Dynamically matched microwave antenna for tissue ablation

<u>BACKGROUND</u>

1. Technical Field

0001The present disclosure relates generally to microwave applicator probes used in tissue ablation procedures. More particularly, the present disclosure is directed to a microwave probe that can be tuned during ablation procedures to obtain a desired impedance match.

2. Background of Related Art

0002Treatment of certain diseases requires destruction of malignant tissue growths (e.g., tumors). It is known that tumor cells denature at elevated temperatures that are slightly lower than temperatures injurious to surrounding healthy cells. Therefore, known treatment methods, such as hyperthermia therapy, heat tumor cells to temperatures above 41°C, while maintaining adjacent healthy cells at lower temperatures to avoid irreversible cell damage. Such methods involve applying electromagnetic radiation to heat tissue and include ablation and coagulation of tissue. In particular, microwave energy is used to coagulate and/or ablate tissue to denature or kill the cancerous cells.

0003Microwave energy is applied via microwave ablation antenna probes which penetrate tissue to reach tumors. There are several types of microwave probes, such as monopole, dipole, and helical. In monopole and dipole probes, microwave energy radiates perpendicularly from the axis of the conductor. Monopole probe (e.g., antenna) includes a single, elongated microwave conductor surrounded by a dielectric sleeve, having a conductor exposed at the end of the probe. Dipole probes have a coaxial construction including an inner conductor and an outer conductor separated by a dielectric portion. More specifically, dipole microwave antennas have a long, thin inner conductor which extends along a longitudinal axis of the probe and is surrounded by an outer conductor. In certain variations, a portion or portions of the outer conductor may be selectively removed to provide for more effective outward radiation of energy. This type of microwave probe construction is typically referred to as a "leaky waveguide" or "leaky coaxial" antenna.

0004In helical probes, microwave energy is directed in a forward direction. This is due to microwave energy radiating perpendicularly from the antenna, which when in helical configuration directs the energy waves in a forward direction. In helical probes the inner conductor is formed in a uniform spiral pattern (e.g., a helix) to provide the required configuration for effective radiation.

0005Conventional microwave probes have a narrow operational bandwidth, a wavelength range at which optimal operational efficiency is achieved, and hence, are incapable of maintaining a predetermined impedance match between the microwave delivery system (e.g., generator, cable, etc.) and the tissue surrounding the microwave probe. More specifically, as microwave energy is applied to tissue, the dielectric constant of the tissue immediately surrounding the microwave probe decreases as the tissue is cooked. The drop causes the wavelength of the microwave energy being applied to tissue to increase beyond the bandwidth of the probe. As a result, there is a mismatch between the bandwidth of conventional microwave probe and the microwave energy being applied. Thus, narrow band microwave probes may detune as a result of steam generation and phase transformation of the tissue hindering effective energy delivery and dispersion.

0006<patcit id="pcit0001" dnum="US20050245919A1"><text>US 2005/0245919 A1</text></patcit> relates to an antenna for microwave tissue ablation. It comprises a coaxial conductor providing a central first conductor surrounded by an insolating dielectric layer in turn surrounded by a second outer coaxial shield. The central conductor with or without the dielectric layer extends a distance L2 out from the conductor of the shield whereas the shield extends a distance L1 out from the conductor.

<u>SUMMARY</u>

0007The present disclosure provides for a microwave ablation probe which can be dynamically matched and/or tuned during ablation. As tissue is ablated, the radiating portion of the probe is actively tuned so that an optimal impedance match is achieved for a desired procedure. This is accomplished by adjusting the shape, size and/or dielectric properties of the components of the probe (e.g., adjusting the length of the conductors, insulating layers, and the like). In monopole and/or dipole antennas, the length of an inner conductor is adjusted to create a more efficient radiator.

0008According to one embodiment of the present disclosure a microwave ablation probe for providing microwave energy to tissue is disclosed. The probe includes a feedline having an inner conductor, a secondary inner conductor, an insulating spacer, and an outer conductor. The inner conductor is slidably disposed within the secondary inner conductor. The feedline also includes a radiating portion having an extruded portion of the inner conductor centrally disposed therein, wherein longitudinal movement of the inner conductor relative to the feedline tunes the radiating portion.

