Abstract:
An apparatus for calibrating a microwave energy delivery device including a body, the body defining a chamber portion. The chamber portion is configured to receive a portion of a microwave energy delivery device and the body is configured to absorb energy transmitted by the microwave energy delivery device at a predetermined absorption rate.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to systems and methods for performing a medical procedure, wherein the medical procedure includes the generation and transfer of energy from an energy source to a dynamically changing device and, more particularly, efficient transfer of energy through a microwave energy delivery, measurement and control system. 
         [0003]    2. Description of Related Art 
         [0004]    During microwave ablation procedures, the electrical performance of a microwave antenna probe changes throughout the course of an ablation treatment. The change in performance may be due to the device or due to changes in tissue properties. The ability to observe parameters indicative of changes in antenna property, antenna performance or tissue properties changes during ablation greatly aids in the understanding of microwave ablation. For example, measuring antenna impedance is a common method for determining antenna performance and/or a change in an antenna property. Microwave systems are typically designed to a characteristic impedance, such as, for example, 50 Ohms, wherein the impedance of the generator, the delivery system, the ablation device and tissue are about equal to the characteristic impedance. Efficiency of energy delivery decreases when the impedance of any portion of the system changes. 
         [0005]    With low frequency RF systems impedance can easily be determined by measuring the delivered current at a known voltage and calculating tissue impedance using well known algorithms. Obtaining accurate measurements of tissue impedance at microwave frequencies is more difficult because circuits behave differently at microwave frequency. For example, unlike an electrode in an RF system, an antenna in a microwave system does not conduct current to tissue. In addition, other components in a microwave system may transmit or radiate energy, like an antenna, or components may reflect energy back into the generator. As such, it is difficult to determine what percentage of the energy generated by the microwave generator is actually delivered to tissue, and conventional algorithms for tissue impedance are inaccurate. 
         [0006]    Therefore, other methods of measuring impedance are typically used in a microwave system. One well known method is an indirect method using measurements of forward and reflected power. While this is a generally accepted method, this method can also prove to be inaccurate because the method fails to account component losses and depends on indirect measurements, such as, for example forward and reflected power measurements from directional couplers, to calculate impedance. In addition, this method does not provide information related to phase, a component vital to determining antenna impedance. 
         [0007]    One alternative method of measuring impedance in a microwave energy delivery system is by determining broadband scattering parameters. Capturing antenna broadband scattering parameters periodically throughout a high power ablation cycle necessitates the use of equipment that requires precise calibration. Unfortunately, this equipment is prone to damage by high power signals and the microwave energy delivery system typically needs to be reconfigured to accommodate and protect such equipment. 
         [0008]    The present disclosure describes a Microwave Research Tool (MRT) that includes a system to measure impedance in a microwave energy delivery system by direct and indirect methods including a system to measure broadband scattering parameters. 
       SUMMARY 
       [0009]    The present disclosure relates to an apparatus for calibrating a microwave energy delivery device including a body defining a chamber portion therein, the chamber portion configured to receive a portion of a microwave energy delivery device and the body is configured to absorb energy transmitted by the microwave energy delivery device at a predetermined absorption rate. 
         [0010]    The chamber partially surrounds the microwave antenna of the microwave energy delivery device. The chamber is formed by the body is an elongate cylindrical chamber, the elongate cylindrical chamber adapted to receive the microwave antenna of the microwave energy delivery device. 
         [0011]    In another embodiment the chamber is configured to engage the microwave antenna of the microwave energy delivery device within the chamber. 
         [0012]    The body further includes a first body portion configured to receive and position the microwave energy delivery device and a second body portion configured to engage the first body portion and form the chamber therebetween. The first body portion and the second body portion may be hingedly engaged. The first and second body portions may include a locking mechanism that locks the calibration device to the microwave energy delivery device. The locking mechanism may be a clip, a latch, a pin, a locking hinge, a self closing hinge, a magnetic lock or an electronic closure mechanism. 
         [0013]    In yet another embodiment the body may includes a positioner to position the microwave energy delivery device in a fixed position relative to the chamber. The positioner on the body may correspond to a substantially similar interface on the microwave energy device. The positioner and the interface may mate with each other to position the microwave energy delivery device in a fixed position relative to the chamber. The positioner may be recessed portion of the body and the interface may be a raised portion of the microwave energy delivery device. The recessed portion and the raised portion mate together and position the microwave energy delivery device. 
         [0014]    The first body portion and the second body portion surround a portion of the microwave energy delivery device in a first condition and are spaced relative to a portion of the microwave energy delivery device in a second condition. 
         [0015]    A system for calibrating a microwave energy delivery device is also disclosed and includes a microwave generator configured to deliver a microwave energy signal to a microwave energy delivery device and a microwave system calibration apparatus. The microwave system calibration apparatus includes a body defining a chamber portion therein, the chamber portion configured to receive a portion of the microwave energy delivery device. The body configured to absorb microwave energy transmitted by the microwave energy delivery device at a predetermined absorption rate. The microwave generator measures a measured parameter related to the microwave energy signal delivered to the microwave energy delivery device and determines at least one calibration parameter related to the calibration of the microwave energy delivery device. 
         [0016]    The chamber in the system may partially surround the microwave antenna of the microwave energy delivery device. The chamber, formed by the body, may be an elongate cylindrical chamber adapted to receive the microwave antenna of the microwave energy delivery device. The chamber may engage the microwave antenna of the microwave energy delivery device within the chamber. 
         [0017]    The measured parameter may be forward power, reflected power or temperature and the calibration parameter may be phase, frequency or impedance. 
         [0018]    In another embodiment of the system the microwave generator may determine engagement of the microwave energy delivery device with the microwave system calibration apparatus. 
         [0019]    A method of calibrating a microwave energy delivery system is also disclosed and includes the steps of: connecting a microwave energy delivery device to a microwave generator; positioning the microwave energy delivery device in a chamber defined in a microwave energy calibration apparatus; delivering microwave energy to the microwave energy delivery device; measuring at least one measured parameter related to the energy delivery; determining at least one calibration parameter related to the calibration of the microwave energy device and utilizing the calibration parameter in a subsequent energy delivery. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a functional block diagram of a microwave energy delivery, measurement and control system in an energy delivery mode according to an embodiment of the present disclosure; 
           [0021]      FIG. 2  is a state machine functional block diagram of the microwave energy delivery, measurement and control system of  FIG. 1 ; 
           [0022]      FIG. 3  is a switch control state machine for the microwave energy delivery, measurement and control system including a precision network analyzer; 
           [0023]      FIG. 4 . is a functional block diagram of a precision network analyzer including passive and active measurements; 
           [0024]      FIG. 5  is a functional block diagram of a microwave energy delivery, measurement and control system including an impedance tuner; 
           [0025]      FIG. 6  is a switch control state machine for the microwave energy delivery, measurement and control system including a precision network analyzer, CPU and a tuner; 
           [0026]      FIG. 7  is a functional block diagram of a microwave energy delivery, measurement and control system according to another embodiment of the present disclosure; 
           [0027]      FIG. 8A  is a schematic representation of an ablation device for use in calibrating the microwave energy delivery, measurement and control system of the present disclosure; 
           [0028]      FIG. 8B  is a cross-sectional schematic representation of the ablation device and switching mechanism for calibrating the microwave energy delivery device; 
           [0029]      FIG. 8C  is an electrical schematic of the switching mechanism of  FIG. 8B ; 
           [0030]      FIG. 9A  is a schematic representation of a stand-alone calibration device for use in calibrating the microwave energy delivery, measurement and control system of the present disclosure; and 
