Abstract:
A method and apparatus is disclosed for the ablation of biological tissue to produce a therapeutic effect. An adjustment is made to the output frequency of an electrosurgical generator during the ablation cycle to accommodate changes in the electrical properties of the apparatus and the biological tissues that occur as energy is transferred to the tissue. The adjustment is made to better align the source impedance with that of the load.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a system and related methods for employing electromagnetic energy in the microwave frequencies to produce a therapeutic effect on targeted tissue at a surgical site. In particular, the present disclosure relates to systems and methods of impedance matching to maximize energy delivered to target tissue. 
         [0003]    2. Background of Related Art 
         [0004]    Electromagnetic radiation may be used to heat cells to produce a therapeutic effect. For example, microwave energy has been used to selectively ablate certain types of cancerous cells found to denature at elevated temperatures slightly lower than temperatures normally injurious to healthy cells. Destroying cellular tissue in place may be less traumatic than removing it in a conventional surgery. Accordingly, a microwave ablation procedure may be an attractive option for many patients who are not good candidates for conventional surgery. 
         [0005]    To denature many malignant growths of cells, temperatures above about 41.5° C. should be achieved. However, because thermal damage to most normal types of cells is commonly observed at temperatures above about 43° C., caution must be taken not to exceed this value. While it is true that electromagnetic energy used in ablative treatments is rapidly dissipated to non-destructive levels by natural processes such as conduction and convection due to circulating fluids, the temperature range suitable for ablative treatment is small, so great care must be taken in the application of microwave energy. 
         [0006]    In a typical microwave ablation procedure, an antenna is positioned in the proximity of the tissue to be treated. For precise control, the antenna may be positioned directly inside the targeted tissue. A generator produces an electromagnetic oscillation, which may be transmitted over a coaxial transmission line to the antenna at its distal end. An electromagnetic field created by the antenna causes friction at a molecular level resulting in elevated temperatures in the vicinity thereof. 
         [0007]    One concern in the management of microwave energy is impedance matching. In order to maximize the power transferred from a source to a load, the output impedance of the source should equal the input impedance of the load. Failure to match impedances may result in standing waves on the transmission line due to reflections of the incident power. In the case of microwave tissue ablation, the source is often configured with impedance throughout the appropriate frequency range approximately equal to that of the load to be ablated, which for most human tissue is approximately 50 ohms. However, as the target tissue is ablated, heating of the transmission line components and changes in the electrical properties of the target tissue tend to vary the load impedance over time. When the load impedances change, a greater portion of the power is reflected and the performance of the antenna system is diminished. 
       SUMMARY 
       [0008]    In light of the foregoing, a need exists for ablation systems and methods not only for preliminarily matching source impedances to load impedances, but also for accommodating changing tissue impedances occurring during the ablation procedure. In one embodiment, a microwave antenna is positioned in close proximity to a targeted tissue. Microwave energy is delivered to the antenna at a particular frequency through a transmission line, and a power signal reflected by the targeted tissue is measured. The particular frequency of the microwave energy is adjusted to reduce the energy reflected. A trocar may be used to position the antenna through the skin of a patient, and an initial frequency adjustment may be made to effect a source impedance to about 50 ohms. An output power may be adjusted in combination with the frequency adjustment. The reflected power signal measured may be compared to a predetermined threshold and frequency adjustments may made only when the reflected power signal exceeds the threshold value. 
         [0009]    In another embodiment, a system for tissue ablation includes a microwave antenna configured for direct insertion into targeted tissue, a generator of microwave energy including a means for adjusting a frequency operatively connected to the antenna by a transmission line, a monitor operatively connected to the transmission line capable of sampling or measuring a reflected signal, and a means of communicating information concerning the reflected power to the generator. The monitor may include a dual directional coupler configured to sample at least one of the reflected signal and a forward signal. The means of communicating the reflected power information may include a visual display on an amplifier configured to amplify a signal output by a generator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate embodiments of the present disclosure and, together with the detailed description of the embodiments given below, serve to explain the principles of the disclosure. 
           [0011]      FIG. 1  is a perspective view of an exemplary embodiment of a microwave probe assembly; 
           [0012]      FIG. 2  is a perspective view with parts separated of the probe depicted in  FIG. 1 ; 
           [0013]      FIG. 3A  is a schematic diagram of an impedance matching circuit of the microwave ablation system of the present disclosure; 
           [0014]      FIGS. 3B through 3D  represent a simplified impedance matching circuit; 
