Patent Publication Number: US-2021177514-A1

Title: Therapeutic microwave ablation devices and methods

Description:
The present application is a non-provisional of, and claims the benefit of US Provisional Patent Application Nos. 62/937,822 (Attorney Docket No. 5191.001PV3) filed Nov. 20, 2019; and 62/937,853 (Attorney Docket No. 5192.001PV3) filed Nov. 20, 2019; the entire contents of each is incorporated herein by reference. 
    
    
     BACKGROUND 
     Microwaves are a form of electromagnetic radiation with wavelengths broadly ranging from about one meter to one millimeter, with frequencies ranging between 300 MHz (1 m) and 300 GHz (1 mm). A more common definition is the range between 1 and 100 GHz (wavelengths between 0.3 m and 3 mm). 
     Microwaves are widely used in modern technology, for example in communication links, wireless networks, microwave relay networks, radar, satellite and space communications, cooking food, etc. 
     Microwave technology has been used as an energy source in many medical devices. Due to several technical and practical reasons, microwave devices are not as common as the devices that use radiofrequency current as the energy source. 
     Additional information about the use of microwave energy is disclosed in Compilation of the Dielectric Properties of Body Tissues at RF and Microwave Frequencies, Report N.AU/OE-TR-1996-037, Occupational and environmental health directorate, Radiofrequency Radiation Division, Brooks Air Force Base, Tex. (USA), 1996 by C. Gabriel; the entire contents of which are incorporated herein by reference. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the drawings, which are not necessarily drawn to scale, like numerals may describe similar components in different views. Like numerals having different letter suffixes may represent different instances of similar components. The drawings illustrate generally, by way of example, but not by way of limitation, various embodiments discussed in the present document. 
         FIG. 1  illustrates a two-wire transmission line. 
         FIG. 2  shows E-field vectors along a two-wire transmission line. 
         FIGS. 3A-3C  illustrate power absorption patterns for a two-wire transmission line. 
         FIGS. 4A-4C  illustrate electric field patterns for a two-wire transmission line. 
         FIGS. 5A-5C  show power absorption plots corresponding to the E-field patterns in  FIGS. 4A-4C . 
         FIGS. 6A-6B  show another example of a two-wire line. 
         FIG. 7  shows S11 vs. frequency for a coaxial fed two wire transmission line. 
         FIG. 8  shows another plot of S11 vs. frequency. 
         FIG. 9  shows still another plot of S11 vs. frequency. 
         FIG. 10  shows yet another plot of S11 vs. frequency. 
         FIGS. 11A-1C  show a three-wire transmission line with corresponding E-field and absorption pattern. 
       FIGS.  12 A 1 - 12 A 5  show various configurations of wire transmission lines. 
       FIGS.  12 B 1 - 12 B 3  show various configurations of wire transmission lines. 
         FIGS. 13A-13B  show a two-wire transmission line and corresponding S11 vs. frequency plot. 
         FIG. 14  shows an absorption pattern. 
         FIGS. 15A-15B  show a loop transmission line and ablation boundary. 
         FIG. 16  shows a two-wire transmission line. 
         FIG. 17  shows a two-wire transmission line. 
         FIG. 18  shows another two-wire transmission line. 
         FIGS. 19A-19B  show a looped transmission line and corresponding magnetic field pattern. 
         FIG. 20  shows dual coaxial conductor needles. 
         FIGS. 21A-21C  show a dual coaxial needle example with absorption patterns. 
         FIGS. 22A-22C  show a microwave applicator example with sheets. 
         FIGS. 23A-23B  show a PCB based loop applicator example and lesion pattern. 
         FIGS. 24A-24C  show an example of anchoring a microwave ablation device to tissue. 
         FIGS. 25-26  show examples of moveable needles. 
         FIG. 27  shows an example cardiac ablation device. 
         FIG. 28  shows a plot of S11 vs. frequency of the example in  FIG. 27 . 
         FIG. 29  shows an example of a three-wire transmission device. 
         FIGS. 30A-30B  show an example of 4-needle microwave transmission probe. 
         FIG. 31  illustrates an example of a 4-needle microwave probe. 
         FIG. 32  shows an example of transmission device formed on a printed circuit board. 
         FIG. 33  shows a transmission device formed on a printed circuit board disposed in tissue. 
         FIG. 34  shows S11 vs. frequency of the tumor ablation design in  FIG. 33   
         FIG. 35A  shows an example of a device with three traces forming a closed loop. 
         FIG. 35B  shows the ablation boundary of the device in  FIG. 35A . 
         FIGS. 36A-36B  show an example of a 3-wire transmission line and ablation boundary. 
         FIG. 37  shows a plot of dB magnitude vs. frequency. 
         FIG. 38  shows an example of a four-needle device. 
         FIG. 39  shows a plot of S11 vs. frequency for the device in  FIG. 38 . 
         FIG. 40  shows S11 dB is minimized when dielectric constant is reduced. 
         FIGS. 41A-41B  show examples of filter circuits. 
     
    
    
