accordingly, in an embodiment, loss information for connector 126 and/or internal cable 126a may be determined and, subsequently, stored in memory 336 and accessed by one or more modules, such as, for example, a calibration module (600) or other suitable module (e.g., AZCM 332) for later use. The loss information for connector 126 and/or internal cable 126a may be determined by any suitable device and/or method. For example, the loss information for connector 126 and/or internal cable 126a may be determined via network analyzer 602. In one particular embodiment, the network analyzer 602 may be an integral part of generator 200 (e.g., part of calibration module 600) or, alternatively, the network analyzer 602 may be a separate handheld device that is in operative communication with generator 200. The network analyzer 602 may be used to perform a diagnostic test of connector 126 and/or internal cable 126a. The network analyzer 602 may function in a fashion similar to most conventional network analyzers that are known in the available art. That is, the network analyzer 602 may determine the properties that are associated with connector 126 and/or internal cable 126a, and more particularly, those properties that are associated with connector 126 and/or internal cable 126a that affect the reflection and/or transmission of an output signal, such as, for example, the characteristic impedance Zo of connector 126 and/or internal cable 126a. 
Known line loss information associated with the connector 126 and/or internal cable 126a may be stored into memory 336 and accessed during an ablation procedure by one or more modules, e.g., AZCM 332, associated with the controller 300 and/or generator 200 and, subsequently, used in determining if a predetermined threshold value, e.g., Pr1-ss, of the reflected power Pr has been met. More particularly, characteristic impedance associated with connector 126 and/or internal cable 126a may be employed to determine a more accurate or comprehensive measurement of the reflected power Pr. For example, a more accurate or comprehensive measurement of the reflected power Pr may be determined using the equation:




  
    
      
        
          
            
              Z
              
                1
                -
                ss
              
            
            -
            
              Z
              o
            
          
          
            
              Z
              
                1
                -
                ss
              
            
            +
            
              Z
              o
            
          
        
        =
        
          
            
              P
              SWR
            
            -
            1
          
          
            
              P
              SWR
            
            +
            1
          
        
      
    
    
      
        (
        1
        )
      
    
  




where, Zo is the characteristic impedance associated with the connector 126 and/or internal cable 126a, Z1-ss is an impedance of the microwave antenna 100 when the microwave antenna 100 is positioned adjacent tissue in a “near field” at times t1-ss, and is a power standing wave ratio that may be calculated using the equation:




  
    
      
        
          P
          SWR
        
        =
        
          
            
              P
              f
            
            +
            
              P
              r
            
          
          
            
              P
              f
            
            -
            
              P
              r
            
          
        
      
    
    
      
        (
        2
        )
      
    
  




where pf is the power associated with the generated signal (i.e., forward signal) and Pr is the power associated with the reflected signal. The characteristic impedance Zo is an accurate measure of the impedance of the connector 126 and/or internal cable 126a and takes into account the line losses associated with the connector 126 and/or internal cable 126a. In this instance, after all the necessary calculations have been carried out, an accurate representation of the reflected power Pr may be transmitted to and measured by the AZCM 332 (or other suitable module associated with either the controller 300 or generator 200).
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.

## Claims

1. A method for monitoring tissue undergoing ablation, the method comprising:
transmitting microwave energy from a power source to a microwave antenna to form a tissue ablation zone;
monitoring reflected power associated with the microwave antenna to generate a reflected power signal;
calculating a derivative indicative of rise and fall portions of the reflected power signal, wherein when the derivative is taken during a rise portion of the reflected power signal the measured reflected power is assigned a positive value and when the derivative is taken during a fall portion of the reflected power signal the measured reflected power is assigned a negative value, the positive and negative values of the reflected power signal indicating when the microwave antenna and tissue in a near field of the ablation zone are approaching a respective steady-state condition or an impedance match between the microwave antenna and tissue in the near field;
communicating a control signal to the power source when a predetermined reflected power is reached at the microwave antenna; and
adjusting the amount of microwave energy from the power source to the microwave antenna.

2. The method according to claim 1, further comprising:
accessing at least one data look-up table including data pertaining to a control curve varying over time and being representative of at least one electrical parameter associated with the microwave antenna; and
generating a signal when a predetermined threshold value of the at least one electrical parameter is corresponding to the radius of the ablation zone.

3. The method according to claim 2, wherein the at least one electrical parameter is selected from the group consisting of impedance, power, voltage and current.

4. The method according to claim 3, further comprising correlating a reflected signal generated by the power source associated with the microwave antenna during transmission of microwave energy to the microwave antenna when the microwave antenna is in a near field state with the tissue ablation zone.