           [0015]      FIG. 4  is a schematic block diagram illustrating the components of a microwave ablation system of the present disclosure; and 
           [0016]      FIG. 5  is a flow diagram illustrating a method, according to the present disclosure, of using frequency tuning in an ablation cycle for impedance matching. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0017]    The attached figures illustrate exemplary embodiments of the present disclosure and are referenced to describe the embodiments depicted therein. Hereinafter, the disclosure will be described in detail by explaining the figures wherein like reference numerals represent like parts throughout the several views. 
         [0018]    The exemplary embodiments of the apparatus disclosed herein are discussed in terms of performing a diagnostic or therapeutic procedure involving collecting or delivering electrical signals relative to a subject. Such procedures are inclusive of, but, not limited to microwave tissue ablation and related treatments of diseases and body ailments of a subject. In the discussion that follows, the term “clinician” refers to a doctor, nurse, or other care provider and may include support personnel. 
         [0019]    Referring initially to  FIG. 1 , an exemplary microwave probe assembly  100  includes a microwave probe  110 , a handle assembly  200 , an electrical hub  250  and an electrical cable  254 . Microwave probe  110  includes an outer jacket  150  and a sharpened trocar tip  112  shaped to permit penetration of skin and intermediate tissue to allow a radiating antenna portion of microwave probe  110  to be positioned adjacent targeted biological tissue. Handle assembly  200  includes a housing  202  adapted to be grasped by a hand of a clinician to handle probe  110 . Electrical cable  254  may be a coaxial cable and together with hub  250  is configured to connect the microwave probe assembly  100  to a source of microwave energy, for example an electrosurgical microwave generator  420  (see  FIG. 4 ). 
         [0020]    As seen in  FIG. 2 , the microwave probe  110  includes an inner conductor  120  and an outer conductor  140  to supply energy to a distal radiating portion  160   a  and a proximal radiating portion  160   b  respectively. A dielectric puck  116  separates the radiating portions  160   a ,  160   b  such that an opposite charge imparted on the radiating portions  160   a ,  160   b  establishes a magnetic field between them. Reference may be made to U.S. patent application Ser. No. 10/482,362, filed on Jul. 20, 2004, for a detailed discussion of the construction and operation of the microwave probe assembly  100 . 
         [0021]    In accordance with the present disclosure, one method of matching the impedance of the antenna  110  to the source impedance is referred to as “active matching.” Such a method may require coupling additional electrical components to the microwave probe assembly  100  to electrically communicate with inner and outer conductors  120 ,  140 . As seen in  FIG. 3A , an active matching control circuit is generally designated as  300 . 
         [0022]    Active matching control circuit  300  includes an input line  303  for receiving an output signal or energy from microwave generator  420  ( FIG. 4 ), and an output line  305  for transmitting an impedance matched output signal or energy to microwave probe or antenna  110 . Control circuit  300  further includes an inductor  313  electrically connected in series between input line  303  and output line  305 . Control circuit  300  includes a first capacitor  321  and a first tuning diode  310  connected in parallel to input line  303  at a location upstream of inductor  313 ; and a second capacitor  322  and a second tuning diode  311  connected in parallel to output line  305  at a location downstream of inductor  313 . Control circuit  300  also includes a first inductor  314  connected to a direct current (“DC”) source via supply line  307  and is connected in series between first capacitor  321  and first tuning diode  310 . Control circuit  300  further includes a second inductor  315  connected to a “DC” source via a supply line  309  and is connected in series between second capacitor  322  and second tuning diode  311 . 
         [0023]    In operation, in order to match the impedance of the load on output line  305  to that of microwave probe  110 , tuning diodes  310 ,  311  may be adjusted as needed or desired. PIN and/or varactor diodes  310 ,  311  may be used such that when a “DC” voltage is applied on supply lines  307 ,  309 , the capacitance exhibited by diodes  310 ,  311  will vary in accordance with the applied voltage thereto. Inductors  314 ,  315  are selected to have high impedances over an appropriate range of frequencies so that inductors  314 ,  315  act as RF chokes, thereby keeping “DC” supply lines  307 ,  309  free from the alternating current supplied to output line  305 . When diodes  310 ,  311  are appropriately tuned, capacitors  321 ,  322  and inductor  313  may compensate for a difference in impedance between the system and the tissue. Active matching control circuit  300  is placed as close as possible to the radiating portions  160   a ,  160   b  to minimize losses therebetween. 
         [0024]    A simplified example of such tuning is described below with reference to  FIGS. 3B through 3D . As seen in  FIG. 3B , at some time after initiating a tissue ablation procedure, the tissue impedance has risen to 200 ohms creating a substantial mismatch with the 50 ohm generator  330 . While the fixed inductor  331  may have an inductance value of 15 nH, the variable capacitor  333  may be tuned as described above to have a capacitance of 1.5 pf. For a predetermined frequency of 915 MHz supplied by the generator  330 , this yields the inductive and capacitive reactance values of:
       x L =j86.6Ω x C =−j116Ω       
 