     DETAILED DESCRIPTION 
     Microwave technology has been used as an energy source in many medical devices. Due to several technical and practical reasons, microwave devices are not as common as the devices that use radiofrequency (RF) current as the energy source. However, microwave technology may offer some unique advantages over RF devices such as avoiding dangerous steam pops in cardiac, tumor or other tissue ablation or may be largely immune to desiccation experienced around RF electrodes which can cause large increases in resistance which deteriorate the ability of RF to deliver power to tissue. At least some of these challenges may be overcome by using microwave energy in a therapeutic procedure using at least some of the examples disclosed herein. 
     The present examples are novel device configurations that transmit the microwave energy deep into the tissue to create large lesions. This can be very useful in medicine where control of large lesions is necessary to treat an illness. Similarly, the present examples may be used to treat tumors (benign and malignant) in various parts of the human anatomy. In general, the examples described herein can be used to selectively kill or denature targeted tissue to bring about a therapeutic change. The change effected may be controllably varied to bring partial damage to regulate a bodily function or response such as neuromodulation, or may remodel the tissue in shape, volume, or another characteristic. 
     Microwave energy offers some unique advantages over the RF devices. At least one of the advantages of the microwave is its ability to penetrate deeper into the tissue. With radiofrequency ablation, most of the resistive heating is confined to about a millimeter of the tissue. Thermal propagation is the only pathway for the RF lesion to grow. When deeper lesions are desired, this can cause a significant problem. Often the tissue in contact with the RF electrode is desiccated. In the worst case, steam pops create an explosion of the tissue with dangerous consequences. With proper design and correct frequency, microwave energy can penetrate and heat a large volume of the tissue at the same or lower power levels compared to RF. Due to this phenomenon, the thermal footprint of the lesion grows faster and safer with microwave treatment. 
     Open-Ended Dual Needle Applicator: Two Wire Transmission Line 
     An ideal two wire transmission line  10  is illustrated in  FIG. 1 . The ideal two wire transmission line  10  may be disposed in muscle-like dielectric material T. An incident microwave signal is launched into the ideal input port  12 . The signal propagates down  16  the transmission line, strikes the open end and reflects 18 toward the input port  12 , causing a standing wave pattern along the line. In addition to the standing wave effect, power is deposited into tissue as the wave is propagating down the line. Length of the transmission line and frequency can be utilized to create a desired absorption/heating pattern. Here, the two-wire transmission line may be formed from any two conductors such as wire filaments or needles  14  with an optional tissue piercing tip 
       FIG. 2  shows a plot of E-field vectors along the two-wire transmission line illustrated in  FIG. 1 , at a frequency of 2 GHz. E-field  20  is focused between the two lines  14 , which can be utilized to create a large absorption zone between the conductors which in this example may be two needles. The regions of intense E-field may be aligned horizontally between the two transmission lines 
       FIGS. 3A-3C  illustrate examples of power absorption patterns (scaled to 1/10 of max) for two wire transmission line needles  14  in a block of muscle tissue T (real part of permittivity (er)=53.5, imaginary part of permittivity (ei)=12.9 @2 GHz). Black/dark coloring near the wires indicates regions of highest absorption. The non-limiting wire parameters in this example are: diameter=0.287 mm, wire spacing=1.5 mm, and wire length=20 mm. 
       FIG. 3A  shows the region of highest absorption may be in two locations  32  along the transmission lines  14  where high absorption occurs at a proximal portion of the transmission lines and a distal portion of the transmission lines. Here, the frequency=1 GHz, and the standing wave pattern creates constructive peaks at the proximal and distal end of the needles. 
       FIG. 3B  shows another example where the region of highest absorption may be in three locations  32  along the transmission lines  14  where high absorption occurs at a proximal portion of the transmission lines, a distal portion of the transmission lines and a region in between the proximal portion of the transmission lines and the distal portion of the transmission lines. Here, the frequency=2 GHz, and the standing wave pattern creates constructive peaks at the proximal and distal ends of the needles, as well as at the center of the needles. 
       FIG. 3C  shows another example where the region of highest absorption  32  is in a proximal portion of the transmission lines  14 . In another example, the region of highest absorption is in a distal portion of the transmission, or the region of highest absorption is between the distal portion and the proximal portion of the transmission lines. Here, the frequency=5.8 GHz, and in this case the losses along the transmission line are so large that the energy exists mainly in the proximal end of the needles. 
     Controlling the location of constructive peaks may be advantageous in the case where a product is supplied in a single length needle electrode (e.g. 20 mm), but the area of the energy delivery is only required at a depth of 5 mm. In this case, the needles can penetrate through the tissue to the full length of 20 mm, but the operator choose the 5.8 GHz frequency to limit the lesion depth. 
     Other aspects of the device in  FIGS. 3A-3C  may be substantially the same as discussed in  FIGS. 1-2  above. 
       FIGS. 4A-4C  shows example Electric Field (E-field) patterns  42  for 20 mm long needles  14  at 2 GHz, with all plots normalized to the same peak E-field value. Smaller spacing between needles  14  has more intense E-field as indicated by the darker color around the electrode needles  14 . 
       FIG. 4A  shows needle spacing=1.5 mm. 
       FIG. 4B  shows needle spacing=3.0 mm. 
       FIG. 4C  shows needle spacing=5.0 mm. 
     The distance between the two wires, needles or other conductors determines how intense of an E-field exists between them. Closer proximity leads to stronger E-fields and a higher absorption level as seen in  FIG. 4A . This may be used for a more narrow but efficient ablation pattern. A larger distance between needles decreases the E-field but spreads the energy over a larger volume which may be used for creating larger lesions as seen in  FIG. 4C .  FIG. 4B  illustrates an intermediate example where E-field and volume are between the examples in  FIGS. 4A and 4C . 
     Other aspects of  FIGS. 4A-4C  may be substantially the same as discussed in  FIGS. 1-2  above. 
       FIGS. 5A-5C  show power absorption plots  52  corresponding to the E-field patterns in the previous figures ( FIGS. 4A-4C ) where  FIG. 5A  corresponds with  FIG. 4A , and  FIG. 5B  corresponds with  FIG. 5B , etc. The outline  52  around and between the electrodes which may be needles or other conductor elements denote regions of equivalent relative energy absorption for each configuration. This demonstrates an increased treatment zone for needles  14  with further separation. 
       FIG. 5A  shows needle spacing=1.5 mm. 
       FIG. 5B  shows needle spacing=3.0 mm. 
       FIG. 5C  shows needle spacing=5.0 mm. 
     Other aspects of  FIGS. 5A-5C  may be substantially the same as in  FIGS. 1-2  above. 
     Open-Ended Dual Needle Applicator: Two Wire Line Fed with Coaxial Cable 
       FIGS. 6A-6B  show how a two-wire line  60  can be formed utilizing a coaxial cable  62 . The center conductor  67  forms one of the arms of the needle  14 , the other arm of the second needle  14   a  is formed by attaching a wire to the outer conductor (shield)  69  of the coaxial cable  62 . The coaxial cable includes a shield  69  disposed over the center conductor  67  with a dielectric  68  disposed in between the two layers  67 ,  69 . The two-wire line  60  has a region of first reflection  66  adjacent the distal end of the coaxial cable  62  and a second region of reflection  64  near the distal end of the needles  14 . 
       FIG. 6A  shows needles  14  extending into tissue T. 
       FIG. 6B  illustrates an enlarged version of  FIG. 6A . 
     When the above device and method seen in  FIGS. 6A-6B  is utilized, a first reflected signal may be present at the transition between the coaxial cable and the two-wire line at the first reflection region  66 . This first reflected signal will interact with the second reflected signal at the open end of the two wires at the second region of reflection  64  to create frequency dependent variation in the impedance of the applicator device  60  (the two-wire line). The amount of the reflected signal at the coax to two-wire junction (first reflection  66 ) is dependent on the surrounding tissue type, the distance between the two wires, the diameter of the wires, and the frequency. The amount of reflected signal at the open end of the two-wire transmission line (second reflection  64 ) is dependent on the surrounding tissue type, the distance between the two wires, the diameter of the wires, the frequency and the length of the transmission line. For a desired frequency of operation and tissue type, the separation distance between the wires, the wire diameter and the length of the transmission line can be selected to optimize the amount of energy coupled into the applicator. Separation and diameter of wires will tend to affect depth of resonance. Length of transmission line will tend to vary the frequency of optimal coupling (resonant frequency). A first wire may be straight and interact with a dielectric layer  68  that is covered by a shield  69 . The second wire may be coupled to the shield  69  and have a curved portion that separates the distance between the first needle  14  and the second needle  14   a.    
