Patent Publication Number: US-10758306-B2

Title: Frequency identification for microwave ablation probes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 12/423,609, filed on Apr. 14, 2009, the entire disclosure of which is incorporated by reference herein. 
    
    
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates generally to microwave antennas used in tissue ablation procedures. More particularly, the present disclosure is directed to optimal frequency identification for microwave ablation antennas. 
     2. Background of Related Art 
     Treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors). It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, known treatment methods, such as hyperthermia therapy, heat tumor cells to temperatures above 41° C., while maintaining adjacent healthy cells at lower temperatures to avoid irreversible cell damage. Such methods involve applying electromagnetic radiation to heat tissue and include ablation and coagulation of tissue. In particular, microwave energy is used to coagulate and/or ablate tissue to denature or kill the cancerous cells. 
     Microwave energy is applied via microwave ablation antennas that penetrate tissue to reach tumors. There are several types of microwave antennas, such as monopole and dipole, in which microwave energy radiates perpendicularly from the axis of the conductor. A monopole antenna includes a single, elongated microwave conductor whereas a dipole antenna includes two conductors. In a dipole antenna, the conductors may be in a coaxial configuration including an inner conductor and an outer conductor separated by a dielectric portion. More specifically, dipole microwave antennas may have a long, thin inner conductor that extends along a longitudinal axis of the antenna and is surrounded by an outer conductor. In certain variations, a portion or portions of the outer conductor may be selectively removed to provide more effective outward radiation of energy. This type of microwave antenna construction is typically referred to as a “leaky waveguide” or “leaky coaxial” antenna. 
     Due to manufacturing tolerance limitations, each microwave antenna assembly has a unique optimal operational frequency. 
     SUMMARY 
     The present disclosure provides for a system and method that allow for identification of specific operational frequency of each microwave antenna assembly. The operational frequency may be encoded as a resistance value or in memory available for measurement or reading by a microwave ablation generator. The generator may then be configured to substantially match the operational frequency to the optimal frequency of each antenna assembly. Matching the output frequency to optimal frequency maximizes antenna assembly&#39;s efficiency and energy delivery to the target tissue, thereby improving ablation size and reducing the ablation time. Utilizing optimal frequency for each antenna assembly also reduces reflected energy from the assembly back to the generator, which in turn, reduces heating of the entire system. Further, the system and method of the present disclosure provide for an additional quality check of the antenna assembly during the manufacturing process, allowing for discarding of any antenna assemblies whose optimal frequency falls outside an expected deviation from a desired operational range. In addition, the system and method according to the present disclosure allow for use of antenna assemblies with a wider range of manufacturing tolerances, since the operational frequency may be used to tune the generator to a desired frequency that best matches the determined frequency of the antenna assembly. Without this tuning capability and using a fixed frequency generator, the optimal operating frequency for a given antenna might be outside the usable frequency range of the generator, which results in inefficient application of microwave energy. 
     According to one embodiment of the present disclosure, a microwave ablation system is provided. The system includes a microwave antenna assembly that includes an identification device configured to store an optimal frequency of the microwave antenna assembly. The system also includes a generator configured to couple to the microwave antenna assembly and to output microwave energy at an operational frequency. The generator is further configured to read the optimal frequency from the identification device and to configure the operational frequency to substantially match the optimal frequency. 
     According to another embodiment of the present disclosure, a microwave ablation antenna assembly is provided. The antenna assembly includes an identification device configured to store an optimal frequency of the microwave antenna assembly and a radiating section coupled to a hub having a cable adapted to couple the microwave antenna assembly to a generator configured to output microwave energy at an operational frequency. The generator is further configured to read the optimal frequency from the identification device and to configure the operational frequency to substantially match the optimal frequency. 