0009According to another embodiment of the present disclosure a microwave ablation probe for providing microwave energy to tissue is disclosed. The probe includes a feedline having an inner conductor, an insulating spacer and an outer conductor, and a radiating portion having an extruded portion of the inner conductor which is centrally disposed therein. The probe also includes a choke disposed around at least a portion of the feedline and configured to confine the microwave energy to the radiating portion.

<u>BRIEF DESCRIPTION OF THE DRAWINGS</u>

0010The above and other aspects, features, and advantages of the present disclosure will become more apparent in light of the following detailed description when taken in conjunction with the accompanying drawings in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a schematic diagram of a microwave ablation system according to the present disclosure;</li><li><figref idref="f0001">Fig. 2</figref> is a perspective cross-sectional view of a microwave ablation probe according to the present disclosure;</li><li><figref idref="f0001 f0002">Figs. 3A-C</figref> are side cross-sectional views of the microwave ablation probe of <figref idref="f0001">Fig. 2</figref>;</li><li><figref idref="f0002">Fig. 4</figref> is a perspective cross-sectional view of the microwave ablation probe having liquid cooled choke according to the present disclosure; and</li><li><figref idref="f0002">Fig. 5</figref> is a perspective cross-sectional view of one embodiment of the microwave ablation probe having a thermally reactive dielectric material therein according to the present disclosure.</li></ul>

0011The invention is defined in the claims.

<u>DETAILED DESCRIPTION</u>

0012Particular embodiments of the present disclosure will be described herein below with reference to the accompanying drawings. In the following description, well-known functions or constructions are not described in detail to avoid obscuring the present disclosure in unnecessary detail.

0013<figref idref="f0001">Fig.1</figref> shows a microwave ablation system 10 which includes a microwave ablation probe 12 coupled to a microwave generator 14 via a flexible coaxial cable 16 that is coupled to a connector of the generator 14. The generator 14 is configured to provide microwave energy at an operational frequency from about 500 MHz to about 2500 MHz.

0014During microwave ablation, the probe 12 is inserted into tissue and microwave energy is supplied thereto. As tissue surrounding the probe 12 is ablated, the tissue undergoes desiccation and denaturization which results in a drop of the effective dielectric constant of the tissue. The drop in the effective dielectric constant, in turn, lengthens the wavelength of the microwave energy. Since the frequency is held constant during ablation, the increase in the wavelength results in the increase of the operational frequency. At the outset the probe 12 is at an initial match point - a predetermined operational frequency that increases to a higher frequency as the ablation continues. Thus, to maintain an impedance match between the probe 12 and the generator 14, the radiating properties of the probe 12 are dynamically adjusted throughout the procedure. This is accomplished by modifying the geometry and/or the dielectric properties of the probe 12.

0015<figref idref="f0001">Fig. 2</figref> shows one embodiment of the probe 12 including a feedline 26, a choke 28 and an adjustable radiating portion 30. The feedline 26 extends between the distal end of the probe 12 where the feedline 26 is coupled to the cable 16, to the radiating portion 30. The feedline 26 is constructed from a coaxial cable having an inner conductor 20 (e.g., wire) surrounded by an insulating spacer 22 which is then surrounded by an outer conductor 24 (e.g., cylindrical conducting sheath). In one embodiment, the feedline 26 may have a diameter of 2.16 mm (0.085 inches) and the insulating spacer 22 may have a dielectric constant of 1.7.

0016The feedline 26 may be flexible or semi-rigid and may be of variable length from a proximal end of the radiating portion 30 to a distal end of the cable 16 ranging from about 2.54 cm to about 25.4 cm (1 to about 10 inches). The inner conductor 20 and the outer conductor 24 may be constructed from a variety of metals and alloys, such as copper, gold, stainless steel, and the like. Metals may be selected based on a variety of factors, such as conductivity and tensile strength. Thus, although stainless steel has lower conductivity than copper and/or gold, it provides the strength required to puncture tissue and/or skin. In such cases, the inner and outer conductors 20 and 24 may be plated with conductive material (e.g., copper, gold, etc.) to improve conductivity and/or decrease energy loss.