       FIG. 7  shows the S11 (amount of reflected signal at the input coaxial cable) on a dB scale vs. frequency for a coaxial fed two wire transmission line embedded in muscle tissue. Three different needle lengths are shown: 5 mm (solid trace), 10 mm (dashed trace) and 20 mm (dotted trace). As shown in the figure, different optimal frequencies of operation (in which the reflected signal reaches a local minima) are demonstrated for the different lengths. For example, the 5 mm long needles have a single optimal operating point at approximately 4.4 GHz. The 10 mm long needles have two optimal operating points at a first resonance of 2 GHz and a second resonance of 4 GHz. The 20 mm long needles have three optimal operating points including a first resonance at 1 GHz, a second near 2 GHz and a third near 3 GHz. Each resonance will create a different standing wave pattern (and corresponding absorption pattern). 
       FIG. 8  shows the S11 (in dB) vs. frequency for 20 mm long needles embedded in muscle tissue with a fixed wire diameter (e.g. 0.3 mm) but varying separation distances. Changing the separation distance causes small shifts in the resonance while the depth of the resonance is affected more dramatically. Deeper resonance at the desired operating frequency translates to more efficient delivery of power into the device. 
       FIG. 9  shows the S11 (in dB) vs. frequency for 20 mm long needles embedded in muscle tissue with a fixed separation distance (e.g. 1.5 mm) but varying wire diameter. Changing the wire diameter causes small shifts in the resonance while the depth of the resonance is affected more dramatically. Deeper resonance at the desired operating frequency translates to more efficient delivery of power into the device. 
     For lower water content tissue types, such as lung or fat tissue, the dielectric constant of the surrounding tissue decreases. This has the effect of decreasing the electrical length of the transmission line and increasing the resonant frequency of the system. 
       FIG. 10  shows the S11 vs. frequency of the coaxial fed two wire line previously shown in  FIGS. 6A-6B , except embedded in lung tissue rather than muscle. The solid trace shows the S11 with the length of the transmission line set to 20 mm. Unlike in muscle tissue, where there is a first resonance at 1 GHz, the first resonance is closer to 2 GHz due to the inherently lower dielectric constant of lung tissue. If the length of the transmission line is increased to 35 mm (dashed trace) the first resonance at 1 GHz is restored and the second resonance is again at 2 GHz. 
     Using the concepts above, the transmission line length, separation distance, wire diameter and frequency can be utilized to create an microwave applicator device that can create adjustable lesion sizes in a variety of tissue types. In one example, the mechanical structure of the applicator can allow in-situ adjustment of the transmission-line length, with corresponding adjustment in frequency to allow optimal efficiency and targeting of variable sized tumors or cardiac tissue targets. In an alternate example, the applicator can have a fixed transmission-line length but be excited with a variable frequency to create an absorption pattern that targets variable sizes of tumor or cardiac tissue. In another example, the applicator can have a fixed transmission line length and the frequency can be changed for optimal efficiency and absorption pattern as the applicator comes into contact with different types of tissue (such as going from muscle tissue to fat or lung tissue). In another example, the applicator can have a variable length such that the length can be changed for optimal efficiency and absorption pattern as the applicator comes into contact with different types of tissue. 
     Example Ranges: 
     Any of the needles disclosed herein can be adjusted to operate in a range of tissue types including fat, muscle, tumorous tissue, lung, cardiac and other soft tissues. This spans a range of dielectric constants from 5 to 100. 
     Separation distances may be 0.5 to 15 mm between needles. Nominal range may be 1 to 10 mm. 
     Wire diameter of a needle may range from 0.1 to 2 mm. Nominal range may be 0.15 to 0.5 mm. 
     Transmission line lengths may range from 1 to 100 mm. Nominal range may be 2 to 30 mm. 
     Transmission line impedance range may be: 10 to 320 Ohms. 
     Frequency range may be: 400 MHz to 10 GHz. Lower frequencies lend themselves to larger absorption pattern, higher frequency is more localized to the region of the antenna. 
     Any of the examples of microwave delivery devices described herein may have any or all of the parameters described above in any combination or permutation. 
     Multi-Pronged Inner or Outer Conductor 
     A two-wire transmission line can be expanded to a multi-line transmission line  1100 . This includes multiple negative or positive lines  1104 ,  1106 . 
       FIGS. 11A-1C  show a three-wire transmission line  1100 , with 20 mm length lines  104 ,  1106 , operating at 2 GHz. This example includes a three-wire transmission line  1100  with two outer needles  1104  and an inner needle  1106  coupled to a coaxial cable  1102 . The outer needles  1104  may vary in length and diameter relative to one another or relative to the inner needle  1106 . 
       FIG. 11A  shows an example where the design has an inner conductor  1106  with an outer conductor  1104  on either side. 
       FIG. 11B  shows the E-fields along the transmission line of  FIG. 11A  and illustrates are largely constrained to the region between the inner and outer conductors as indicated by the arrows. This is similar to the two-wire transmission lines previously discussed above. 
       FIG. 11C  shows the absorption pattern (scaled to 1/10 of max) indicated by the darkened regions of the example in  FIGS. 11A-11B . 
     FIGS.  12 A 1 - 12 A 5  and  12 B 1 - 12 B 3  show a cross section through various configurations of wire transmission lines. 
     FIG.  12 A 1  shows a two-wire configuration  1202  with one positive and one negative wire  1220  in a linear array. 
     FIG.  12 A 2  shows a three-wire configuration  1204  with two negative and one positive wire  1220  in between the two negative wires, and in a linear array. 
     FIG.  12 A 3  shows four-wire configuration  1206  with two positive and two negative wires  1220  circumferentially disposed about ninety degrees apart from one another. The positive and negative wires alternative circumferentially. 
     FIG.  12 A 4  shows a five-wire configuration  1208  with one positive wire in a center position surrounded by four negative wires  1220  forming a cross-shaped pattern. The four negative wires are arranged approximately ninety degrees apart from one another circumferentially. 
     FIG.  12 A 5  shows another five-wire configuration  1210  with a central positive wire  1220  surrounded by four alternating positive and negative wires  1220  circumferentially disposed about ninety degrees apart from one another. 
     FIGS.  12 B 1 - 12 B 3  shows that polarity may be reversed in the three- and five-wire examples of FIGS.  12 A 2 ,  12 A 4 , and  12 A 5 . 
     In FIG.  12 B 1  a three-wire configuration  1212  with a negative polarity wire in between two positive polarity wires  1220  arranged in a linear array. 
     FIG.  12 B 2  shows a negative central wire  1220  surrounded by four positive polarity wires  1220  disposed about ninety degrees circumferentially apart from one another to form a five-wire configuration  1214 . 
     FIGS.  12 B 3  shows another five-wire configuration  1216  with a negative central wire  1220  surrounded by four wires with alternating positive and negative polarity and disposed approximately 90 degrees circumferentially apart from one another. 
     Multiple conductors can be utilized to tailor the field configuration and corresponding absorption pattern of the transmission line applicator as shown in  FIGS. 12A - 12 A 5  and  12  B 1 - 12 B 3 . The center conductor may be attached to the positive terminals and the negative terminals may be attached to the outer terminals, or the opposite connections may be used. 
     Loop—Shorted Two-Wire (or Multi-Wire) Transmission Line 
     The transmission lines described above can be terminated in a short circuit, rather than an open circuit. In this case there still exists a first reflection at the coax-to-two-wire transition (near the distal end of the coax cable) and a second reflection at the short circuit termination near the distal end of the transmission lines. This second reflection due to a short circuit has a different phase than an open circuit and therefore the length of line that optimizes the efficiency at a particular frequency is different. Thus, the use of a shorted line can be utilized to create an absorption pattern of a desired length/size while maintaining an optimal efficiency. 
       FIG. 13A  shows a two-wire  1304  shorted transmission line  1300  design that is 15 mm long. The two wires  1304  may be needles for piercing tissue and one wire is coupled to the inner conductor of the coaxial cable  1302  while the other wire is coupled to the outer conductor of the coaxial cable. A connection  1306  creates a short between the two wires  1304  at or near their distal end. 
       FIG. 13B  shows the S11 vs frequency for the design of  FIG. 13A . The design has a second resonance near 2 GHz as previously shown for the open two-wire line, but in this case the length of the line required to achieve this resonance is 15 mm rather than 20 mm. The result is a more compact heating pattern, as shown in  FIG. 14 . 
       FIG. 14  shows the absorption pattern of a 15 mm long, shorted two-wire coaxial fed line  1300  in muscle tissue T. The darker regions between the wires show greater absorption compared to the lighter regions. 
     Loop—Standard Loop Antennas, Coupled to Magnetic Field 
     In another example, a loop antenna can be formed by forming a loop with e.g. the center conductor of a coaxial cable, bending it back toward the outer conductor and electrically attaching it to the outer conductor. The loop structure causes a large amount of current to flow in a circularly shaped (or partially circularly shaped) pattern, creating a magnetic field and subsequent electric field that gets absorbed in the surrounding tissue. 
       FIGS. 15A-15B  show an example of a loop antenna  1500 . 
       FIG. 15A  shows construction of loop from a coaxial cable, here the loop  1502  may be formed from the center conductor of coaxial cable  1504  or any other conductor that is electrically coupled with the outer conductor of the coaxial cable  1504  to form a loop of any size or shape and disposed at the distal end of the coaxial cable. A cable connector or other connector element  1506  may be coupled to the proximal end of the coaxial cable to allow connection with a microwave generator or another instrument. 