           [0031]      FIG. 9B  is a schematic representation of a interfacing calibration device for use in calibrating the microwave energy delivery, management and control system of the present disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0032]    Detailed embodiments of the present disclosure are described herein; however, it is to be understood that the disclosed embodiments are merely exemplary and may be embodied in various forms. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present disclosure in virtually any appropriately detailed structure. 
         [0033]    Referring to  FIG. 1 , a Microwave Research Tool (MRT) including a measurement and control system for use in performing a medical procedure or medical procedure testing, employing embodiments of the present disclosure is generally designated  100 . MRT  100  may provide all the functionality of a microwave generator typically used to deliver microwave energy in a medical procedure but with improved functionality as described herewithin. MRT  100  includes individual components, as illustrated in  FIG. 1 , or the functionality of individual components may be combined or included in one or more components. Components are interconnected with suitable cables and/or connectors. 
         [0034]    MRT  100  includes a microwave energy delivery system, a measurement system and a supervisory control system. Each system is described individually although each system may share common components as discussed hereinbelow. 
         [0035]    The microwave energy delivery system includes a signal generator  105  capable of generating and supplying a high frequency microwave signal to an amplifier  110 . Signal generator  105  may be a single frequency generator or may include variable frequency capability. Signal generator  105  may also be capable of providing a signal including two or more frequencies wherein the device under test  115  (DUT) resonates at two or more frequencies. Supervisory control system may control various aspects of the signal generator  105  such as, for example, the signal delivery timing, the frequency (or frequencies) of the output and the phase of the signal. 
         [0036]    Amplifier  110  receives and amplifies the signal from the signal generator  105  to a desirable energy level. Amplifier  110  may be a single or multi-stage amplifier  110  and may include one or more signal conditioning circuits or filters (not shown) such as, for example, a low, high or bandpass circuits. Amplifier  110  gain may be fixed or controlled by a suitable controller, such as, for example, a control algorithm in the supervisory control system, a central processing unit  120  (CPU) or by manual adjustment (not shown). 
         [0037]    Amplifier  110  supplies a continuous, amplified microwave signal to a hot switch relay  125 . Hot switch relay  125  is controlled by the supervisory control system or CPU  120  and switches the amplified microwave signal to one of an amplifier burn-off load resistor  130  and a circulator  135 . The hot switch relay  125  in Position A delivers energy to the DUT  115  through the circulator  135 . The hot switch relay  125  in Position B delivers energy away from the DUT  115  and into an amplifier burn-off load resistor  130 . 
         [0038]    Hot switch relay  125  may be any suitable solid-state high power switch capable of switching a high power microwave energy signal. Hot switch relay  125  receives the high power microwave signal from the signal generator  105  and amplifier  110 , and passes the signal between the amplifier burn-off load resistor  130  or the circulator  135  without powering down the signal generator  105  or amplifier  110 . One suitable device is a JFW 50S-1552-N, which is a 150 watt 915 MHz dual pole single-throw solid-state switch that can be powered by two DC supply lines and controlled with a single TTL signal line from a supervisory control system or CPU  120 . In use, the JFW 50S-1552-N allows the MRT  100  to provide near instantaneous power (i.e. can provide nearly continuous power with very rapid on/off capabilities) without creating amplifier transients, by eliminating the need to power down the signal generator  105  or amplifier  110 . 
         [0039]    At times, the MRT may provide two sources of electrical isolation between the microwave energy signal and the measurement devices. For example, the first source of electrical isolation may be provided by the electrical isolation in the hot switch relay  125  between the output of Position A and the output of Position B. This electrical isolation prevents unacceptable amounts of energy from the high power microwave energy signal from being passed to the Position A output and to the measurement system connected thereto. For example, at 915 MHz the JFW 50S-1552-N switch (discussed above) provides about 45 dB of electrical isolation between outputs. The second source of electrical isolation is provided by the transfer switch  140  and the electrical isolation between Port 4 and Port 2 of the transfer switch  140  discussed hereinbelow. 
         [0040]    Continuous operation of the signal generator  105  and amplifier  110  prevents the introduction of amplifier  110  transients into the microwave energy delivery system. To maintain continuous operation, the switching time between Positions A and B on the hot switch relay  125  should be sufficiently fast to allow continuous operation of the signal generator  105  and amplifier  110 . For example, at 915 MHz the JFW 50S-1552-N switches between Position A and B in about 360 ns and between Positions B and A in about 370 ns. 
         [0041]    Amplifier burn-off load resistor  130  may be any suitable coaxial terminator capable of dissipating microwave energy while generating a minimal amount of VSWR, or reflective energy, over the bandwidth of the signal generator  105 . One such device is a 1433-3 50-ohm 250-watt coaxial terminator sold by Aeroflex/Weinschel and intended for operation over the bandwidth of DC to 5 GHz. Over the entire bandwidth of the 1433-3 the VSWR is less than 1.1. 
         [0042]    Circulator  135  is a passive three port device that eliminates standing waves between the hot switch relay  125  and the transfer switch  140 . Circulator  135  passes signals received on Port A to Port B, signals received on Port B to Port C and signals received on Port C to Port A. When hot switch relay  125  is in Position A, the microwave energy signal is passed from Port A of the circulator  135  to the transfer switch  140  connected to Port B. Reflected energy from the transfer switch  140  or the DUT  115 , received on Port B, is passed to Port C and dissipated through the reflected energy burn-off load resistor  142 . Reflected energy burn-off load resistor  142  is similar in function to the amplifier burn-off load resistor  130  as discussed hereinabove. 
         [0043]    Hot switch relay  125  and transfer switch  140 , when switching from Positions A to Positions B, appears as open circuits to the circulator  135 . During and after switching occurs, the circulator  135  clears the system of any residual power left in the system by directing the residual power into the reflected energy burn-off load resistor  142 . 
         [0044]    In addition, when hot switch relay  125  switches from Position A to Position B energy from dual directional coupler  145  and the DUT  115  is directed through the transfer switch  140 , to the circulator  135  and is dissipated by the reflected energy burn-off load resistor  142 . With the hot switch relay  125  and the transfer switch  140  both in Position B the MRT  100  connects to the DUT  115  and performs active measurements thereof. Interaction between the hot switch relay  125 , the transfer switch  140  and active testing of the DUT  115  is further described hereinbelow. 
         [0045]    Transfer switch  140  provides sufficient electrical isolation between the measurement system and the microwave energy delivery system. In Position A, the high power microwave energy signal is received on Port 4, passed to Port 3 and to the directional coupler  145 . The precision network analyzer  150 , connected to Port 2 of the transfer switch  140 , connects the transfer switch load resistor  155  on Port 1. In Position B, energy received on Port 4 is passed to Port 1 and dissipated by the transfer switch load resistor  155 , and the precision network analyzer  150  on Port 2 is connected to through Port 3 to the directional coupler  145  and the DUT  115 . The transfer switch  140  maintains electrical isolation between Ports 4 and 2 (and electrical isolation between the high power microwave energy and the precision network analyzer  150 ) regardless of the transfer switch  140  position. 
         [0046]    In operation, microwave energy is switched to the amplifier burn-off load resistor  130  by the hot switch relay  125  before the transfer switch  140  switches from Position A to Position B. As such, the transfer switch  140  does not operate as a “hot switch” because it is not under a load from the signal generator  105  or amplifier  10  when switching occurs. 
         [0047]    One suitable device that may be used as a transfer switch  140  is a TNH1D31 coaxial transfer switch sold by Ducommun of Carson Calif. The TNH1D31 displays less than 1.05 VSWR, better than 0.1 dB insertion loss and less than 80 dB electrical isolation for all states at 915 MHz. The hot switch relay  125  switches out the high energy microwave energy signal before the transfer switch  140  transitions, therefore, transition times for the transfer switch  140  are not critical. High-to-low transition times for the TNDH1D31 are about 75 ms and low-to-high transitions times are about 25 ms. 
         [0048]    Directional coupler  145  may be configured to operate like most conventional directional couplers known in the available art. As illustrated in  FIG. 1 , directional coupler  145  passes the high power microwave energy signal received on Port 1 to Port 2 with minimal insertion loss. Energy reflected back from the DUT  115  and received on Port 2 of the directional coupler  145  is passed through the transfer switch  140  to Port B of the circulator  135 . Energy received from the transfer switch  140  on Port B of the circulator  135  is passed to Port C of the circulator  135  and dissipated by the reflected energy burn-off load resistor  142 . 
         [0049]    Directional coupler  145  samples a small portion of each of the signals received on Port 1 and Port 2 and passes a small portion of the signals to Ports 3 and 4, respectively. The signals on Port 3 and 4 are proportional to the forward and reverse power, respectively. The measurement system measures the signal samples and provides the measurements to the supervisory control system. 
