Microwave apparatus for controlled tissue ablation

[0001] 1. Field of the Invention

[0002] This invention relates to a microwave apparatus for controlled tissue ablation. It relates especially to such apparatus which incorporates a minimally invasive microwave antenna catheter which can transmit and receive signals at different frequencies.

[0003] It is well known in the medical field to transmit electromagnetic energy to internal tissue by way of an antenna placed close to the tissue in order to heat the tissue sufficiently to ablate or necrose same by hyperthermia. Invariably the antenna is incorporated into a catheter which can be inserted into the body and positioned near the tissue of interest. In many cases, catheter ablation of tissue is the preferred treatment for cardiac arrhythmia.

[0004] 2. Description of the Prior Art

[0005] Heretofore, various antenna types have been used for ablation/necrosis treatment applications. These include monopole, dipole and helical antennas as well as capacitor tip antennas. See, for example, my U.S. Pat. Nos. 4,583,556, 5,683,382 and 6,932,776. Although those prior antennas and the apparatus associated therewith are certainly capable of heating tissue, they are not particularly suitable for carrying out minimally invasive procedures such as the ablation of heart tissue. This is because the prior antennas are relatively large and stiff making it difficult to place the antenna in the position necessary to treat the problem of cardiac arrhythmias. Such treatment usually requires that the antenna be placed close to one of the major blood vessels where it connects to the heart and to deliver a uniform, predictable and controllable heating pattern to ablate or necrose tissue in the vessel wall that will produce a lesion of just the right size to treat the arrhythmias.

[0006] Also, the antenna catheters commonly used for this purpose do not monitor the actual temperature of the tissue being heated by the catheter. At most, they may incorporate a thermister or thermocouple in the catheter tip that is used to control the output of the transmitter that produces the microwave energy radiated by the antenna. However, those devices simply sense the temperature at the catheter, not the actual temperature of the tissue being treated. This can result in the tissue being underheated in which case the procedure will take longer than it should, or being overheated resulting in unintended damage to the tissue adjacent to the desired lesion.

[0007] Accordingly, it is an object of the present invention to provide an improved microwave apparatus for producing controlled tissue ablation.

[0008] Another object of the invention is to provide such apparatus which includes a minimally invasive antenna catheter that is particularly suitable for treating cardiac arrhythmias.

[0009] Still another object of the invention is to provide apparatus such as this which is capable of heating tissue in a controlled manner.

[0010] A further object of the invention is to provide a minimally invasive microwave antenna particularly adapted to ablate or necrose tissue at or near the heart.

[0011] A further object of the invention is to provide such an antenna which can operate over a large frequency range.

[0012] A further object of the invention is to provide a microwave antenna which can simultaneously transmit and receive signals having substantially different frequencies.

[0013] Still another object of the invention is to provide an antenna catheter which has an elongated uniform antenna pattern which can be directed toward the tissue being treated.

[0014] A further object of the invention is to provide an antenna catheter having one or more of the above characteristics and which can be made relatively inexpensively using known etched or printed circuit techniques.

[0015] Other objects will, in part be obvious, and will, in part, appear hereinafter.

[0016] The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the construction hereinafter set forth, and the scope of the invention will be indicated in the claims.

[0017] In general, my apparatus for controlled tissue ablation comprises an elongated flexible antenna including a first, strip-like, conductor having a proximal end and a distal end and a second, serpentine conductor, composed of a series of similar segments and having a distal end connected to the distal end of the first conductor and a proximal end. Preferably, the second conductor segments are wider than the first conductor so that those segments overhang the sides of the first conductor. A dielectric spacer is interposed between the second conductor segments and the first conductor to electrically isolate same. Preferably also, the first and second conductors as well as the spacer are enclosed within an electrically insulating enclosure so that the antenna conductors cannot directly contact the tissue of a patient being treated.

[0018] In use, a microwave transmitter/receiver unit is connected by way of the inner and outer conductors of a coaxial cable to the proximal ends of the first and second conductors, respectively. The transmitters/receiver unit includes a microwave transmitter which transmits a heating signal at a first frequency to the antenna via the cable which causes the antenna to radiate energy in a uniform pattern capable of heating tissue adjacent to the antenna. The antenna is designed to operate over a wide frequency range so that it can simultaneously detect much higher frequency emissions from the tissue being treated which signal is a direct indication of the temperature of that tissue. The temperature-indicating measuring signal is applied via the cable to the transmitter/receiver unit which includes a radiometer whose output provides a real time indication of the tissue temperature.

[0019] As we shall see, the transmitter/receiver unit also includes a diplexer which prevents the heating signal from being applied to the radiometer and also prevents the measuring signal from being coupled to the transmitter. The transmitter/receiver unit may also include a processor which responds to the output of the radiometer to apply a control signal to the transmitter causing the antenna to heat the adjacent tissue in a controlled manner. Thus the apparatus as a whole is able to controllably heat the tissue and provide a direct indication of the temperature of the tissue being heated. This enables the apparatus to be placed in proximity to the tissue being treated and to ablate or necrose that tissue to produce a lesion of just the right size, e.g. to treat a cardiac arrhythmia.