       FIG. 15B  show an example lesion  1508  created with the loop  1500  in  FIG. 15A  and disposed in tissue T (such as the liver). 
     The absorption zone of such a loop antenna is focused in the region where the loop is, as well as inside the loop. As such it may be adapted such that the loop encircles or partially encircles e.g. a tumor or other structure to preferentially target that structure and protect surrounding normal tissues. 
     Floating Outer Conductor/Floating Inner Conductor 
     Wires can be left electrically floating (from a low frequency standpoint), with microwave currents capacitively or inductively coupled onto the wires. This can be done for the center conductor and/or outer conductor of a coaxial cable. 
       FIG. 16  shows a coaxial fed two wire transmission line  1600 , with the outer wire  1608  isolated from the shield of the coaxial cable  1602  with an insulator  1606 . The inner wire  1604  may be directly connected to the inner conductor of the coaxial cable  1602 . Microwave energy can be capacitively or inductively coupled onto the outer wire  1608  from the shield of the coax cable. The insulator  1606  may be created from plastic or ceramic materials. 
     Additionally, rather than using a plastic or ceramic insulator, the outer wire can be embedded in tissue and placed near the shield of the coaxial cable. In this configuration microwave energy can couple onto the outer wire across the tissue. This is shown in  FIG. 17  with a coaxial fed two wire transmission line  1700 , where the outer needle  1708  is coupled to shield  1710  through tissue  1706 . Other aspects of the transmission line  1700  are similar to those of  FIG. 16  where an inner needle  1704  is coupled to the center conductor of coax cable  1702 . 
     Leave-in-Place Conductor/Fusible Link 
     A fine flexible platinum or other metallic conductor can be looped around the target site. This can be accomplished number of ways. In an example, a nickel-titanium needle reshaped in the form a circle is advanced from a catheter. Once the penetrating needle encircles the target tissue and returns to the catheter tip, a conductor is fed through the need and recaptured. Microwave power is applied to the conductor causing the ablation of the target tissue. Once the ablation is successfully completed, a fusible link connecting the conductor to the catheter is melted by applying high current. 
     Differential Length Conductors 
       FIG. 18  shows a two-wire transmission line  1800  with unequal length lines  1802 ,  1804 . The lengths of the wires for the two-wire transmission line can be set to unequal lengths. Having unequal length creates variation in the open-circuit effect at the end of the line. The variation can serve to change the magnitude or phase of the second reflection previously described above, potentially leading to a different optimal length for the transmission line. Additionally, fringing fields that naturally exist at the open ends of the transmission line may be varied to change the size/volumetric geometry of the absorption pattern in tissue. Here, the center needle or line  1802  is coupled to the center conductor of the coaxial cable  1806  and the outer needle or conductor  1804  is coupled to the shielding of the coaxial cable. The outer needle  1804  is shorter than the inner needle  1802 . In another example, the inner needle  1802  may be shorter than the outer needle  1804 . 
     Loop with Floating Loop 
       FIGS. 19A-19B  show a loop antenna  1900  with a first loop  1902  and a secondary floating antenna  1904 .  FIG. 19A  shows a front view of the example and  FIG. 19B  shows a side view of  FIG. 19A . 
     In  FIG. 19A , and outer loop  1902  may be formed with a wire or other conductive filament coupled to the center conductor of a coaxial cable  1906  at one end of the loop and the opposite end of the loop coupled to the outer conductor of the coaxial cable  1906 . The loop may be any shape or size such as a round or circular loop. The inner loop or floating loop  1904  may also be a round loop or any other size and shape and may be disposed entirely or partially within the outer loop and in the same plane or in a separate plane. 
       FIG. 19B  shows an example where the outer loop in in a first plane and the floating loop in in a second plane that may be parallel with the first plane. The resulting magnetic field  1908  between the outer loop and floating loop forms to loops which flow in opposite directions. For example, the two loops may be semi-circular in shape with one loop on top of the other loop. The top magnetic field runs counterclockwise while the bottom magnetic field runs clockwise. The two magnetic field share a common section of their perimeter. 
     The loop antenna geometry described previously in  FIGS. 15A-15B  creates currents traveling along the wire in a circular path, and the corresponding magnetic field created by these currents may be coupled into a floating loop of wire present in the tissue as shown in  FIGS. 19A-19B . This magnetic coupling mechanism can result in induced currents in the floating loop, with resulting heating occurring in the region of both the main loop and the floating loop. This mechanism may be utilized for increasing the heating zone of a loop antenna, as well as implementing a leave in place conductor for identification of tumor margin. 
     Multi-Frequency Generator 
     A multi-frequency generator can be utilized for exciting any of the applicator devices described herein with a variable frequency signal for creating adjustable sized lesions. The generator may have a switchable frequency (e.g. in which one frequency can be output at a time) or may have the ability to generate a multi-frequency signal (e.g. in which multiple frequencies can be output at a time). A multi-frequency generator could be readily implemented with a combination of programmable or fixed synthesizers/oscillators, amplifiers, diplexers/multiplexers, combiners and filtering. 
     In addition to exciting different frequencies, the generator may be designed to measure reflected power at one or more frequencies. By measuring the magnitude and/or phase of the reflected signal and comparing it to the forward power signal, diagnostic information about the current length of the applicator transmission line and/or the type of tissue in contact with the tissue may be obtained. 
     Dual Coaxial (In/Out Phase) 
       FIG. 20  shows a microwave applicator device  2000  with dual coaxial cables  2006  (also referred to herein as coax) and dual conductor needles  2002  in a perspective view. 
     An alternate to the single coax fed two wire line discussed previously is a device  2000  with dual coax  2006 . Here each needle  2002  is coupled to the central conductor of the coax cable. A dielectric  2004  is disposed over the central conductor. The dual coax configuration allows for a more balanced design than the single coax that uses a wire attached to the shield of the coax. In the balanced design the two wires (or needles)  2002  are fed 180 degrees out of phase such that one wire is positive while the other is negative. This achieves an absorption zone in between the two needles that is similar to the previous two-wire configurations shown above, with better symmetry between the needles possible. 
       FIGS. 21A-21C  show absorption patterns at 2 GHz for a dual-coax needle configuration such as the example in  FIG. 14 .  FIG. 21A  shows a cross section of the dual coaxial cables  2006  coupled to dual needles  2002  which are disposed in tissue T. 
       FIG. 21B  shows 180 degree out of phase drive of the coaxial cables  2006 , with a peak absorption zone is focused in between the two needles  2002  (black/dark areas indicate peak absorption). Lighter areas indicate less absorption. 
       FIG. 21C  shows in phase drive of the coaxial cables  2006 —there is a null absorption zone in between the two needles  2002 . 
     In an alternate example shown in  FIG. 21C , the two needles  2002  are coupled to two coaxial cables  2006  and may be setup to create two individual absorption zones by driving both coaxial needles  2002  in-phase. This results in a null absorption zone in-between the two needles  2002 . 
     Phase Changing Microwave Generator 
     A microwave generator may be setup to drive two or more coaxial lines with a varying phase relationship. Thus, allowing an in-phase or 180 degrees out of phase type of absorption to be created as described above. The generator could also be utilized to drive the coaxial lines using a zero to 360-degree variable phase shift between coaxial lines. This could readily be done using a digital phase shifter, varactor or mechanical phase shifter. 
     Active Heat Management of the Coaxial Cable 
     In microwave technology, a medical device used to ablate tissue in the body cavity or in the vascular system often requires a long coaxial cable. Even when impedance is matched properly, a substantial amount of energy is lost in transmission through the cable. This causes heating of the cable. For treatments of a long duration, the coaxial cable could get hot causing unintended consequences. It may be desirable to provide a heat exchanger or cooling mechanism to alleviate excessive heating. In one example, a tight flexible sleeve is attached to the outside of the coaxial cable. The sleeve is fused/glued to the ends of the coaxial cable in such a way that a cooling element such as a fluid like a solvent can be introduced in the annular space between the outer shield and the sheath, and be fully sealed and contained within that space. The proximal end of the cable is cooled with cryogen or coolant. Material such as industrial diamond can be used to improve the efficiency of the cooling. During the application of the microwave energy, the heat generated in the coax cable will cause the liquid trapped to heat up and evaporate. The transfer of the energy will cool down the cable. The heated solvent or the vapor will reach the cooled proximal section of the device and condense back to liquid. The coaxial cable with the trapped solvent acts as a heat pipe transferring heat from the cable. 
     Flared Conductors 