         [0050]    Directional coupler  145  samples a small portion of each of the signals received on Port 1 and Port 2 and passes a small portion of the signals to Ports 3 and 4, respectively. The signals on Port 3 and 4 are proportional to the forward and reverse power, respectively. The measurement system measures the signal samples and provides the measurements to the CPU  120 . The forward and reverse power measurements from the directional coupler  145  are passively measured and the samples may be taken continuously or at a periodic sample frequency. Unlike the broadband scattering parameter measurements, the directional coupler  145  measurements are indirect measurements of the delivered energy. As such, the measurements from the directional coupler  145  are limited to the bandwidth of the microwave energy supplied to the ablation device  115  from the signal generator  100  (i.e., feedback is fixed to the frequency of the high power microwave energy signal). A single frequency measurements, or narrowband measurement, can be used to calibrate amplitude and phase at a single frequency. By calibrating and/or compensating for the return loss to the antenna feedpoint and phase for ‘open’ or ‘short’ we are able to obtain a characteristic representation of the antenna&#39;s behavior (i.e., a Smith Chart representation of the antenna behavior). 
         [0051]    One suitable directional coupler  145  is a directional coupler sold by Werlatone of Brewster, N.Y. The directional coupler  145  may be a 40 dB dual directional coupler with 30 dB directivity and less than 0.1 dB insertion loss from 800 MHz to 3 GHz. 
         [0052]    DUT  115  includes a microwave ablation device that connects to Port 2 of the directional coupler  145  and may be any suitable microwave device capable of delivering microwave energy to tissue. DUT  115  may also include the tissue or surrounding medium in which the microwave ablation device is inserted or deployed. 
         [0053]    Supervisory control system includes a central processor unit  120  (CPU) capable of executing instructions and/or performing algorithms, configured to receive one or more inputs and may be configured to control one or more devices in the MRT  100 . Inputs may include analog inputs, such as, for example, signals from the forward and reverse coupling ports, Port 3 and Port 4 of the directional coupler  145 , respectively. Inputs may also include digital inputs, such as, for example, communication with one or more devices (i.e., precision network analyzer  150 ). 
         [0054]    CPU  120  may control one or more components of the MRT  100 . The signal generator  105  may receive at least one of an enabled/disabled control signal from the CPU  120  and reference signal. Enable/disable control signal indicates that the MRT system is in a condition to receive a microwave signal (i.e., the hot switch relay  125  and/or the transfer switch  140  are in a suitable position to receive a microwave signal). Reference signals may include the desired microwave frequency and a gain setting. CPU  120  may also provide control signals to the precision network analyzer  150 . 
         [0055]    The functionality of the measurement system may be performed in the CPU  120  and the precision network analyzer  150 . As illustrated in  FIG. 1 , the CPU  120  receives the passive inputs of power measurements (i.e., forward and reflected power signals from the directional coupler  145 ) and the precision network analyzer  150  performs active measurements of the DUT  115 . 
         [0056]    The measurement system may include other inputs, such as, for example, temperature sensors, cooling fluid temperature or flow sensors, movement sensors, power sensors, or electromagnetic field sensors. For example, an array of temperature sensors (not shown) configured to measure tissue temperature surrounding the DUT may be connected to the CPU  120  or the precision network analyzer  150 . Tissue temperatures may be used to generate an estimation of an ablation size or to generate an alarm or fault condition. Cooling fluid temperature or flow sensors may be used to indicate proper operation of a cooled DUT  115 . In another embodiment, the CPU  120  or precision network analyzer  150  may include all of the functionality of the supervisory control system, measurement system or any combination thereof. For example, in another embodiment of the present disclosure, as disclosed hereinbelow, the precision network analyzer  150  may receive the passive inputs, performs the active measurements and then report information to the supervisory system. 
         [0057]    In yet another embodiment, the precision network analyzer  150  is part of a modular system, such as, for example, a PXI system (PCI eXtensions for Instrumentation) fold by National Instrument of Austin, Tex. A PXI system (not shown) may include a chassis configured to house a plurality of functional components that form the MRT  100  and connect over a CPI backplane, across a PCI bridge or by any other suitable connection. 
         [0058]    Precision network analyzer  150  of the measurement system may connect to Port 2 of the transfer switch  140 . Precision network analyzer  150  may be any suitable network analyzer capable of performing scattering parameter measurements of the DUT and/or determining loss information for transmission system. Alternatively, precision network analyzer  150  may be a computer or programmable controller containing a module, program or card that performs the functions of the precision network analyzer  150 . 
         [0059]    In the embodiment in  FIG. 1 , precision network analyzer  150  is a stand-alone device or member that is in operative communication with transfer switch  140  and/or CPU  120 . In another embodiment, the functionality of the precision network analyzer  150  may be an integral part of the supervisory control system (i.e., a function of the CPU  120 ). 
         [0060]    Precision network analyzer  150  may function in a fashion similar to most conventional network analyzers that are known in the available art. That is, precision network analyzer  150  may determine various properties that are associated with the energy delivery system of the MRT  100 , such as, for example, the transmission line, the DUT  115  or the medium surrounding the DUT  115  (i.e., tissue). More particularly, the precision network analyzer  150  determines at least one property or conditions associated with increases in reflected energy (i.e., properties that can be correlated to reduction in energy transmission or decreases in overall system efficiency, such as, a change in the characteristic impedance (Z o ) of at least a portion of the microwave energy delivery system). One suitable precision network analyzer  150  is a four port precision network analyzer sold by Agilent of Santa Clara, Calif. 
         [0061]    Precision network analyzer  150  may connect to the transfer switch  140  through an attenuator  160  or other suitable protection device. In another embodiment attenuator  160  may scale the signal from the transfer switch  140  to one of a suitable power, current and voltage level. 
         [0062]    Attenuator  160  may be a limiting device, such as, for example, a fuse-type device that opens a circuit when a high power signal is detected. Limiting device may appear transparent to the precision network analyzer  150  until the limiting device is hit with a high power signal. One such device is a power limiter sold by Agilent of Santa Clara, Calif., that provides a 10 MHz to 18 GHz broadband precision network analyzer input protection from excess power, DC transients and electrostatic discharge. The attenuator  160  limits RF and microwave power to 25 dBm and DC voltage to 30 volts at 25° C. at 16 volts at 85° C. with turn-on times of less than 100 picoseconds. 
         [0063]    Limiting device may function as one of a fuse and a circuit-breaker type device. Fuse device may need to be removed and replaced after failure while a circuit-breaker type device may include a reset that reinitializes the circuit breaker after a failure. Reset may be a manual reset or MRT  100  may include a reset circuit that is initiated and/or performed by the supervisory control system or the like. 
         [0064]    In an energy delivery mode, as illustrated in  FIG. 1 , the MRT  100  is configured to delivery energy to the DUT  115 . The microwave energy signal from the signal generator  105  and amplifier  110  passed between the hot switch relay  125  in Position A, the circulator  135 , the transfer switch  140  in Position A, the directional coupler  145  and the DUT  115 . The measurement system (i.e., the CPU  120 ) passively measures forward and reflected energy at Port 3 and 4 of the dual directional coupler  145 . The precision network analyzer  150  is electrically isolated from the high energy microwave signal by the transfer switch  140 . 
         [0065]    In another embodiment of the present disclosure, electrical isolation between the ports of the transfer switch  140  allows a portion of the signal at Ports 3 and 4 to pass to Ports 1 and 2 wherein the passed signal is proportional to the high energy microwave signal from the signal generator  105  and amplifier  110 . The energy of the passed signal is either sufficiently attenuated by the transfer switch  140  to prevent damage the precision network analyzer  150  or the precision network analyzer  150  may be protected from excessive energy, (i.e., transients and current or voltage spikes) by the attenuator  155 , or alternatively, a limiter. The passed signal is shunted to a matched or a reference load and dissipated, through the transfer switch load resistor  155  connected to Port 1 and measured at Port 2 by the precision network analyzer  150 . 
         [0066]    Precision network analyzer  150  may be configured to passively measure the forward and reflected voltages from the directional coupler  145  and the energy waveform from transfer switch  140 . Power parameters, including the magnitude and phase of the microwave signal, may be obtained or calculated from the measured signals, by conventional algorithms or any suitable method known in the available art. In one embodiment, the forward and reflected measurements of power and phase can be used to determine impedances and admittances at a given frequency using a Smith Chart. 
         [0067]    In another embodiment, the impedance at the MRT  100  may be calculated as follows: First, the forward and reflected voltages, V fwd  and V ref , respectively, are measured. Then, the voltage standing wave ratio (V SWR ) may be calculated using the equation: 
         [0000]    
       