         [0026]    Now rotating the 200 ohm tissue impedance through the matching network, the tissue is in parallel with the 1.5 pf capacitor. First equivalent impedance Z L1  may be calculated using the parallel impedance formula. 
         [0000]    
       
         
           
             
               Z 
               
                 L 
                  
                 
                     
                 
                  
                 1 
               
             
             = 
             
               
                 
                   
                     ( 
                     
                       200 
                        
                       
                           
                       
                        
                       Ω 
                     
                     ) 
                   
                   · 
                   
                     ( 
                     
                       
                         - 
                         j 
                       
                        
                       
                           
                       
                        
                       116 
                        
                       Ω 
                     
                     ) 
                   
                 
                 
                   
                     200 
                      
                     
                         
                     
                      
                     Ω 
                   
                   - 
                   
                     j 
                      
                     
                         
                     
                      
                     116 
                      
                     
                         
                     
                      
                     Ω 
                   
                 
               
               ≅ 
               
                 
                   ( 
                   
                     50 
                     - 
                     
                       j 
                        
                       
                           
                       
                        
                       86.8 
                     
                   
                   ) 
                 
                  
                 Ω 
               
             
           
         
       
     
         [0027]    Representing the tissue load and variable capacitor with the first equivalent impedance yields the circuit depicted in  FIG. 3C . It can be seen that a second equivalent impedance Z L2  may now be calculated for the fixed inductor and the first equivalent impedance in series. 
         [0000]        Z   L2 =(50− j 86.8)Ω+ j 86.6Ω≅50Ω
 