     In addition to the two-wire or multi-wire transmission lines shown above, which are parallel and thus have a uniform impedance. A design with flared wires is also possible. A design with flared wires will have a non-uniform impedance, as well as a non-uniform electric field vs. distance down the line. The non-uniformity may be utilized to alter the impedance/efficiency and absorption pattern along the transmission line. 
     The flare angle can vary anywhere from 0 to 60 degrees. The nominal flare angle would be 0 to 30 degrees to maintain the transmission line effect as the signal propagates down the line. 
     Conductive Sheets, Conductors on PCB 
     As an alternative to wires, needles or other elements, the transmission lines presented above may be created with metal or otherwise conductive sheets of material. The sheets can be fed with a coaxial cable coupled in a perpendicular or transverse direction to the sheets of material, or in parallel alignment. In a perpendicular feeding configuration, the upper region of the transmission line may be open or short circuited, with the length from the feed to the open or short circuit being adjusted for coupling efficiency between the coax and transmission line. This feeding configuration may also be used for the wire-type of transmission line. The sheets may have uniform width or may have a step or taper as shown in the figures. Additionally, the separation distance between the sheets may also be tapered. 
       FIGS. 22A-22C  show an applicator device design created using metal sheets rather than wires. 
       FIG. 22A  shows a side view of applicator  2200  which also includes a perpendicularly aligned coax cable  2206  coupled to one end of the sheets  2202  which may be flat planar sheets of conducting material that form transmission line  2204 . The applicator also may include an adjustable opening or short  2208  for tuning. 
       FIG. 22B  shows an alternate version of applicator device  2220  with flared sheets  2222  to create non-uniform separation distance. The coaxial cable  2206  may be coupled perpendicular or transfer to the flared sheets, or parallel thereto. 
       FIG. 22C  shows a perspective view of another alternate version of applicator  2230  with sheets  2232  of non-uniform width. The coaxial cable  2206  may be coupled parallel, perpendicular or transverse to the sheets  2232 .  FIG. 22C  shows a perpendicular connection. 
     The examples of  FIGS. 22A-22C  can be designed to operate when embedded or pressed into tissue. However, they can also be designed to operate in a balloon or other media in which there aren&#39;t significantly lossy materials in-between the two arms of the transmission line. In this fashion, an end-fire type device that mainly creates absorption at the distal end of the device, may be created. 
     An alternate example can include traces on a substrate such as a Printed Circuit Board (PCB) rather than wires to form a two-wire, multi-wire or loop type of applicator. The PCB may be attached to a coaxial cable, which feeds into traces on the PCB rather than wires. This may be utilized for creating a particular mechanical structure, for example a more rigid structure for puncturing a tumor or other desired target tissue. The PCB can be conventional circuit board such as FR4, high frequency circuit board such as Rogers or Taconic laminates, or may also be built from ceramic materials such as Macor or Alumina. Examples are shown in the figures. 
       FIGS. 23A-23B  show a PCB based loop applicator design  2300 . 
       FIG. 23A  shows a perspective view of the applicator  2300  showing the coaxial cable  2304  to PCB  2308  transition  2306  and the loop  2302  implemented as traces on the PCB  2308 . Optionally one end of the trace may be coupled to the center conductor of the coaxial cable and the other end of the trace may be coupled to the shielding of the coaxial cable. 
       FIG. 23B  shows an example of a heating/lesion pattern  2310  of PCB loop applicator  2300  previously illustrated in  FIG. 23A . 
     Temperature Controlled Microwave Ablation 
     With proper filtering, thermocouples or thermistors can be used with the microwave devices disclosed herein to measure temperature of tissue during ablation. This configuration can work in most cases, but in some special cases an optical temperature sensing method can be used with the examples disclosed herein instead of a thermocouple or thermistor. In one example, hollow conductors are used to form the electrode elements of any of the above-mentioned configurations. Further, a fiber optic temperature probe (such as Luxtron) is placed in the inside of the hollow conductors. This allows the continuous monitoring of the ablation temperature without distorting the microwave field. 
     A smart microwave generator may be used where temperature-controlled ablation can be accomplished using the temperature reading from the fiber optic probes or any other temperature sensor used (e.g. thermistor, thermocouple). The generator can be programmed to vary the power output to maintain a preset temperature target. 
     Method of Inserting Electrodes into the Tissue 
     The electrode of any example herein can be inserted into the target tissue by mechanical means or by the means of an electromechanical feature integrated into the device. A tissue piercing tip may be included in any of the examples to facilitate introduction into the tissue. 
     The coaxial cable used for the transmission can be chosen such that it has sufficient pushability and control to force electrodes with sharpened tips to enter the tissue. The outer shaft of the device can be made steerable as to allow the electrodes to pass through the target tissue in the correct orientation. This would allow the user to ablate any desire aspect of the tissue. 
     The electromechanical insertion of the electrode needle(s) can be achieved by incorporation solenoid like feature at an appropriate location near the distal end of the catheter. When actuated, the inner core with the electrode is advanced. The rate of advancement can be controlled by the amount of current applied to the coil. 
     Penetrating a tumor mass in soft tissue such as lung or liver with large probes is often difficult as the tumor mass tends to move away. In any example, a central thin sharp needle with barbs may be first advanced from the tip of a catheter to penetrate and stabilize the tumor or other target tissue. This central needle may be a passive element, or it may be a conductor for delivery of microwave energy into the tissue. Subsequently, the additional conductors are advanced into the stabilized tumor for ablation. 
       FIGS. 24A-24C  show an ablation catheter  2300  penetrating the target ablation tissue T. The ablation catheter has an anchor element  2302  which allows the ablation catheter to be stabilized in the tumor. In an example, there may be one anchor element to stabilize the target tissue. In another example, more than one anchor element may penetrate the target ablation region and stabilize the tumor. In any example, the one or more anchor elements  2302  penetrate the target ablation zone region before the one or more transmission lines  2302  penetrate the target ablation region. In another example, the transmission lines penetrate the target ablation region before the one or more anchor elements penetrate the target ablation region. In another example, the transmission lines penetrate the target ablation region concurrently with the one or more anchor elements.  FIG. 23A  shows the catheter advanced adjacent to the treatment region T.  FIG. 23B  shows advancement of the anchor element  2302  distally into the target treatment region, and  FIG. 23C  shows subsequent deployment of the transmission lines  2304  into the target treatment area. Anchoring elements may be included with any of the devices or methods disclosed herein. 
       FIGS. 25 and 26  show a schematic view of retractable needle electrode catheters  2500  and  2600 . At the distal tip of the device, the coax cable  2506  is connected to two tubular needle electrode guides or contacts  2510 . A needle electrode  2504 ,  2508  is connected to a non-conductive shaft  2502  is inserted through the needle guide  2510 . The inner diameter of the needle guide has a spring loaded contact to maintain good electrical connection between the needle guide  2510  and the needle  2504 ,  2508 . Further, the needle guides may be designed to pivot in plane (independently or in unison) to change of angle of entry of the needle electrode into the tissue T. At the proximal end the needle actuator shafts  2502  are connected to an actuator mechanism in the handle (not shown). The needle electrodes can be actuated together or independently of one another such that one needle  2512  may be disposed in the target tissue T while the other needle  2504  remains in a retracted configuration. 
     In  FIG. 25  the needle electrode  2504 ,  2508  may be actuated individually or together in a proximal or distal direction that is substantially parallel with the longitudinal axis of the device. The needle electrode actuator shafts may be actuated manually by an operator or they may be coupled to an actuation mechanism such as a motor, piston or other mechanism for moving the actuator shafts. One electrode needle may be coupled to the shielding of the coaxial cable  2506  and the other electrode needle may be coupled to the center conductor of the coaxial cable  2506 , or the electrode needles may be coupled in any other configuration such as those described herein. More than two electrode needles may be employed in this device.  FIG. 25  shows one needle deployed while the second needle is still disposed in the electrode guide. 
       FIG. 26  shows a variation of the example in  FIG. 25  with the major difference being that the electrode needles may be pivoted relative to one another to change the angle of insertion. This may be done manually or an actuation mechanism such as a motor or other pivoting mechanism may be used. Other aspects of the example in  FIG. 26  are generally the same as those in  FIG. 25 . 
     Example Devices for Ablation 
     An example of a design for tissue (e.g. cardiac, lung, tumor, or other tissue) ablation applications has a four wire transmission line with two positive conductors attached to the inner conductor of a coaxial cable and two negative conductors connected to the outer conductor of a coaxial cable as shown below. The coaxial cable has an outer diameter of 0.047″ (a standard “047” coaxial cable). 