     A method for microwave ablation is also contemplated by the present disclosure. The method includes the steps of: immersing at least a portion of a microwave antenna assembly in a model tissue, determining optimal frequency of the microwave antenna assembly within the model tissue and recording the optimal frequency in an identification device associated with the microwave antenna assembly. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a schematic diagram of a microwave ablation system according to an embodiment of the present disclosure; 
         FIG. 2  is a schematic diagram of a system for determining optimal operational frequency of a microwave antenna assembly according to an embodiment of the present disclosure; 
         FIG. 3  is a plot of scattering parameter measurements for a plurality of microwave antenna assemblies; and 
         FIG. 4  is a flow diagram of a method for determining optimal operational frequency of a microwave antenna assembly according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     Particular embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail. 
     Microwave antenna assemblies are typically resonant structures, which operate most efficiently at a particular frequency. In other words, due to manufacturing tolerance limitations, each microwave antenna assembly has a unique optimal operational frequency. The present disclosure provides for a system and method for determining the operational frequency of the microwave antenna assembly and providing the optimal frequency to a microwave generator, which then adjusts output of the microwave energy accordingly to substantially match the optimal frequency. 
       FIG. 1  shows a microwave ablation system  10  that includes a microwave antenna assembly  12  coupled to a microwave generator  14  via a flexible coaxial cable  16 . The generator  14  is configured to provide microwave energy at an operational frequency from about 500 MHz to about 10,000 MHz. In the illustrated embodiment, the antenna assembly  12  includes a radiating section  18  connected by feedline  20  (or shaft) to the cable  16 , the radiating section  18  having a tip  48  at its distal end. More specifically, the feedline  20  is connected to a hub  22  which is connected to the cable  16  through a cable connector  17 . The hub  22  may have a variety of suitable shapes, e.g., cylindrical, rectangular, etc. In one embodiment, the feedline  20  may be formed from a coaxial, semi-rigid or flexible cable having a wire with a 0.047″ outer diameter rated for 50 Ohms. 
       FIG. 2  illustrates a system  100  for determining optimal operational frequency of the microwave antenna assembly  12 . The microwave antenna assembly  12  is inserted into a tissue model  110  at least to fully submerge the radiating section  18  therein. The tissue model  110  may be any suitable material that models targeted tissue of interest. In particular, the material may closely approximate complex dielectric properties of targeted tissue. The material may be actual tissue, e.g., liver tissue, muscle tissue, etc. or synthetic variant thereof (e.g., ceramic, tissue phantom, etc.). Tissue phantoms may be manufactured from various types of gels, e.g., hydrogel. 
     The system  100  also includes a network analyzer  120  configured to measure scattering parameters of the microwave antenna assembly  12 . The network analyzer  120  acts as a microwave generator and supplies a simulation pulse to the microwave antenna assembly  12 , to provide a simulation of operating parameters reflective of actual application of microwave energy to tissue. During simulated treatment, the scattering of the microwave energy is measured by the network analyzer  120 . In one embodiment, the network analyzer  120  measures a reflected signal of the measurement pulse, which is reflective of the scattering parameters of the microwave antenna assembly  12 , such as optimal frequency, electrical length, phase, and the like. Measurement of scattering parameters may be accomplished during production of the antenna assembly  12  (e.g., testing the response of the antenna assembly  12  in the model tissue  110  after assembly thereof). 
     As discussed above, individual microwave antenna assemblies  12  display varying scattering parameters, such as optimal operating frequencies.  FIG. 3  shows a plot of scattering parameter measurements for three different microwave antenna assemblies  12 , marked as a, b and c, respectively, and a simulation plot d. The plots illustrate resonant performance of each of the microwave antenna assemblies  12  in terms of decibels (shown from 0 to −40 dB to illustrate a reflected signal) of the reflected signal across a 1 GHz frequency range from about 0.8 GHz to about 1.8 GHz. A simulated graph d is also shown, which illustrates expected optimal frequency response from the microwave antenna assembly  12  at about 1.04 GHz with a signal strength response of about −20 dB. In comparison, the graphs a, b and c illustrate that the actual signal and frequency responses vary for each of the microwave antenna assemblies  12 . In particular, the graph a shows an optimal frequency response at about 1.04 GHz, the graphs b and c show optimal frequency responses at about 1.05 GHz, whereas graph b has a signal response similar to the graph a at about −27 dB and the graph c has a signal response similar to the graph c at about −33 dB. 