         
           
             
               V 
               SWR 
             
             = 
             
               
                 
                   V 
                   fwd 
                 
                 + 
                 
                   V 
                   ref 
                 
               
               
                 
                   V 
                   fwd 
                 
                 - 
                 
                   V 
                   ref 
                 
               
             
           
         
       
     
         [0068]    The magnitude of the load impedance (Z L ) may be determined by first computing the reflection coefficient, Γ, from V SWR  using the following equation: 
         [0000]    
       
         
           
             
                
               Γ 
                
             
             = 
             
               
                 
                   V 
                   SWR 
                 
                 - 
                 1 
               
               
                 
                   V 
                   SWR 
                 
                 + 
                 1 
               
             
           
         
       
     
         [0069]    Then, based on intrinsic system impedance, the load impedance Z L  is: 
         [0000]    
       
         
           
             
               Z 
               L 
             
             = 
             
               
                 
                   Z 
                   0 
                 
                  
                 
                   ( 
                   
                     1 
                     + 
                     Γ 
                   
                   ) 
                 
               
               
                 ( 
                 
                   1 
                   - 
                   Γ 
                 
                 ) 
               
             
           
         
       
     
         [0070]    Phase must be determined by the measured phase angle between the forward and reflected signals. 
         [0071]    Those skilled in the relative art can appreciate that the phase may be determined with calibrated or known reference phases (e.g., measurements with a short or open at the antenna feedpoint) and with measured values of V fwd  and V ref . The magnitude and the phase of Z L  can then be communicated or relayed to the supervisory control system that may be designed to make adjustments to the MRT as discussed hereinbelow. 
         [0072]      FIG. 2  displayed the MRT system state machine  200 . The six states, defined as State S, State C and States  1 - 4 , show the various states of the MRT  100  in  FIG. 1  and are designated as  210 - 260 , respectively. The operating states of the MRT  100  of  FIG. 1  are determined by the position of the two switches, the hot switch relay  125  and the transfer switch  140 , and the previous operating state of the MRT  100 . In use, the operation of the MRT  100  flows between the six states. Multiple states end in the same switch orientation but are shown as different states to illustrate a unique control sequence. The utility of each state during the ablation cycle are described hereinbelow. 
         [0073]    State S  210  is the Standby State  210  of the MRT. When power is removed both switches  125 ,  140  default to this condition, therefore, this condition is also the failsafe position (i.e., the default condition when power is removed or on power failure directs energy away from the patient or medical personnel). As such, the system provides for safe operation in the case of power failure, fault detection or when the system is not in use. A failsafe Standby State  210  also ensures that on startup, transient power spikes or other potentially dangerous power surges from the amplifier  110  are directed into the amp burn-off matched load resistor  130  thereby protecting equipment downstream from the hot switch relay  125 . 
         [0074]    State C  220  is the Calibration State  220  of the MRT. During the Calibration State  220  the hot switch relay  125  directs microwave power from the signal generator  105  and amplifier  110  to the amp burn-off load resistor  130  and the transfer switch  140  connects the precision network analyzer  150  to the DUT  115 . One or more calibrations are performed during this state. In one first calibration the precision network analyzer  150  may be calibrated to the DUT  115  reference plane, through the attenuator  160 , transfer switch  140  and directional coupler  145 , for broadband scattering parameter measurements. A second calibration may involve the measurement of line attenuation between the directional coupler  145  output ports and the DUT  115  reference plane. Determining line attenuation may require a second calibration value that may be obtained by replacing the DUT with an ‘open’ or ‘short’ at the exact reference path length. Alternatively, a second calibration value may be obtained by operating the antenna in air and comparing this value with a known value of the antenna operating in air. This attenuation value is used to calibrate power measurements at the directional coupler  145  to power delivered to the DUT  115 . An initial broadband scattering parameter measurement may be made during the Calibration State  220  to capture the DUT  115  impedance within uncooked tissue. 
         [0075]    State  1   130  begins post calibration or after State  4   260 . During State  1   130 , the transfer switch  140  is activated which connects the DUT  115  load to Port 2 of the circulator  140  and the precision network analyzer  150  to the terminal switch load resistor  155 . In State  1   230 , the only high power signal present in the system is flowing between the signal generator  105 , the amplifier  110 , the hot switch relay  125  in Position B and the amplifier burn-off resistor  130 . State  1   230  may include a delay to ensure that the transfer switch  140  has transitioned from Position B to Position A. A fault condition in State  1   230  returns the system to State S  210 , the Standby State  210 . 
         [0076]    State  2   240  begins after the transfer switch  140  has completed the transfer switch&#39;s  140  switching cycle in State  1   230 . A high control signal, delivered to the hot switch relay  125  from the CPU  120 , directs power from the signal generator  105  and amplifier  110  through the circulator  135 , transfer switch  140 , directional coupler  145  and into the DUT  115 . State  2   240  is the period during which an ablation is generated and generally represents the majority of system time. A fault condition in State  2   240  returns the system to State S  210 , the Standby State  210 . 
         [0077]    State  3   250  ends a period of power delivery to the DUT  115  in preparation for a precision network analyzer  150  scattering parameter measurement. A low signal is presented to the hot switch relay  125  directing power from the signal generator  105  and amplifier  110  into the amplifier burn-off load resistor  130 . A period of clear line wait time is added to the end of State  3  to allow the system to clear the circuit of high power signals. A fault condition in State  3  returns the system to State S, the Standby State  210 . 
         [0078]    State  4   260  is initiated after the clear line wait time at the end of State  3   250  expires. State  4   260  is initiated by activating the transfer switch  140 . Activation of the transfer switch  140  restores the system to the calibration configuration allowing the precision network analyzer  150  to perform broadband scatter parameter measurement of the DUT  115 . The only high power signals present in the system flow between the signal generator  105 , the amplifier  110 , the hot switch relay  125  and the amplifier burn-off load resistor  130 . After the precision network analyzer  150  completes a measurement cycle the system leaves State  4   260 , re-enters State  1   230 , and the MRT  100  repeats the cycle unless the ablation cycle has ended or a fault occurs, in which case the system enters State S  210 , the Standby State  210 . 
         [0079]    The MRT system state machine  200  essentially eliminates the risk of high power signals from potentially damaging sensitive microwave equipment, such as, for example, the precision network analyzer  150 . Additional switching and clear line delay times may be added into the system to ensure this safety aspect of the system architecture. 