       FIG. 27  shows an example ablation device  2700  which includes two positive needles  2702  and two negative needles  2704  offset roughly ninety degrees apart from one another in a circumferential direction. The needles  2702 ,  2704  are coupled to a coaxial cable  2710 . A scale  2708  in millimeters show an example size of the working tip of the device. The device may be designed for cardiac or other tissue ablation with the following non-limiting specifications: 
     Separation distance: 5 mm between needles. 
     Needle wire diameter: 0.0287 inches (0.729 mm). 
     Transmission line length: 20 mm. 
     Transmission line impedance: 20 to 100 Ohms. 
     Operational frequency: 900 to 1100 MHz, operates at the first resonance (second or third is also possible). 
       FIG. 28  shows S11 vs. frequency of a nominal design for cardiac ablation. First resonance shown near 1 GHz, second resonance shown near 2 GHz. 
       FIG. 29  shows an example of a transmural lesion  2604  created in tissue T such as cardiac ventricular tissue, lung tissue, or any other tissue, with an example device which includes three needles  2606  coupled to a coaxial cable  2602 . In this example the two outer needles may be coupled to the shielding of the coaxial cable and the middle needle may be coupled to the center conductor of the coaxial cable. The scale in this figure is shown in centimeters to demonstrate relative size of the device but it not intended to be limiting. Any figure in this specification which may have a scale to show relative size of the example is not intended to be limiting. 
       FIGS. 30A  (front view)- 30 B (back view) and  FIG. 31  show an example of a deployable needle device having a four-needle  3004  microwave probe. A coaxial cable&#39;s outer braid  3010  is soldered to a trace, separating it from the core  3006  wire with a dielectric  3008 . The core wire  3006  is soldered to a separate trace. On the front face of the PC board  3014 , are soldered  2  needles  3004 , isolated from one another, but connected electrically to either the core wire  3006  or the coax braid  3010 . The other two needles  3004  are coupled to the other of the core wire or coax braid. Also attached to the PC board is a plunger mandrel  3002  that will control the sliding of the needles in and out of a catheter tip  3016  (best seen in  FIG. 31 ) and into the tissue. The mandrel is a solid rod and the coax cable follows the position of the mandrel. 
     On the back of the PC board, vias  3012  connect the two electrical pathways to traces on the back of the PC board and corresponding needles are soldered into place. 
     The needles  3004  may be hypodermic needles or sharpened mandrels. The needles can be coated with a lubricant, such as parylene to facilitate tissue penetration. 
     For electrophysiology applications, an ECG electrode can be threaded through a separate lumen and an ECG electrode placed at the distal tip of the electrode. So can a thermocouple or thermistor. A temperature sensor such as an optical fiber can be threaded through the central lumen of a hypodermic needle to measure temperature. The catheter tip has angled exit holes for the needles to launch at angles spreading their distances between each other in the tissue, or the needles may be pre-set in angle and take on their shape when exiting the catheter tip. 
       FIG. 32  shows a perspective view of nominal tumor ablation device  3200 . The example has a three-wire transmission-line, with two outer conductors  3208  and one center conductor  3210 . The center conductor may be coupled with the core of the coaxial cable and the two outer conductors may be coupled with the shield in the coaxial cable. The line is terminated in a short circuit  3212 . It is constructed on a substrate  3206  such as a Printed Circuit Board (PCB) or ceramic and formed into a tip  3214  capable of penetrating tissues such as normal or tumorous lung tissue. The PCB is fed with 0.047″ sized coaxial cable  3204  that is flexible. The transmission-line is printed on both sides of the PCB such that the line is in close contact with tissue on both sides. The traces may be coated with an insulating layer such as parylene to help with the removal of the device from coagulated tissue. An example nominal configuration is shown below. 
       FIG. 33  shows a front view of an example of a tumor ablation device  3300  with non-limiting, example dimensions disposed in tissue T. The nominal frequency of operation for the device is 5.8 GHz, which is the second resonance for a line length of 6.8 mm. The impedance of the transmission line for the optimal design is 32 ohms (it can range between 15 and 45 ohms). Here the device includes three transmission lines with two outer and one inner trace disposed a substrate such as a PCB. The trace width may be about 0.25 mm. A coaxial cable  3302  is coupled to the transmission line and the inner conductor of the coaxial cable may be coupled to the inner conductor and the two outer conductors may be coupled to the outer shield conductor of the coaxial cable, or the connections may be reversed. The PCB may have a pointed tip to facilitate with tissue piercing. The loop width  3306  may be about 1.75 mm, the line length  3308  about 6.8 mm and the PCB length  3310  about 8.7 mm. These are example dimensions are not intended to be limiting. 
       FIG. 34  shows S11 vs. frequency of the tumor ablation design in  FIG. 33 , showing second resonance near 5.8 GHz. 
       FIGS. 35A-35B  show an example of the heating/lesion profiles from the tumor ablation device  3300  in  FIGS. 32-33 , operating at a frequency=5.8 GHz. 
       FIG. 35A  shows a front view of the heating profile  3302  forming a partially elliptical profile. 
       FIG. 35B  shows a side view of the heating profile  3302 . 
     In variation of the example in  FIGS. 32-33 , the line length may be decreased to 2.8 mm such that the device operates at the first resonance for a frequency of 5.8 GHz, as shown  FIGS. 36A-36B . 
       FIG. 36A  shows the device  3600  with non-limiting example line length of 2.8 mm, and frequency=5.8 GHz. The device includes three conductors, two outer and one inner conductor that are coupled with a coaxial cable. The inner conductor may be coupled to the inner conductor of the coaxial cable and the two outer conductors may be coupled to the shield of the coaxial cable, or the connections may be reversed. The three conductors may be mounted on a substrate such as a PCB with a pointed tip to facilitate tissue penetration. Other aspects of the device  3600  are generally the same as the device in  FIGS. 32-33 . 
       FIG. 36B  shows aside view of the heating/lesion pattern  3604  for the device  3600  of  FIG. 36A . Here, the heating pattern is partially elliptically shaped. 
       FIG. 37  shows S11 vs. frequency of the 2.8 mm design in  FIG. 36A . The first resonance is near 5.8 GHz. 
     Tuning for Optimal Energy Transfer 
     Tissue properties change as microwave energy is applied. Often this results in a mismatch of electromagnetic wave penetration into the body and high reflected power. Not only does the efficiency of the power transfer to the tissue go down but, the reflected power manifests as excessive heating of the coaxial cable. A hot coaxial cable can cause unintended burns in adjacent tissue or unwanted equipment damage. 
     In one example, the conductor lengths are optimized deliberately for changing (denaturing, heating and losing moisture) desiccated target tissue as function of microwave application. Though there may be a mismatch for the native tissue, as soon as the ablation starts causing desiccation around the conductor, the match improves, and highly efficient transfer of the energy ensues for the rest of the ablation time. For a given frequency, the ideal conductor lengths for ablating desiccated tissue can be theoretically or empirically calculated. Conductors designed in this fashion are more efficient and produce minimal heating of the coaxial (also referred to herein as coax) cable. In one specific example, a design with 24 mm long conductors and a 25-degree flare angle is designed to ablate high water content tissue (such as liver, cardiac or muscle tissue) as shown in  FIG. 38 . This configuration was designed for operation at 2.45 GHz and the device  3800  includes four needles  3804  coupled to a coaxial cable  3802 . In this example the needles flare radially outward and away from the coaxial cable. 
       FIG. 39  shows an example S11 vs. frequency plot for the device  3800  in  FIG. 38 , with the dielectric properties of the tissue reduced from the nominal value (corresponding to unablated tissue at regular human body temperature) to a value corresponding with ablated tissue. The figure shows that the resonance of the applicator is shifting upward as the dielectric properties change during an ablation. As a result of this shifting, the value of S11 at the operational frequency of 2.45 GHz varies. 
       FIG. 40  shows that the S11 dB is minimized (and thus efficiency maximized) when the dielectric constant is reduced by around 35%. As the ablation progresses, the S11 starts to increase again (with efficiency dropping). This phenomenon may be utilized as a means of monitoring the progress of an ablation. 
     Injection of Solution to Target Site 
     In microwave ablation of tissue, good matching or tuning of the antenna is essential for efficient transfer of energy to the target tissue. Interestingly, the biological tissue properties vary widely, and a single antenna operating at chosen frequency may not be ideal for all tissue types. Table 1 below shows the representative values for heart, liver and lung tissues at an operating frequency of 2.4 GHz. In the current examples disclosed herein, an antenna made of hypotubes capable of delivering fluid to the tissue tuned to work most efficiently in the presence of a biocompatible solution is used to perform ablation of a variety of tissues. The biocompatible solution can be chosen from a variety of material known to medical professionals such as normal saline, phosphate buffered saline, dextrose solution (e.g. D5W), Lactate Ringer&#39;s solution, etc. The antenna can be designed for any chosen operating frequency. In an example, the antenna is tuned to work at one of the following frequencies: 915 MHz, 2.4 GHz or 5.8 GHz. 
     