[0020] In a preferred embodiment of the invention, the antenna enclosure comprises a housing having a portion adjacent to the first conductor made of a material having a low dielectric constant. Thus, when the second conductor is placed in close proximity to a high dielectric material such as tissue which has a relatively high water content, the antenna pattern will be directed toward that tissue. Such a dielectric enclosure thus increases the heating efficiency of the apparatus and optimizes its ability to detect small changes in the temperature of the tissue being heated.

[0030] Referring to FIG. 1 of the drawings, my apparatus comprises a microwave antenna catheter shown generally at 10 which includes a co-axial cable 12 terminated by a connector 13 which is adapted to be coupled to the connector 14a of a remote transmitter/receiver unit 14.

[0031] Unit 14 includes a microwave transmitter 15 which transmits a signal at a first frequency approved by the Federal Communications Commission, e.g. 2450 or 915 MHz, via a diplexer 16 to catheter 10. This causes the antenna to radiate electromagnetic energy in a pattern capable of heating tissue adjacent to the catheter. The antenna catheter 10 also detects thermal radiation emitted by that tissue and applies a corresponding measuring signal via cable 12 to the diplexer 16. The diplexer routes that signal to a radiometer 18 in unit 14. The radiometer operates at a center frequency in the order of 1 to 4 GHz so that the apparatus can detect thermal emissions from locations relatively deep in the tissue being treated. The output of radiometer 18 is processed by a processor 19 in unit 14 which controls a display 20 therein.

[0032] The diplexer 16 and the other elements in unit 14 are known and not part of this invention. Suffice it to say here that the diplexer 16 isolates the radiometer 18 from the signal being transmitted by transmitter 15 to the antenna catheter 10 and isolates the transmitter 15 from the temperature-indicating signal from catheter 10 being routed to the radiometer 18. The processor 19 receives the signal from radiometer 18 and uses that signal to control transmitter 15 so that the antenna catheter radiates sufficient energy to maintain the adjacent tissue at a selected temperature. The processor 19 may also control display 20 to display a variety of different parameters such as tissue temperature, transmitter frequency, output power, reflected power, elapsed time, etc. In FIG. 1, all of the external elements of the apparatus are shown in a single unit 14. It should be understood that the diplexer 16 and radiometer could be placed in a small subunit 14_p proximal to the catheter 10 implanted in a patient to minimize losses and signed errors due to the coaxial cable, while the other external elements may be placed in a remote subunit 14_R far removed from the patient.

[0033] As best seen in the FIGS. 1 and 2, the antenna catheter 10 comprises a first elongated conductor 26 having a proximal end 26a and a distal end 26b. Preferably, the conductor is in the form of a flat metal strip having a generally rectangular cross section as shown. The cable 12 includes an inner conductor 12a which is connected to the proximal end 26a of first conductor 26 via a weld or solder bead 27.

[0034] Antenna catheter 10 also includes a second conductor 28 in the form of a serpentine wire that overlies conductor 26 and is lengthwise coextensive therewith. Thus conductor 28 is made up of a series of sinuous segments or cycles 28a which extend laterally beyond the side edges of conductor 26. The proximal end 28b of conductor 28 is connected to an outer conductor 12b of cable 12 via a solder or weld bead 30.

[0035] As best seen in FIG. 2, the distal end 28c of conductor 28 overlaps the distal end 26b of conductor 26 and those two ends are connected together by a weld bead 32. Thus, the conductor 26 is returned to ground via conductor 28 and the cable conductor 12b to assure patient safety.

[0036] Between their ends, the conductors 26 and 28 are electrically insulated from one another by a dielectric spacer 34 of a material having a low dielectric constant and a low loss tangent and which preferably, but not necessarily, extends the full width of the conductor segments 28a as shown in FIG. 2. A suitable spacer material is PTFE or silicone.

[0037] In a catheter 10 exemplar, the conductor 26 may be a 0.060×0.030 mm rectangular rod, the conductor 28 may be a 0.020 mm diameter wire, the insulating spacer 34 may be a 0.008 mm PTFE film and cover 46 may be a 0.005 mm film of the same material. The illustrated catheter 10 has an antenna length, i.e. the distance between conductor ends 28a and 28b, of 4 cm with the conductor segments 28a having a width of 0.016 mm, a spacing between adjacent segments of 16 mm, and a lateral overhang of 0.40 mm.

[0038] Desirably the antenna conductors 26 and 28 are electrically insulated from the patient. This may be accomplished by placing the conductors within an electrically insulating enclosure 42. The enclosure may be a flexible envelope or encapsulation of a dielectric material. More preferably, the enclosure 42 is, as shown, a box-like housing of a low dielectric material such as aerated PTFE or silicone and formed with a recess 44 which is wide and deep enough to accommodate the conductor segments 28a and the dielectric spacer 34, and with a slightly deeper axial recess extension 44a (FIG. 2) to receive conductor 26. The opening into recess 44 may be closed by a cover 46 consisting of a thin sheet of dielectric material, e.g. PTFE, which may be adhered to the top of enclosure 42.