         [0080]      FIG. 3  is a switch control state machine  300  for the microwave energy delivery, measurement and control system of the present disclosure. With reference to  FIG. 1 , the position of the hot switch relay  125  is indicated in the upper timing diagram of  FIG. 3  and the position of the transfer switch  140  is indicated in the lower timing diagram. A measurement period  310  includes an energy delivery period  320 , a clear line period  330 , a first transfer transient period  340 , a precision network analyzer sweep period  350  and a second transfer transient period  360 . The energy delivery period  320  is the period in which energy is delivered to the DUT  115  and initializes the start of a new measurement period  310 . The clear line period  330 , which follows the energy delivery period  320 , provides a delay in which the standing waves and transients in the system are allowed to dissipate through the circulator  135  and load  142  or the DUT  115 . The first transfer transient period  340  provides a delay to allow the transfer switch  140  to transition from Position A to Position B. The precision network analyzer sweep period  350  provides time for the precision network analyzer  150  to perform broadband scattering parameter measurements. The second transfer transient period  360  provides a delay to allow the transfer switch  140  to transition from Position B to Position A. 
         [0081]    The time intervals of the timing diagrams in the switch control state machine  300  of  FIG. 3  are not necessarily to scale. For example, if the system is providing a continuous waveform, the energy delivery period  320 , or the “on-time” in which microwave energy is delivered to the DUT  115 , is a majority of the measurement period  310 . The remaining portion of the measurement period  310 , or “off-time”, is split between the clear line period  330 , the first transfer transient period  340 , the precision network analyzer sweep period  350  and second transfer transient periods  360 . The clear line period  330  and the first and second transfer transient periods  340 ,  360  may be fixed in duration and based on the specific hardware used in the MRT system  100 . The precision network analyzer sweep period  350  is based on one or more sampling parameters. Sampling parameters include the sweep bandwidth, the number of steps within the bandwidth, the number of samples taken at each step and the sampling rate. 
         [0082]    The clear line period  330  must be sufficient in duration to allow all transients in the system to dissipate after the hot switch relay  125  switches from Position A to Position B. Transient, such as, for example, standing waves or reflective energy, may “bounce” between components before eventually being dissipated or shunted by the reflected energy burn-off load resistor  142 , dissipated in the system  100 , or expended by the DUT  115 . For example, the hot switch relay  125  may switch from Position A to Position B in as little as about 360 ns, thereby leaving energy in the MRT  110  between the circulator  135  and the DUT  115 . The energy may be sufficiently high to damage the precision network analyzer  150  if energy is not dissipated. 
         [0083]    After switching occurs energy remains in the system for an amount of time. The amount of time is related to the cable length, or path distance, between the antenna and the hot switch relay  125 . For a typical system using conventional cables having a transmission line with a dielectric value (ε) of about 2, the signal speed is about 1.5 ns/ft for each direction. For example, a circuit and cable length of about 10 feet between the DUT and the switch, a signal traveling away from the hot switch relay  125  would travel once cycle, or the 20 feet between the hot switch relay  125 , the DUT  115  and back to the hot switch relay  125 , in about 30 ns. Without dissipating the standing waves, the signal may ringing, or remain in the system, for as many as 5 cycles between the hot switch relay  125  and the DUT  115 , or about 150 ns. Circulator may dissipate the standing waves to an acceptably low energy level in as little as one or two cycles between the DUT and the hot switch relay  125 . Transfer switch  140  remains in Position A until the energy has dissipated to acceptably low energy levels. 
         [0084]    In another embodiment of the present disclosure, the clear line period  330  is variable and determined by measurements performed by the precision network analyzer  150  or the CPU  120 . For example, measurements from the forward coupling port (Port 3) or the reverse coupling port (Port 4) of the directional coupler  145  may be used to determine if energy remains in the system. The hardware design, or at low microwave energy levels, the amount of transient energy remaining in the MRT  100  after the hot switch relay  125  transitions from Position A to Position B, may be minimal and may allow the clear line period  330  to be equal to, or about equal to, zero. 
         [0085]    First transfer transient periods  340  provide a delay before initiating the precision network analysis sweep  350 . The first transfer transient period  340  allows the transfer switch  140  to switch from Position A to Position B before the precision network analyzer  150  begins the broadband scattering parameter sweep. 
         [0086]    Second transfer transient period  360  provides a delay before the subsequent measurement period begins (i.e., the next energy delivery period). The second transfer transient period  360  allows the transfer switch  140  to switch from Position B to Position A before the hot switch relay  125  transitions from Position B to Position A and energy delivery to the DUT  115  resumes. 
         [0087]    During the precision network analyzer sweep  350 , the precision network analyzer  150  determines broadband small-signal scattering parameter measurements. The sweep algorithm, and the amount of time to perform the sweep algorithm, is determined by the specific control algorithm executed by the CPU  120 . Unlike the passive forward and reflected power measurements, the measurements taken during the precision network analyzer sweep period  350  are active measurements wherein the precision network analyzer  150  drives the DUT  115  with a broadband signal and measures at least one parameter related to the signal (i.e., S 11 , reflection coefficient, reflection loss). The CPU  120  uses at least one of an active measurement taken by the network analyzer  350  during the broadband small signal scattering parameter measurements or a passive measurements from the directional coupler  145  in a feedback algorithms to control further energy delivery and/or subsequent MRT  100  operation. 
         [0088]    Energy delivery time, or “on-time”, as a percentage of the measurement period, may be adjusted. For example, the initial duration of the energy delivery may be based on historical information or based on at least one parameter measured during the calibration or start-up states,  220   210 , discussed hereinabove. The “on-time” may be subsequently adjusted, either longer or shorter, in duration. Adjustments in the “on-time” may be based on the measurements performed by one of the precision network analyzer  150  and the CPU  120 , from historical information and/or patient data. In one embodiment, the initial duration of an energy delivery period  320  in the ablation procedure may be about 95% of the total measurement period  310  with the remaining percentage, or “off-time”, reserved for measurement (“on-time” duty cycle approximately equal to about 95%). As the ablation procedure progresses, the “on-time” duty cycle may be reduced to between 95% and 5% to reduce the risk of producing tissue char and to provide more frequent measurements. The “off-time” may also be used to perform additional procedures that provide beneficial therapeutic effects, such as, for example, tissue hydration, or for purposes of tissue relaxation. 
         [0089]    In another embodiment of the present disclosure, as illustrated in  FIG. 4 , the MRT  400  includes a signal generator  405 , a microwave amplifier  410 , a directional coupler  445 , a transfer switch  440 , an attenuator  455 , a precision network analyzer  450  and a DUT  415 . In the present embodiment, the precision network analyzer  450  performs active and passive measurements of various system parameters of the MRT  400 . 
         [0090]    MRT  400  includes a signal generator  405  and amplifier  410  to generate and supply a high energy microwave signal to the directional coupler  445 . In an energy delivery mode the directional coupler  445  passes the signal to Port 2 of the transfer switch  440  and the transfer switch  440  passes the signal to the DUT  415  through Port 3. In a measurement mode, the high energy microwave signal is passed to a terminator  155  connected to Port 1 of the transfer switch  440 . Precision network analyzer  450  connects the first and second passive ports  451 ,  452  to the forward and reflected power ports, Ports 3 and 4, of the directional coupler  445 , respectively. The active port  453  of the precision network analyzer  450  connects to Port 4 of the transfer switch  440 . Precision network analyzer  450  may connect to Port 4 of the transfer switch  440  through a suitable attenuator  455  as illustrated in  FIG. 4  and discussed hereinabove. 