         [0028]    Second equivalent impedance Z L2  may be calculated and represented in the circuit depicted in  FIG. 3D . It can be seen that the second equivalent impedance is substantially matched to the generator impedance. Thus, appropriately tuning the variable capacitor allows for maximum power delivery to the tissue load. 
         [0029]    In accordance with the present disclosure, a second method of matching the impedance of the microwave probe  110  to the source impedance is referred to as “frequency tuning.” As seen in  FIG. 4 , a schematic illustration depicts a microwave ablation system  400  that may be used for frequency tuning. 
         [0030]    Microwave ablation system  400  includes a transmission line including a microwave frequency generator  420 , an amplifier  430  electrically coupled to generator  420 , a monitor such as coupler  440  electrically connected to amplifier  430 , and a microwave energy delivery device or antenna  460  electrically coupled to  440  via a transmission cable  450 . Microwave ablation system  400  further includes a measurement board  470  electrically coupled to coupler  440  and a microprocessor  480  in a communication line connecting the measurement board  470  with the frequency generator  420 . 
         [0031]    Frequency generator  420  may take any suitable form and should be configured to adjust the frequency of the output signal. The optimal frequencies for microwave tissue ablation are generally in the neighborhood of those frequencies best suited for heating water. By way of example, frequency generator  420  may be capable of producing output frequencies in the range of about 850 MHz to about 1.35 GHz, although higher frequencies are contemplated by the present disclosure. Amplifier  430  should be capable of amplifying the relatively low energy signal generated by frequency generator  420 . Amplifier  430  should also be capable of communicating information about both the forward power and any reflected power signals present in system  400 . Amplifier  430  may include a 300 Watt amplifier operating in the frequency range of 800 to 1000 MHz. Coupler  440  should be capable of sampling forward power from amplifier  430  and also the power reflected by the targeted tissue. Coupler  440  may include a 40 dB dual directional coupler having operating parameters suitable for use in this application. Measurement board  470  is in communication with coupler  450  and is capable of monitoring forward and reflected power signals and/or communicating impedance measurements. 
         [0032]    Cable  450  and antenna  460  may take any suitable form for use in a frequency tuning application. Here it is contemplated that the coaxial cable  254  and microwave probe  110  discussed with reference to  FIG. 1  above may be used as cable  450  and antenna  460 , respectively. Cable  450  may be selected to provide sufficient flexibility to allow antenna probe  110  to be conveniently positioned, and may also be selected to minimize power losses therethrough. 
         [0033]    Turning now to  FIG. 5 , a flow diagram of a method of performing a tissue ablation in accordance with the present disclosure is shown. In accordance with the present disclosure, a microwave probe  110  may be directly inserted into the target tissue in accordance with any suitable method known in the art (Process  510 ). Insertion of microwave probe  110  to a depth of about 5.5 to about 6 cm or any other suitable depth may be appropriate. Next, an initial impedance match is made by adjusting the output frequency of generator  420  to effect a source impedance of about 50 ohms, the approximate impedance of the target tissue (Process  515 ). Amplifier  430  may be set according to the preference of a clinician or in one embodiment, to output from about 30 Watts to about 45 Watts. At such a setting, the target tissue will begin to ablate due to the delivery of microwave energy thereto via antenna  460  (Process  520 ). The forward power at antenna  460  may be calculated by applying a correction factor based on the energy expected to be lost in the cable  450  to the output power of the amplifier  430 . 
         [0034]    The cable loss is partly a function of the characteristics of cable  450  (e.g., length diameter, materials of construction, etc.) and the frequency of the energy delivered therethrough. The cable loss may be readily calculated from such known characteristics. After a predetermined amount of time, an indication of the power reflected (i.e. not delivered to the target tissue) may be communicated through amplifier  430  to the operator by a display  435  on the amplifier  430  (Process  525 ). The correction factor determined for the cable loss may be applied to the value displayed for the reflected power at the amplifier  430  to determine the amount of energy reflected at the antenna  460 . 
         [0035]    A large reflected power, at antenna  460  in relation to the forward power at antenna  460 , is representative of a substantial impedance mismatch. Conversely, smaller reflections are characteristic of having achieved a matched impedance between the target tissue and the load source or between the reflected power and the forward power. Some reflected power may be acceptable and thus not require any adjustments to the frequency output by generator  420  (Decision  535 ). 
         [0036]    During operation, if some threshold value of reflected power is exceeded (Decision  535 ), the output frequency of generator  420  may be adjusted (upwardly or downwardly) to reduce the reflected power to a level below the threshold value (Process  540 ). This frequency adjustment may be accompanied by an adjustment to the output power of amplifier  430  if it is deemed necessary in order to deliver the necessary power to the target tissue (Process  545 ). 
         [0037]    Even a single adjustment to the frequency of the output signal of generator  420  during the ablation procedure may have a substantial or significant effect on the effectiveness of the ablation procedure. In some embodiment, microwave ablation system  400  may include, for example, a microcontroller  480  capable of automatically making many frequency adjustments during a given period of time. The ablation cycle may continue, continuously monitoring the reflected power and making frequency adjustments as necessary, until the target tissue has been sufficiently ablated (Decision  530 ). When the tissue has been sufficiently ablated, energy delivery to the tissue may cease (Process  550 ). 
         [0038]    An ablation procedure performed on cow liver tissue yielded the exemplary values presented in Table 1 below. Initially the output frequency of generator  420  was set at 925 MHz, and the amplifier  430  was set to output 31 Watts. The output power from the amplifier was transmitted through a cable  450  known to have a loss of 1.25 dB. A cable loss (dB) is related to a correction factor (P) by the equation dB=10(log P). A correction factor of about 1.33 was thus determined for the cable  450 . Dividing the 31 Watt output power of the amplifier  430  by the 1.33 correction factor yielded the initial value recorded for the load power. This initial value of 23.25 Watts represents the power delivered to the cow liver tissue load or the output power of the amplifier  430  less the power lost in the cable  450 . 
         [0039]    After the initial value of load power was thus calculated, subsequent values recorded for load power were based on a value recorded for the power reflected by the tissue. The values for power reflected were observed on the display  435  of amplifier  430  and thus represent the power reflected by the tissue load not lost in the cable  450  as the signal returned from the tissue to the amplifier  430 . To determine the amount of power actually reflected by the tissue, the 1.33 correction factor was multiplied by the power reflected and recorded as the corrected power reflected. The values listed for load power subsequent to the initial value were calculated by subtracting the corrected power reflected from the initial load power calculated. The values for load power then represent the power delivered to the tissue and not reflected by the tissue. Initially the power reflected was recorded as Low because the power reflected was below the range detectable by the amplifier  430 . Whenever possible, an impedance reading was recorded as illustrated in Table 1 below. Impedance readings may be used to determine a frequency needed for tuning. 
         [0000]    
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                 Corrected 
                   