     As shown in  FIG. 3 , each microwave antenna assembly  12  has a specific operational frequency. To achieve maximum efficiency from the microwave antenna assembly  12 , it is desirable to supply microwave energy thereto at the predetermined operational frequency as determined by the network analyzer  120 . Conventionally, the microwave generator  14  supplies the energy at a stated frequency designated for an entire type (e.g., model) of the microwave antenna assembly without accounting for frequency variations between each specific microwave antenna assembly  12 . The present disclosure provides for a system and method to provide the predetermined optimal frequency of the microwave antenna  12  to the generator  14 , such that the generator  14  tunes the operational output frequency to the optimal frequency. 
     With reference again to  FIG. 1 , the antenna assembly  12  includes an identification device  50  disposed thereon for encoding the optimal frequency. More specifically, during production of the microwave assembly  12 , the frequency is determined as discussed above with respect to  FIG. 3 . The determined optimal frequency is then encoded in the identification device  50 . During operation, the identification device  50  is read by the microwave generator  14  to determine the optimal frequency, and the generator  14  then adjusts the output to suit that frequency. In one embodiment, the identification device  50  may also include other information, such as model number, energy delivery characteristics and physical characteristics (e.g., length of the radiating section  18 ) of the microwave antenna assembly  12 . This information may also be used by the generator  14  to adjust the output. 
     In one embodiment, the identification device  50  may be a storage device such as a microcontroller, microprocessor, non-volatile memory (e.g., EPROM), radio frequency identification tags. Information can be transmitted to the generator  14  via a variety of communication protocols (e.g., wired or wireless) between the microwave assembly  12  and the generator  14 . In this embodiment, the optimal frequency is stored in the storage device which is extracted by the generator  14  through a communication port (e.g., serial or parallel data bus). 
     In another embodiment, the identification device  50  may be any suitable identifier, such as optical, displacement, magnetic or electrical (e.g., conductance, resistance, capacitance, impedance) component. In this embodiment, the optimal frequency is encoded as a resistance, capacitance, etc. The generator  14  supplies an electrical current signal through the identification device  50 , which allows the generator  14  to measure the resistance or another electrical property of the identification device  50  and then determine the optimal frequency that corresponds to the measured resistance. The generator  14  may also include a storage device having a lookup table or a microprocessor adapted to process the resistance value to determine the corresponding optimal frequency. 
     In a further embodiment, the identification device  50  may be a barcode or another type of optically encoded storage device. The optimal frequency may be read by scanning the barcode using various types of barcode readers. The barcode may store the actual optimal frequency or a code associated therewith, which when read by the generator  14  may be then determined to correspond to the frequency. 
     Once the generator  14  determines the optimal frequency from the identification device  50 , the output of the microwave energy to the antenna assembly  12  is tuned to the optimal frequency. The generator  14  is a tunable microwave generator that may operate at a variable output frequency. The generator  14  may include a phase lock loop (PLL) to set an operational frequency to the optimal frequency. The PLL may be implemented as a digital or analog circuit. The PLL of the generator  14  controls the operational frequency throughout the procedure, maintaining the operational frequency within the desired range of the operational frequency. 
       FIG. 4  illustrates a flow diagram of a method for determining optimal operational frequency of a microwave antenna assembly  12 . In step  200 , the antenna assembly  12  is inserted into model tissue  110  and is also coupled to the network analyzer  120 . In step  210 , the network analyzer  120  provides one or more test pulses to the antenna assembly  12  to determine the optimal frequency thereof. In step  220 , the optimal frequency is recoded in the identification device  50  of the antenna assembly  12 . During use of the antenna assembly  12 , in step  230 , the optimal frequency is read from the identification device  50  by the generator  14 . In step  240 , the generator  14  configures the operational frequency thereof to match the optimal frequency of the antenna assembly  12  as read from the identification device  50 . 
     The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Various modifications and variations can be made without departing from the spirit or scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law.