         [0091]    In an energy delivery mode, the precision network analyzer  450  of the MRT  400  passively measures forward and reflected power of the high energy microwave signal from the forward and reflected power ports, Ports 3 and 4, respectively, of the directional coupler  445 . 
         [0092]    In a measurement mode, the precision network analyzer  450  of the MRT  400  actively performs broadband scattering parameter measurements by connecting to the DUT  415  through Ports 3 and 4 of the transfer switch  440 . The precision network analyzer  450  drives the DUT  415  with a signal at a range of frequencies and measures at least one parameter related to the DUT  415  at a plurality of frequencies. 
         [0093]    Transfer switch  440  may be a single-pole, dual-throw coaxial switch that provides sufficient electrical isolation between Port 2 and Port 4 of the transfer switch  440  thereby preventing the high energy signal from damaging the precision network analyzer  450  in either the energy delivery mode, the measurement mode and while switching therebetween. Attenuator  455  provides sufficient signal attenuation to prevent the high energy signal from damaging the precision network analyzer  450 . Alternatively, attenuator may be a limiting-type device as discussed hereinabove. 
         [0094]    In yet another embodiment of the present disclosure, as illustrated in  FIG. 5 , the MRT  500  includes a tuner  565  positioned between the dual directional coupler  545  and the DUT  515 . The tuner  565  may be a tuning network or tuning circuit configured to match the impedance of the delivery system with the impendence of the DUT  515  or, alternatively, the tuner  565  is configured to match the impedance of the DUT  515  to the impedance of the delivery system. Tuner  565  may include a variable stub tuning network, a diode network or any other automated tuning network or circuit capable of high power operation and having the ability to match the DUT  565  impedance variations to the MRT  500  system impedance over the cooking cycle. 
         [0095]    In calculating a tuner adjustment, the CPU  520  characterizes the tuner  565  and removes the tuner  565  from the signal measured in the active measurement portion of the measuring cycle. 
         [0096]    Tuner  565  may be incorporated into the DUT  515  wherein the CPU  520  directs the tuner  565  to actively changes one or more properties of the antenna (not shown) in the DUT  515  such that the antenna impedance appears to be about equal to a characteristic impedance, e.g. 50 Ohms. For example, the CPU  520  may instruct the tuner  565  to change the effective antenna length or change one or more dielectric properties. 
         [0097]    The CPU  520  may use feedback from the measurement system to optimize energy delivery to the DUT  515  during at least a portion of the ablation procedure. Optimization may include: changing the frequency of the delivered microwave energy to better match the impedance of the DUT  515 , using the tuner  565  to adjust the output impedance of the MRT  500  to match the impendence of the DUT  515  or a combination thereof. 
         [0098]    In one embodiment the supervisory control system uses a forward power measurement from directional coupler  545 , a reverse power measurement from the directional coupler  545 , or one or more broadband scattering perimeter measurements to optimize energy delivery. 
         [0099]      FIG. 6  is a switch control state machine  600  for the microwave energy delivery, measurement and control system  500  illustrated in  FIG. 5 . The position of the hot switch relay  525  is indicated in the upper timing diagram and the position of the transfer switch  540  is indicated in the lower timing diagram. A measurement period  610  includes an energy delivery period  620 , a clear line period  630 , a first transfer transient period  640 , a measurement, CPU processing and tuner control period  650  and a second transfer transient period  660 . The clear line period  630  is after the energy delivery period  620  and provides a delay in which the standing waves and transients in the MRT  500  are allowed to dissipate. The first transfer transient period  640  provides a delay to allow the transfer switch  540  to transition from Position A to Position B. The measurement, CPU processing and tuner control period  650  allows the precision network to perform broadband scattering parameter measurements, perform control algorithms in the CPU and to perform adjustments to system tuning. The second transfer transient period  660  provides a delay to allow the transfer switch  540  to transition from Position B to Position A. 
         [0100]    The time intervals of the timing diagrams in the switch control state machine  600  of  FIG. 6  are not to scale. For example, the energy delivery period  620 , or “on-time” in which microwave energy is delivered to the DUT  515 , is typically equal to a majority of the measurement period  610 . The remaining portion of the measurement period, or “off-time”, is split between the clear line period  630 , the first transfer transient period  640 , the measurement, CPU processing and tuner control period  650  and second transfer transient periods  660 . The clear line period  630  and the first and second transfer transient periods  640 ,  660 , respectively, may be fixed in duration and based on specific hardware in the system. The measurement, CPU processing and tuner control period  650  is based on the sampling parameter, processing time or tuner control time. Sampling parameters include the sweep bandwidth, the number of steps within the bandwidth, the number of samples taken at each step and the sampling rate. The CPU processing includes the execution of the tuner algorithm and the tuner control time includes a frequency adjustment, a tuner adjustment or any related system settling time. 
         [0101]    The clear line period  630  must be sufficient in duration to allow all transients in the system to dissipate after the hot switch relay  625  switches from Position A to Position B. Transient, such as, for example, standing waves or reflective energy, may “bounce” between components before eventually being dissipated or shunted through the reflected energy burn-off load resistor  642 , dissipated in the system, or expended by the DUT  615 . For example, the hot switch relay  625  may switch in from Position A to Position B in as little as about 360 ns, thereby leaving energy in the circuit between the circulator  635  and the DUT  615 . The energy present in the MRT  500  circuitry and the DUT  515  may be sufficiently high to damage the precision network analyzer  550 , therefore, the transfer switch  540  remains in Position A until the energy has dissipated to acceptably low energy levels. As discussed hereinabove, the amount of time for the energy to dissipate is dependent on the circuit and cable length in which the standing waves must travel. In one embodiment (dielectric value, ε,=2) the length of time is equal to: 
         [0000]      dissipation time=(2×distance*1.5 ns/ft)*safety factor; 
         [0000]    wherein the distance equals the circuit length plus the cable length, safety factor equals 2 or 3 and the speed of 1.5 ns/ft is based upon approximately ε r =2 for typical transmission line cables 
         [0102]    In another embodiment of the present disclosure, the clear line period  630  is variable and determined by the precision network analyzer  550  or the CPU  520  measurements. For example, measurements from the forward coupling port (Port 3) and the reverse coupling port (Port 4) of the directional coupler  545 , may be used to determine if energy remains in the system. The hardware design, or at low microwave energy levels the amount of transient energy remaining in the system after the hot switch relay  625  transitions from Position A to Position B, may be minimal and may allow the clear line period to be equal to, or about equal to, zero. 
         [0103]    First transfer transient period  640  provides a delay before initiating the measurement, CPU processing and tuner control period  650 . The first transfer transient period  640  allows the transfer switch  540  to switch from Position A to Position B before the precision network  550  begins the broadband scattering parameter sweep. 
         [0104]    Second transfer transient period  360  provides a delay before the subsequent measurement period begins (i.e., the next energy delivery period). The second transfer transient period  660  allows the transfer switch  640  to switch from Position B to Position A before the hot switch relay  525  transitions from Position B to Position A and energy delivery to the DUT  515  resumes. 