       
         
           
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Elec. 
               
               
                   
                   
                   
                 Cond.  
               
               
                   
                 Tissue 
                 Permittivity 
                 (S/m) 
               
               
                   
                   
               
             
            
               
                   
                 Heart Muscle 
                 5.49E+1 
                 2.22E+0 
               
               
                   
                 Liver 
                 4.31E+1 
                 1.65E+0 
               
               
                   
                 Lung (inflated) 
                 2.05E+1 
                 7.90E−1 
               
               
                   
                   
               
            
           
         
       
     
     In any example, the target tissue (e.g. tumor, infarcted heart tissue, aberrant or otherwise diseased or damaged tissue) is identified using appropriate diagnostic imaging methods such as CT scan, MRI, electro-anatomical mapping, ultrasound, etc. Using a fine hypodermic needle, a chosen solution (e.g. saline or D5W) is injected to cover the entire target tissue. The solution may also contain other ingredients such as dyes or radiopaque contrast media to aid the operator to identify the target region clearly. This can be done during the diagnosis or immediately before the ablation. A catheter containing the microwave antenna (any one of the antenna configurations (e.g. needle or conductor configurations) disclosed herein) is advanced to the target site using appropriate sheaths or scopes or visualizing techniques. The antenna is then deployed into the target tissue by penetration. Microwave energy of appropriate power and duration is then applied to denature the target tissue. The use of the coupling solution may be utilized to optimize the impedance match (and thus efficiency) in the target region and help optimize ablation of the border zone around a tumor tissue or infarcted cardiac tissue, or other treatment tissue. Additionally, the coupling solution may be utilized dynamically during energy delivery to stabilize the microwave properties of the target zone as the lesion matures. A coupling solution may be used with any of the examples of devices or methods disclosed herein. 
     In addition to improved efficiency via impedance matching, the coupling solution may be utilized to increase the conductivity (and absorption coefficient) of the target tissue. For example, inflated lung tissue has a conductivity of 0.79 S/m. Injection of a coupling solution such as saline into a specific target zone, with the target zone surrounded by inflated lung tissue, will increase the conductivity in that target region significantly. As a result, the target zone may undergo preferential heating due to increased absorption in that region compared with the surrounding tissue. 
     Microwave Antenna for Recording EGM During Ablation 
     It may be desirable during ablation procedures such as cardiac ablation procedures to monitor electrical activity in cardiac tissue. Performing simultaneous power delivery and electrogram recording can provide useful feedback to the operator regarding the formation of lesions during cardiac ablation. 
       FIG. 41A  shows a filter circuit  4100  that is connected to any of the applicator devices disclosed herein to allow simultaneous application of microwave energy and electrogram recording with the needles. The filter  4100  consists of two DC blocking capacitors  4104 ,  4116  and two Microwave Chokes  4114 ,  4112  (inductors). The DC blocking capacitors create a low impedance path for microwave energy and a high impedance path for low frequency cardiac electrical signals. The Microwave Chokes create a high impedance path for microwave energy and a low impedance path for low frequency cardiac electrical signals. With such a filter circuit, microwave energy that is input from a microwave generator  4102  will be transmitted into the microwave applicator  3508  with minimal leakage into the electrogram recording system. An example nominal design for operation at 2.45 GHz may have a minimum DC Blocking capacitor value of 60 pF to ensure an impedance of approximately of 1 ohm or less is presented to the microwave energy. In the same nominal design, a Microwave Choke of a minimum 68 nH inductance ensures an impedance of approximately 1000 ohms or greater is presented between the microwave generator and electrogram recording system. In the frequency range of normal cardiac electrical activity, this value of inductance also provides an impedance of less than 1 ohm between the applicator and the recording system. This nominal design will provide approximately −30 dB of isolation between the microwave generator end recording system, while having negligible microwave loss between the microwave generator and applicator. Other values of capacitance and inductance may also be possible depending on the isolation and loss requirements for the system. 
       FIG. 41B  shows that the filter circuit in  FIG. 41A  can also be implemented as a single sided filter  4100   a , with a DC Block and Microwave Choke  4104 ,  4114  only attached to the positive line  4106  of the applicator. In this configuration the negative line  4110  of the applicator is attached directly to the generator  4102  and recording system  4118 . 
     Anchoring and Tip Stabilization 
     As previously discussed above with respect to  FIGS. 24A-24C , any microwave ablation device disclosed herein may include some or all of the anchoring features described herein. The ablation device may include an anchor element which allows the ablation catheter to be stabilized in the tumor or other target treatment area. In an example, there may be one anchor element to stabilize the target tissue. In another example, more than one anchor elements may penetrate the target ablation region and stabilize the tumor. In any example, the one or more anchor elements penetrate the target ablation zone region before the one or more transmission lines penetrate the target ablation region. In another example, the transmission lines penetrate the target ablation region before the one or more anchor elements penetrate the target ablation region. 
     Any of the treatment devices disclosed herein may be delivered to the target treatment tissue in any number of ways. For example, for treating cardiac tissue, the treatment device may be disposed in a vascular catheter that can be advanced transvascularly to the treatment tissue. For treatment of lung tissue, the device may be delivered via a bronchoscope. The device may be advanced through skin into the body to treat the target tissue from outside the body, or a surgical incision may be used to provide access to the treatment tissue and the device may be advanced from outside the body through the incision. Other scopes or access routes may also be used. 
     NOTES AND EXAMPLES 
     The following, non-limiting examples, detail certain aspects of the present subject matter to solve the challenges and provide the benefits discussed herein, among others. 
     Example 1 is a device for treating tissue, said device comprising: a plurality of conductors forming one or more transmission lines configured to deliver microwave energy to target tissue. 
     Example 2 is the device of Example 1, wherein the plurality of conductors comprises two conductors, three conductors, or four conductors. 
     Example 3 is the device of any of Examples 1-2, wherein the one or more transmission lines are configured such that power is reflected from an end of the transmission lines and overall reflected power decreases during at least a portion of an ablative procedure. 
     Example 4 is the device of any of Examples 1-3, wherein the one or more transmission lines are designed such that the reflected power decreases, reaches a minimum, and begins increasing again as ablation progresses. 
     Example 5 is the device of any of Examples 1-4, wherein the reflected power reaches a minimum when an ablation region is surrounded by a biocompatible solution. 
     Example 6 is the device of any of Examples 1-5, wherein the biocompatible solution is a normal saline solution, buffered saline solution, dextrose solution, lactate Ringer&#39;s solution, or a mixture thereof. 
     Example 7 is the device of any of Examples 1-6, wherein the biocompatible solution comprises radiopaque dyes, regular dyes, tissue stains, or a combination thereof to improve visualization of the target tissue. 
     Example 8 is the device of any of Examples 1-7, further comprising a coaxial cable having a center conductor and a shield, wherein the plurality of conductors comprise a first conductor and a second conductor, and wherein the first conductor is electrically coupled with the center conductor and the second conductor is electrically coupled with the shield. 
     Example 9 is the device of any of Examples 1-8, further comprising a coaxial cable having a center conductor and a shield, wherein the plurality of conductors comprise a first conductor, a second conductor, and a third conductor, and wherein the first conductor is electrically coupled with the center conductor and the second and the third conductors are electrically coupled with the shield, and wherein the first conductor is disposed between the second and third conductors. 
     Example 10 is the device of any of Examples 1-9, further comprising a coaxial cable having a center conductor and a shield, wherein the plurality of conductors comprise a first conductor, a second conductor, and a third conductor, and wherein the second and third conductors are electrically coupled with the center conductor and the first conductor is electrically coupled with the shield, and wherein the first conductors is disposed between the second and third conductors. 
     Example 11 is the device of any of Examples 1-10, wherein the plurality of conductors comprise a first conductor and a second conductor, each conductor having a proximal end and a distal end, and wherein the first and second conductors are electrically coupled together adjacent their distal ends to form an electrical short between the first and second conductors. 
     Example 12 is the device of any of Examples 1-11, wherein the plurality of conductors comprise a first conductor and a second conductor, and wherein the first and second conductors are coupled together to form a loop. 