[0039] Since the present apparatus provides a heating signal at a first frequency e.g. 2450 MHz (2.45 GHz), and detects thermal emissions at a second widely different frequency, e.g. 4 GHz, the antenna catheter 10 must operate over a wide frequency range allowing use at multiple frequencies with a good impedance match at those frequencies

[0040] FIG. 4 is a graph plotting the return loss or reflection coefficient of the above catheter exemplar across a range of frequencies. The curve C allows one to determine if the antenna is reasonably “matched” to the desired frequencies. To obtain the curve C, a network analyzer sends a small amount of energy to antenna catheter 10 at a number of different frequencies from 0-5 GHz and records the amount of energy that returns. The recorded energy levels are shown on a graph of energy level (in dB) versus frequency (in GHz). The graph starts at 0 dB and the lesser the return energy, the more negative the curve C. A well matched antenna should have a return loss less than 20 dB.

[0041] As can be seen from FIG. 4, curve C has relatively small dB numbers at the 2.45 GHz heating frequency (point 1) and the 4.0 GHz measuring frequency (between points 2 and 4), indicating that the antenna catheter is reasonably well matched at those frequencies. Ideally, the catheter 10 should be matched to operate at or near one of the points 1-4 of minimal energy return on curve C which points occur at regular intervals along the curve.

[0042] A major factor influencing the position of the minimum curve points in the frequency spectrum is the length of the antenna catheter 10. When antenna length is increased by increasing the number of sections 28a in conductor 28 (and lengthening conductor 26 correspondingly) the negative points 1 to 4 in curve C are shifted to the left in FIG. 4. This is apparent from a comparison of FIG. 4 with FIG. 5 which is a graph showing the return loss for an antenna that is 6 cm, vs. 4 cm, long with a 5 mil. spacer 34. The is opposite effect may be obtained by shortening the antenna.

[0043] Also, changing the cross-sections of conductors 26 and 28 and the thickness of the spacer 34 and the spacing of the conductor 28a segments similarly affects the frequency of the antenna. Therefore, once the catheter 10 length has been selected, one or another of the above antenna parameters may be varied to provide an antenna catheter which operates with maximum efficiency at the desired heating and measuring frequencies.

[0044] FIGS. 6A and 6B illustrate longitudinal and cross-sectional views respectively of the antenna pattern of catheter 10 operated at a heating frequency of 2.45 GHz. FIGS. 7A and 7B are similar views of the antenna pattern of catheter 10 operated at a frequency of 4.0 GHz. It should be noted that antenna patterns are usually obtained in the transmit or radiate mode rather than the receive mode. However, reciprocity dictates that the two patterns be identical. In any event, it is readily apparent from these figures that the antenna patterns for both the heating and measuring frequencies are relatively uniform along the catheter and are directed uniformly away from the cover 46; i.e. toward the tissue being treated.

[0045] FIG. 8 is a graph demonstrating the actual performance of catheter 10 during simultaneous heating and measuring of tissue placed opposite the catheter. In this example, power at 80 Watts and 2.45 GHz is applied to the catheter at time 15 seconds and held for 30 seconds at which time the power is reduced to 20 Watts. Radiometric data is then taken represented by curve R, along with temperature data from conventional Luxtron probes placed in the tissue, represented by curves T₁ and T₂. As seen from FIG. 8, the radiometric data are quite consistent with the temperatures measured by the probes imbedded in the tissue indicating that the subject apparatus provides an accurate real time measurement of the actual temperature of the tissue being treated.

[0046] When, as shown in FIGS. 1 and 2, the enclosure 42 is in the form of a housing made of a material having a low dielectric constant such as silicone, aerated PTFE and the like, the antenna has a directional antenna pattern. That is, when the antenna conductor 28 is placed in close proximity to tissue which has a relatively high water content and thus a relatively high dielectric constant, the microwave energy from the antenna will be directed into that tissue. Thus unlike the case with some prior catheters of this general type, no reflective shield is required which could reduce the overall flexibility of the antenna catheter. In other words, the present catheter has minimal or no field coupling in the direction away from the tissue being ablated. This feature improves both the safety and efficiency of the apparatus. It also makes it safe for the surgeon to handle and manipulate, i.e. he/she is less apt to suffer radiation burns.

[0047] It will be appreciated from the forgoing that antenna catheter 10 has several distinct advantages. More particularly, its conductors 26 and 28 can be made quite thin using conventional printed or etched circuit techniques thereby minimizing manufacturing cost. Also, as shown in FIG. 3, when those conductors are encased within a flexible enclosure 42, such as a plastic envelope 42′, the overall antenna catheter is flexible enough so that during a surgical procedure to treat a cardiac arrhythmia, it can be positioned around an artery A close to the heart H of a patient. Since the antenna conductor 28 is returned to ground and the antenna as a whole is electrically isolated by its enclosure from the tissue to be heated, the antenna should satisfy all FDA safety requirements.

[0048] It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained. Also, since certain changes may be made in the above constructions without departing from the scope of the invention, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

[0049] It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention described herein.