         [0105]    During the measurement, CPU processing and tuner control period, the precision network analyzer  550  determines broadband small-signal scattering parameter measurements. The measurement algorithm is determined by the specific control algorithm used by the supervisory control system and is similar to the precision network analyzer sweep algorithm discussed hereinabove. The supervisory control system, or CPU  520 , the active measurements of the broadband small signal scattering parameter measurements or the passive measurements from the directional coupler  545  in a tuning algorithm. The tuning algorithm checks for the presence of a mismatch in impedance between the MRT  500 , the DUT 515 , and/or any combination thereof, and determines if an adjustment is necessary to correct the impedance mismatch. 
         [0106]    Energy delivery time, or “on-time”, as a percentage of the measurement period, may be adjusted. For example, the initial duration of the energy delivery may be based on historical information or based on at least one parameter measured during the calibration or start-up states,  220   210 , discussed hereinabove. The “on-time” may be subsequently adjusted, either longer or shorter, in duration. Adjustments may be based on the measurements performed by the precision network analyzer  550  and/or the CPU  510  or from historical information and/or patient data. In one embodiment, the initial duration of an energy delivery period in the ablation procedure may be about 95% of the total measurement period with the remaining percentage, or “off-time”, reserved for measurement (“on-time” duty cycle approximately equal to about 95%). As the ablation procedure progresses, the “on-time” duty cycle may be reduced to between 95% and 5% to reduce the risk of producing tissue char and to provide more frequent measurements. 
         [0107]    The “off-time” may also be used to perform additional procedures that provide beneficial therapeutic effects, such as, tissue hydration, or for purposes of tissue relaxation. For example, tuning algorithm may initiate a re-hydration of tissue to reduce tissue impedance instead of adjusting the frequency or re-tuning the MRT. 
         [0108]    Another embodiment of the MRT is illustrated in  FIG. 7  and is shown as MRT  700 . MRT  700  includes a variable attenuator  770  that replaces the hot switch relay  125  in the MRT  100  in  FIG. 1 . In  FIG. 7 , the MRT  700  includes a signal generator  705  that supplies a microwave frequency signal to the variable attenuator  770 . Variable attenuator  770  includes a variable network or circuit that scales the signal from the signal generator  705  between 0% and 100% and provides the scaled signal to the amplifier  710 . Amplifier  710  amplifies the signal by a fixed amount and provides the signal to the circulator  735 . 
         [0109]    The MRT  100  in  FIG. 1  controls the energy output (i.e., the power of the microwave signal) by adjusting the output of the signal generator  105  and/or the gain of the amplifier  110  (i.e., signal from the signal generator  105  amplified by the gain of the amplifier  710 ). In the MRT  700  of  FIG. 7 , the energy output is controlled by one or more of the signal generator  705 , the variable attenuator  770  and the amplifier  710 . The output energy of the MRT  700  in  FIG. 7  is equal to the signal generator  705  output scaled by variable attenuator  770  attenuation percentage and amplified by the gain of the amplifier  710 . 
         [0110]    With reference to the hot switch relay  125  in  FIG. 1  and the variable attenuator  770  in  FIG. 7 , Position A of the hot switch relay  125  is equivalent to the variable attenuator  770  is Position A (i.e., a scaling factor of 100%). In both  FIGS. 1 and 7 , Position A provides microwave energy to Port A of the circulator  135  and  735 , respectively. Similarly, Position B of the hot switch relay  125  is equivalent to the variable attenuator  770  in Position B (i.e., a scaling factor of 0%). Position B in both  FIGS. 1 and 7 , no microwave energy signal is provided to Port A of the circulator  135  and  735 , respectively. 
         [0111]    The hot switch relay  125  in the MRT  100  of  FIG. 1  includes a switch that switches between Position A and Position B and is capable of executing the transition in a minimum amount of time to prevent transients or spikes in the waveform. The variable attenuator  770  in the MRT  700  of  FIG. 7  may includes an automated variable attenuator, such as, for example, a rheostat-like circuit that does not switch but transitions between Position A and Position B thereby generating fewer transients compared to the switch in  FIG. 1 . 
         [0112]    Atenuator activation time would be added to the dissipation time calculation for safe switching and measurement. 
         [0113]    In yet another embodiment of the present disclosure, the DUT includes a MRT calibration device configured to measure the length of the transmission path from the antenna feedpoint to the directional coupler and each respective signal to the network analyzer.  FIG. 8  is a schematic representation of an ablation device for use in calibrating a microwave energy delivery, measurement and control system of the present disclosure. 
         [0114]    As is known in the art, calibration of a microwave energy delivery system may be preformed by various calibration procedures. For example, one of a Short-Open-Load (SOL), a Short-Open-Load-Thru (SOLT), a Short-Short-Load-Thru (SSLT) and a Thru-Reflect-Line (TRL) calibration technique may be used. 
         [0115]    In one embodiment the system is calibrated with a Short-Open (SO) calibration technique. The SO calibration provides a determination of the relative performance of the DUT. The Short-Open calibration technique is known in the art and is generally described hereinbelow. 
         [0116]    The first step of the SO calibration is preformed by running the microwave generator with a “short” at the output of the microwave generator (i.e., the coaxial cable connector). The second step of the SO calibration is preformed by running the microwave generator with the output of the microwave generator “open”. The two steps of the SO calibration, which is often referred to as “shifting a reference plane” allows the generator to analyze the system up to the output of the directional coupler. One shortcoming of performing this calibration by placing the “open” and the “short” at the output of the generator is that the calibration fails to account for any portion of the transmission line beyond the microwave generator. 
         [0117]      FIG. 8A  illustrates the output portion of a microwave generator  810  and a coaxial cable  820  that connects the microwave generator  810  to an MRT calibration device  800  of the present disclosure. The MRT calibration device  800  includes a transmission portion  830  and an antenna portion  840 . 
         [0118]      FIG. 8B  illustrates the transition between the transmission portion  830  and the antenna portion  840 . Switching mechanism  850  is located adjacent on the proximal portion of the antenna under test  840  and on the distal portion of the transmission portion  830  of the MRT calibration device  800 . Switching mechanism  850  allows the system to perform an SO calibration without replacing the DUT. 
         [0119]    Switching mechanism  850  is further illustrated in  FIG. 8C  and includes an open circuit switch  850   a , a short circuit switch  850   b  and a short circuit load  840   a.    
         [0120]    The switching mechanism  850  in the MRT calibration device  800  allows the reference plane to be shifted to a point proximal the antenna thereby accounting for a majority of the transmission path in the calibration procedure. An open circuit is first obtained by actuating the open circuit switch  850   a  to an open position thereby disconnecting the inner conductor  832  and outer conductor  834  from the antenna under test  815 . 
         [0121]    A short circuit between the inner conductor  832  and the outer conductor  834  through a short circuit load  840   a  is obtained by transition the short circuit switch  850   b  from Position A to Position B. The short circuit load  840   a  is a fixed load that replaces the antenna under test  815 . For example, in one embodiment the short circuit load  840   a  is an antenna with a feedpoint equivalent to the antenna under test  815  thereby providing a known antenna response that can be used to calibrate the antenna under test  815 . 
         [0122]    With the short circuit switch  850   b  in Position B the system yields a known phase and amplitude of the reflected energy at the antenna feed. The antenna under test  840  is replaced with a short circuit load  840   b  that may include an equivalent path-length and/or an equivalent antenna. Energy provided to the short circuit load  840   a  is reflected at the short circuit load  840   a  with a specific phase for the returned signal. 
         [0123]    In test, the short circuit load  840   a  returns energy at a first phase and the open returns energy at a second phase. The short circuit load  840   a  places a voltage minimum at the short and full standing waves at every λ/4 and 3λ/4 wavelengths on the transmission line proximal the short circuit load  840   a . The open circuit  850   a  places full standing waves at the open and every λ/2 wavelengths on the transmission line proximal the open circuit  850   a.    