                   
               
               
                   
                   
                 Power 
                 Power 
                 Load 
               
               
                 Time 
                 Frequency 
                 Reflected 
                 Reflected 
                 Power 
                 Impedance 
               
               
                 (minutes) 
                 (MHz) 
                 (Watts) 
                 (Watts) 
                 (Watts) 
                 (ohms) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 0.0 
                 925 
                 Low 
                 N/A 
                 23.25 
                 43 + j8 
               
               
                 0.5 
                 925 
                 Low 
                 N/A 
                 23.25 
                 43 + j0 
               
               
                 1.0 
                 925 
                 Low 
                 N/A 
                 23.25 
                 43 − j2 
               
               
                 1.5 
                 930 
                 Low 
                 N/A 
                 23.25 
                 — 
               
               
                 2.0 
                 935 
                 Low 
                 N/A 
                 23.25 
                 — 
               
               
                 2.5 
                 940 
                 0.6 
                 0.8 
                 22.45 
                 — 
               
               
                 3.0 
                 940 
                 0.6 
                 0.8 
                 22.45 
                 33 − j12 
               
               
                 3.5 
                 940 
                 0.7 
                 .93 
                 22.32 
                 33 − j12.9 
               
               
                 4.0 
                 940 
                 0.7 
                 .93 
                 22.32 
                 33.5 − j14 
               
               
                 4.5 
                 940 
                 0.7 
                 .93 
                 22.32 
                 33.6 − j14 
               
               
                 5.0 
                 940 
                 0.85 
                 1.13 
                 22.12 
                 33.4 − j17.7 
               
               
                 5.5 
                 940 
                 1.0 
                 1.33 
                 21.92 
                 — 
               
               
                 6.0 
                 940 
                 1.8 
                 2.4 
                 20.85 
                 — 
               
               
                 6.5 
                 940 
                 2.0 
                 2.66 
                 20.59 
                 — 
               
               
                 7.0 
                 940 
                 2.2 
                 2.93 
                 20.32 
                 — 
               
               
                 7.5 
                 940 
                 2.8 
                 3.72 
                 19.53 
                 33 − j39 
               
               
                 8.0 
                 940 
                 3.2 
                 4.3 
                 18.95 
                 33 − j42 
               
               
                 8.5 
                 940 
                 3.5 
                 4.7 
                 18.55 
                 33 − j43 
               
               
                 9.0 
                 940 
                 3.5 
                 4.7 
                 18.55 
                 33 − j44 
               
               
                 9.5 
                 940 
                 4 
                 5.32 
                 17.93 
                 33 − j46 
               
               
                 10.0 
                 940 
                 4 
                 5.32 
                 17.93 
                 33 − j48 
               
               
                   
               
             
          
         
       
     
         [0040]    As can be seen in Table 1, the power reflected by the tissue generally increased over time as the tissue was heated, just as expected. The output frequency of generator  420  was adjusted to 930 MHz after 1.5 minutes, to 935 MHz after 2 minutes, and 940 MHz after 2.5 minutes of the ablation procedure. These adjustments yielded a reflected power of 4 Watts after 10 minutes. With no frequency adjustments, a max reflected power of about 8 Watts could be expected around 10 minutes. Thus, a comparison of these reflected power values demonstrates the effect of frequency adjustments on the delivery of energy to the tissue. 
         [0041]    Although the foregoing disclosure has been described in some detail by way of illustration and example, for purposes of clarity or understanding, it will be obvious that certain changes and modifications may be practiced within the scope of the appended claims.