0017In one embodiment, the feedline 26 includes a secondary inner conductor 23, as shown in <figref idref="f0001">Fig. 3A</figref>, having a tubular structure which surrounds the inner conductor 20. The inner conductor 20 is slidably disposed within the secondary inner conductor 23 (e.g., moves within the secondary inner conductor 23 while maintaining smooth continuous contact therewith), such that the inner conductor 20 can be slid in either the proximal and/or distal direction to tune the inner conductor 20 to a desired operational frequency. The inner conductor 20 and the secondary inner conductor 23 are in electromechanical contact, allowing the inner conductor 20 to slide in and out of the feedline 26 during tuning while continuing to conduct microwave energy.

0018As shown in <figref idref="f0001">Fig. 3B</figref>, the feedline 26 includes one or more grooves 25 which mechanically interface with one or more corresponding stop members 27 disposed on the inner conductor 20. The groove 25, may be disposed in the secondary inner conductor 23 and/or the insulative spacer 22. The groove 25 in conjunction with the corresponding stop member 27, guides and limits the movement of the inner conductor 20 as the inner conductor 20 is slid within the feedline 26. Further, the groove 25 and stop member 27 combination provides for additional conductive contact between the secondary inner conductor 23 and the inner conductor 20. In embodiments, the location of the groove 25 and the stop member 27 may be interchanged, such that the groove 25 may be disposed within the inner conductor 20 and the stop member 27 may be disposed on the secondary inner conductor 23.

0019With reference to <figref idref="f0001">Fig. 2</figref>, the choke 28 of the probe 12 is disposed around the feedline 26 and includes an inner dielectric layer 32 and an outer conductive layer 34. The choke 28 confines the microwave energy from the generator 14 to the radiating portion 30 of the probe 12 thereby limiting the microwave energy deposition zone length along the feedline 26. The choke 28 is implemented with a quarter wave short by using the outer conductive layer 34 around the outer conductor 24 of the feedline 26 separated by the dielectric layer 32. The choke 28 is shorted to the outer conductor 24 of the feedline 26 at the proximal end of the choke 28 by soldering or other means. In embodiments, the length of the choke 28 may be from a quarter to a full wavelength. The choke 28 acts as a high impedance to microwave energy conducted down the outside of the feedline 26 thereby limiting energy deposition to the end of the probe. In one embodiment, the dielectric layer 32 is formed from a fluoropolymer such as tetrafluorethylene, perfluorpropylene, and the like and has a thickness of 0.127 mm (0.005 inches). The outer conductive layer 34 may be formed from a so-called "perfect conductor" material such as a highly conductive metal (e.g., copper).

0020As shown in <figref idref="f0002">Fig. 3C</figref>, the choke 28 is configured to slide atop the feedline 26 along the longitudinal axis defined by the probe 12. Sliding the choke 28 in either proximal and/or distal direction along the feedline 26 provides for adjustment of the length of the radiating portion 30. The choke 28 includes a groove 33 disposed within the dielectric layer 32. The groove 33 is configured to mechanically interface with a stop member 35 that is disposed on the outer conductor 24. The stop member 35 guides the sliding of the choke 28 along the length of the groove 33.

0021Moving one or both of the inner conductor 20 and the choke 28 relative to the feedline 26 allows for adjustment of the length of the radiating portion 30, such as adjusting the choke 28 and the inner conductor 20 to be ¼ wavelength long as the ablation continues to maintain ½ wavelength dipole. In embodiments, the inner conductor 20, the feedline 26 and the choke 28 may have markings and/or indicia thereon to indicate desired wavelength adjustment positions.

0022In one embodiment, the grooves 25 and 33 and/or the stop members 27 and 35 may include one or more detents (not explicitly shown) which provide tactile feedback when the choke 28 and/or inner conductor 20 are slid along the feedline 26. This allows for more precise movement of the components and tuning of the radiating portion 30.

0023The probe 12 further includes a tapered end 36 which terminates in a tip 38 at the distal end of the radiating portion 30. The tapered end 36 allows for insertion of the probe 12 into tissue with minimal resistance. In cases where the radiating portion 12 is inserted into a pre-existing opening, the tip 38 may be rounded or flat. The tapered end 36 may be formed from any hard material such as metal and/or plastic.