         [0124]    Using known open or short parameters and the present open and short parameters the phase angle and returned power of the antenna may be determined. An active tuning circuit may use one or more of these parameters to determine one or more system tuning parameters. For example, an active tuning circuit may be placed in the generator, the handle of the microwave energy delivery device or any other suitable location. Active tuning circuit may determine a range of mismatch and/or provide one or more calibration parameters to the system or may properly calibrate to the antenna feedpoint. 
         [0125]    For example, the antenna and/or the tissue may be behaving inductively (i.e., 50Ω+20 Ωj wherein the positive 20 Ωj is inductive) or capacitively (i.e., 50Ω−20 Ωj wherein the negative 20 Ωj is inductive). Calibrating to the antenna feedpoint the system can identify if the antenna and/or tissue is behaving inductively or capacitively. As such, the system can incorporate a matching network to offset the impedance mismatch. 
         [0126]    In yet another embodiment of the present disclosure calibration is performed by placing the antenna  940  of a microwave energy delivery device  915  in a calibration apparatus  900 . Calibration apparatus  900  includes a chamber  910   a  configured to produce a known reflection and phase shift in an antenna  940   a  when the antenna  940   a  is placed adjacent the chamber  910   a . Calibration is performed by placing the antenna  940   a  in a fixed position relative to the chamber  910   a  and driving the antenna  940   a  with a predetermined signal. The microwave generator  905   a  measures one or more parameters indicative of the performance of the antenna  940   a  and compares the measured parameters with one or more predetermined parameters. The microwave generator  905   a  then determines one or more calibration parameters or one or more tuning parameters for the antenna  940   a  under test. 
         [0127]    Chamber  910   a  may be a cylindrical shaped chamber configured to receive the antenna  940   a . Chamber  910   a  may receive the distal end of the microwave energy delivery device  915   a , including the antenna  940   a , as illustrated in  FIG. 9A , or chamber  940   b  may be configured to receive the microwave energy delivery device  915   b , as illustrated in  FIG. 9B . A positioning mechanism or stop mechanism may provide consistent placement of the antenna in the chamber. Stopping mechanism may include a sensing mechanism to sense the placement in the chamber. Sensing mechanism may provide a signal to the system to indicate that the antenna is in position. System, after receiving the signal from the sensing mechanism, may be configured to switch to a test mode in which the system drives the antenna with a predetermined microwave signal. 
         [0128]    Calibration device  940   a  may be configured as a stand-alone device as illustrated in  FIG. 9A , configured to interface with the microwave energy delivery device (not shown), configured to interface with the microwave generator, as illustrated in  FIG. 9B  or any combination thereof. Calibration device  900   a  may be a passive device that provides a load on the antenna  940   a  wherein the antenna response  940   a  to the load  900   a  (the calibration device) is known to the microwave generator  905   a.    
         [0129]    With reference to  FIGS. 9A-9B , calibration device  900   a ,  900   b  may include a chamber  910   a ,  910   b  configured to receive at least a portion of the microwave energy delivery device  915   a ,  915   b . Chamber  910   a ,  910   b  may be configured to receive the antenna  940   a ,  940   b  or the antenna and a portion of the device transmission line  930   a ,  930   b . Chamber  910   a ,  910   b  is configured to position a microwave energy absorbing load relative to the antenna  940   a ,  940   b.    
         [0130]    In use, a clinician mates together the calibration device  900   a ,  900   b  and the microwave energy delivery device  915   a ,  915   b , respectively. The antenna  940   a ,  940   b  of the microwave energy delivery device  915   a ,  915   b  is positioned relative to calibration device  900   a ,  900   b , respectively, and a calibration procedure is performed. The calibration procedure may be initiated manually, by the clinician, via a microwave generator input  906   a ,  906   b  or interface screen  907   a ,  907   b  or by an input on the microwave energy delivery device (not shown). Alternatively, the calibration procedure may be automatically initiated by the microwave generator  905   b . For example, placement of the antenna  940   b  relative to the load in the calibration device  900   b  may trigger a sensor  901   b  or input to the microwave generator  905   b  (not shown) and a calibration procedure may be automatically initiated. 
         [0131]    In one embodiment, the calibration procedure includes the steps of driving the antenna with a microwave energy signal, measuring at least one parameter related to the antenna and generating at least one antenna calibration parameter. The microwave energy signal may be a predetermined signal, a signal selected by the clinician or a signal selected for the specific antenna. The one or more parameters related to the antenna may include one of forward power, reflected power, impedance and temperature. The at least one antenna calibration parameter is related to the operation of the antenna, such as, for example, a parameter related to antenna tuning, a parameter related to the resonance of the antenna, a parameter related to antenna construction or any other suitable parameter related to microwave energy delivery. 
         [0132]    Calibration device may be configured to interface with one of the microwave energy delivery device or the microwave generator. As illustrated in  FIG. 9B , calibration device  900   b  may connect to the microwave generator  905   b  via a cable  820   b . In another embodiment, the calibration device  900   b  may include a connector (not shown) that interfaces with the microwave energy delivery device  915   b  when mated together. Connection between the calibration device  900   b  and microwave generator  905   b  or microwave energy delivery device  915   b  may also be configured as a wireless connection. Connection may include one or more digital or analog connections or may include a suitable communication means, such as, for example, TCP/IP, OSI, FTP, UPnP, iSCSI, IEEE 802.15.1 (Bluetooth) or Wireless USB. Calibration device  900   b  may provide one or more parameters related to the calibration device  900   b  and/or the calibration procedure to one of the microwave energy delivery device  915   b  and the microwave generator  905   b.    
         [0133]    Calibration device  900   b  may further include a positioner  902   b  to position the microwave energy delivery device  915   b  in one or more positions relative to the calibration device  900   b . As illustrated in  FIG. 9B , positioner  902   b  aligns with notch  916   b  on the microwave energy delivery device  915   b  such that the calibration device  900   b  and microwave energy delivery device  915   b  mate in position. Positioner  902   b  and notch  916   b  are configured to position the antenna  940   b  in a desirable position relative to chamber  910   b . Positioner may be any suitable means of positioning the microwave energy delivery device  915   b  relative to the calibration device  900   b  such as, for example, a latch, a catch, a locking clam-shell, a clip, a locking or positioning pin, an unique shaped appendage and matching recessed portion configured to receive the appendage and any other suitable positioning device. 
         [0134]    Calibration device  900   b  may further include a locking mechanism  903 ,  904 ,  909  for locking the calibration device  900   b  to the microwave energy delivery device  915   b . As illustrated in  FIG. 9B , catches  904  align with slots  909  when chamber  910   b  is in a closed position. Slide  903  actuates catches  904  within the slots thereby locking the chamber in a closed position. Any suitable locking mechanism may be used such as, for example, a clip, a latch, a pressed fit pin, a locking or self-closing hinge, a magnetic or electronic closure mechanism or any other suitable locking mechanism. Slide  903  or other locking release mechanism may be configured to be disabled when the antenna  940   b  is activated thereby preventing the calibration device  900   b  from releasing the microwave energy delivery device  915   b  during calibration or energy delivery. 
         [0135]    As various changes could be made in the above constructions without departing from the scope of the disclosure, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense. It will be seen that several objects of the disclosure are achieved and other advantageous results attained, as defined by the scope of the following claims.