     Example 13 is the device of any of Examples 1-12, further comprising a coaxial cable having a center conductor and a shield, wherein the plurality of conductors comprise a first conductor and a second conductor, and wherein the first conductor is electrically coupled with the center conductor and the second conductor is electrically insulated from the shield. 
     Example 14 is the device of any of Examples 1-13, wherein the plurality of conductors comprise a first conductor and a second conductor, and wherein the first conductor forms a loop, and wherein the second conductor is discrete from the first conductor, and the second conductor forms a second loop electromagnetically coupled with the first loop. 
     Example 15 is the device of any of Examples 1-14, wherein the plurality of conductors comprise a first conductor and a second conductor, and wherein the first conductor and the second conductor are discrete from one another, and wherein the second conductor is configured to be inserted into the target tissue separately from the first conductor, and wherein the first and second conductors are electromagnetically coupled with one another. 
     Example 16 is the device of any of Examples 1-15, wherein the plurality of conductors comprise a first conductor having a first length and a second conductor having a second length, and wherein the first length is different than the second length. 
     Example 17 is the device of any of Examples 1-16, further comprising a first coaxial cable having a first center conductor and a second coaxial cable having a second center conductor, and wherein the plurality of conductors comprise a first conductor coupled with the first center conductor, and a second conductor coupled with the second center conductor. 
     Example 18 is the device of any of Examples 1-17, wherein the plurality of conductors comprises a first plate and second plate. 
     Example 19 is the device of any of Examples 1-18, further comprising a printed circuit board, and wherein at least some of the plurality conductors comprise one or more traces disposed on the printed circuit board. 
     Example 20 is the device of any of Examples 1-19, wherein the plurality of conductors comprises a plurality of needles. 
     Example 21 is the device of any of Examples 1-20, wherein the plurality of needles comprise four needles disposed approximately 90 degrees circumferentially apart from one another. 
     Example 22 is the device of any of Examples 1-21, further comprising a temperature monitoring element. 
     Example 23 is the device of any of Examples 1-22, wherein at least one of the plurality of conductors comprise a fusible link that is melted or otherwise decoupled from the device by applying current therethrough. 
     Example 24 is the device of any of Examples 1-23, further comprising an actuatable anchor element having a collapsed configuration, and an extended configuration in which the anchor element is configured to anchor the device to the tissue. 
     Example 25 is the device of any of Examples 1-24, wherein the anchor element is independently advanceable and retractable relative to the plurality of conductors, and wherein the anchor element is configured to penetrate the target tissue without displacement thereof. 
     Example 26 is the device of any of Examples 1-25, wherein the anchor element is an active element of the one or more transmission lines. 
     Example 27 is the device of any of Examples 1-26, wherein the anchor element is a passive element of the one or more transmission lines. 
     Example 28 is the device of any of Examples 1-27, wherein at least some of the plurality of conductors are independently advanceable and retractable relative to one another. 
     Example 29 is the device of any of Examples 1-28, wherein at least some of the plurality of conductors are pivotable relative to one another. 
     Example 30 is a system for treating tissue, said system comprising the device of any of Examples 1-29; and a microwave generator. 
     Example 31 is the system of Example 30, wherein the microwave generator comprises a multi-frequency generator or a phase changing generator. 
     Example 32 is the system of any of Examples 30-31, further comprising a coaxial cable coupled to the device and a cooling element thermally coupled to the coaxial cable, and wherein the cooling element is configured to cool the coaxial cable during operation of the device. 
     Example 33 is the system of any of Examples 30-32, wherein the device is operably coupled to the filter circuit to allow simultaneous application of microwave energy and recording of an electrogram. 
     Example 34 is a method for treating tissue, said method comprising: providing an energy delivery apparatus having a plurality of conductors forming one or more transmission lines; inserting at least a portion of the plurality conductors into target tissue; delivering microwave energy to the target tissue with the plurality of conductors; and ablating the target tissue. 
     Example 35 is the method of Example 34, wherein delivering the microwave energy comprises delivering the microwave energy from a coaxial cable to the plurality of conductors. 
     Example 36 is the method of any of Examples 34-35, wherein delivering the microwave energy comprises delivering the microwave energy through the plurality of conductors, and wherein the plurality of conductors have different lengths. 
     Example 37 is the method of any of Examples 34-36, wherein the plurality of conductors comprises a separate conductor, and wherein inserting the plurality of conductors comprises inserting the separate conductor separately from the insertion of the other of the plurality of conductors. 
     Example 38 is the method of any of Examples 34-37, wherein inserting the plurality of conductors comprises disposing one or more flat plates against the target tissue. 
     Example 39 is the method of any of Examples 34-38, further comprising monitoring a temperature of the target tissue. 
     Example 40 is the method of any of Examples 34-39, further comprising passing a current through at least one of the plurality of conductors and melting the at least one of the plurality of conductors or otherwise separating the at least one of the plurality of conductors into a plurality of segments. 
     Example 41 is the method of any of Examples 34-40, wherein delivering the microwave energy comprises delivering multiple frequencies or multiple phases of microwave energy. 
     Example 42 is the method of any of Examples 34-41, further comprising cooling the energy delivery apparatus or a coaxial cable coupled thereto. 
     Example 43 is the method of any of Examples 34-42, further comprising actuating an anchor element on the energy delivery apparatus and anchoring the energy delivery apparatus to the target tissue or tissue adjacent thereto. 
     Example 44 is the method of any of Examples 34-43, wherein the anchoring is achieved prior to inserting the plurality conductors into the target tissue. 
     Example 45 is the method of any of Examples 34-44, further comprising advancing or retracting at least some of the plurality of conductors independently of one another. 
     Example 46 is the method of any of Examples 34-45, further comprising pivoting at least some of the plurality of conductors relative to one another. 
     Example 47 is the method of any of Examples 34-46, further comprising injecting a biocompatible solution into the target tissue and altering a property of the target tissue to facilitate microwave ablation of the target tissue, or to facilitate visualization of the target tissue. 
     Example 48 is the method of any of Examples 34-47, wherein injecting the biocompatible solution occurs during ablation of the target tissue. 
     Example 49 is the method of any of Examples 34-48, wherein injecting the biocompatible solution occurs before ablation of the target tissue. 
     In Example 50, the apparatuses or methods of any one or any combination of Examples 1-49 can optionally be configured such that all elements or options recited are available to use or select from. 
     The above detailed description includes references to the accompanying drawings, which form a part of the detailed description. The drawings show, by way of illustration, specific embodiments in which the invention can be practiced. These embodiments are also referred to herein as “examples.” Such examples can include elements in addition to those shown or described. However, the present inventors also contemplate examples in which only those elements shown or described are provided. Moreover, the present inventors also contemplate examples using any combination or permutation of those elements shown or described (or one or more aspects thereof), either with respect to a particular example (or one or more aspects thereof), or with respect to other examples (or one or more aspects thereof) shown or described herein. 
     In the event of inconsistent usages between this document and any documents so incorporated by reference, the usage in this document controls. 
     In this document, the terms “a” or “an” are used, as is common in patent documents, to include one or more than one, independent of any other instances or usages of “at least one” or “one or more.” In this document, the term “or” is used to refer to a nonexclusive or, such that “A or B” includes “A but not B,” “B but not A,” and “A and B,” unless otherwise indicated. In this document, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Also, in the following claims, the terms “including” and “comprising” are open-ended, that is, a system, device, article, composition, formulation, or process that includes elements in addition to those listed after such a term in a claim are still deemed to fall within the scope of that claim. Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. 
     The above description is intended to be illustrative, and not restrictive. For example, the above-described examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments can be used, such as by one of ordinary skill in the art upon reviewing the above description. The Abstract is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Also, in the above Detailed Description, various features may be grouped together to streamline the disclosure. This should not be interpreted as intending that an unclaimed disclosed feature is essential to any claim. Rather, inventive subject matter may lie in less than all features of a disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description as examples or embodiments, with each claim standing on its own as a separate embodiment, and it is contemplated that such embodiments can be combined with each other in various combinations or permutations. The scope of the invention should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.