0024<figref idref="f0002">Fig. 4</figref> shows another embodiment of the probe 12 having a liquid-cooled choke 40 that includes a cylindrical conducting housing 42 having a chamber 44 and defining a cylindrical cavity 46 which surrounds the feedline 26. The housing 42 is formed from a conducting metal such as copper, stainless steel, and/or alloys thereof. The housing 42 includes one or more inlet tubes 48 and outlet tubes 50 which cycle a cooling dielectric liquid 52 (e.g., water, saline solution, and the like) through the chamber 44. The liquid 52 may be supplied by a pump (not explicitly shown) configured to adjust the flow rate of the liquid 52 through the chamber 44. As the liquid 52 is supplied into the choke 40, the heat generated by the feedline 26 is removed. Further, compounds used in the liquid 52 may be adjusted to obtain a desired dielectric constant within the choke 28. This may be useful in multifrequency probes allowing the resonant frequency of the choke 28 to be adjusted by filling the chamber 44 with varying fluid volume and/or varying the ratio of air and liquid therein.

0025The housing 42 also includes an O-ring 54 having an opening 56 allowing the O-ring 54 to fit within the chamber 44. As the chamber 44 is filled with the liquid 52, the liquid 52 pushes the O-ring 54 in the distal direction within the chamber 44. The O-ring 54 fits the walls of the chamber 44 in a substantially liquid-tight fashion preventing the liquid 52 from seeping into a distal portion 58 of the chamber 44. This allows selective or automatic adjustment of the cooling temperature of the choke 28 by limiting the volume of the chamber 44 being filled with the liquid 52.

0026More specifically, the O-ring 54 is formed from rubber, silicone rubber and other elastomer material such that the frictional forces between the O-ring 54 and the housing 42 maintain the O-ring 54 in position until the flow rate of the liquid 52 is sufficient to shift the O-ring 54 in the distal direction. In one embodiment, the distal portion 58 includes sloping or chamfered walls 60 inside the chamber 44. As the O-ring 54 is pushed in the distal direction, the sloping walls 60 compress the O-ring 54 which requires an increase in the flow rate of the liquid 52. This provides for a counter-force that pushes back against the flow of the liquid 52 requiring an increase in the flow rate if additional filling of the chamber 44 (e.g., additional cooling of the choke 28) is desired. Once the liquid 52 is withdrawn from the choke 28, the O-ring 54 is moved back into its original position (e.g., in the proximal direction) by the compression of the walls 60.

0027<figref idref="f0002">Fig. 5</figref> shows a further embodiment of the probe 12 having a ferroelectric material therein. More specifically, the probe 12 includes an internal ferroelectric loading 70 at a distal end of the feedline 26 and an external ferroelectric loading 74 at the distal end of the inner conductor 20. In one embodiment, the internal ferroelectric loading 70 may be have a length corresponding to the quarter wave of the microwave frequency and act as a dynamic quarter-wave transformer.

0028The ferroelectric loadings 70 and 74 include ferroelectric material such as lead zirconate, lead titanate, barium titanate, and the like. Ferroelectric materials provide for dynamic matching of the probe 12 to the tissue due to changing dielectric properties of such materials when DC electric field is applied across thereof during application of microwave energy to the probe 12 such that the DC electric field biases the ferroelectric material. The DC electric field is supplied to the loadings 70 and 74 through the outer conductor 24 and inner conductor 20 respectively. As the DC electric field is supplied to the loadings 70 and 74, the dielectric constant thereof is varied. The "+" and "-" illustrate one possible polarity of DC electric field within the probe 12. As the wavelength of the frequency of operation increases due to desiccation of the tissue, the DC electric field is supplied to the loadings 70 and 74 is also adjusted accordingly to increase the dielectric constant accordingly. This counteracts the detuning of the probe 12 due to the changes in the tissue. In one embodiment, the DC electric field supply (not explicitly shown) may be controlled via a feedback loop by the generator 14 based on impedance measurement of the probe 12 and the cable 16 and other methods within purview of those skilled in the art. In another embodiment, the supply of the DC current may be varied in a predetermined fashion over time based on empirical laboratory measurements.

0029The described embodiments of the present disclosure are intended to be illustrative rather than restrictive, and are not intended to represent every embodiment of the present disclosure. Various modifications and variations can be made without departing from the scope of the disclosure as set forth in the following claims both literally and in equivalents recognized in law.