Integrated heating/sensing catheter apparatus for minimally invasive applications

[0001] This invention relates to a medical catheter apparatus for minimally invasive applications. It relates more particularly to an integrated antenna catheter system which relies on electromagnetic radiation to simultaneously controllably heat, and detect the temperature of, fluid or tissue adjacent to the catheter. By placing the catheter at the region of interest in the body, one can detect, diagnose and treat certain abnormalities associated with tumors, cardiac arrhythmias, benign prosthetic hyperplasia (BPH) and the like. When placed in a patient's vascular system, the catheter can be used to measure temperature or even to raise tissue temperature during heart surgery. Examples of known antenna catheter apparatus of this general type are described in my patents U.S. Pat. Nos. 5,683,382 and 6,932,776.

[0002] Obviously in order to perform their functions, such catheters must be small in diameter and quite flexible so that they can be threaded into the body to the target site via various natural passages in the body. They may also be required to facilitate various ancillary processes such as maintenance of the catheter at a selected or constant temperature, display of the target site, irrigation of the target site, etc.

[0003] Antenna catheters of the above type are invariably connected via a long cable to an external control unit which includes a transmitter for transmitting electromagnetic energy to the antenna catheter in order to heat fluid or tissue adjacent to the catheter, and a receiver in the form of a radiometer which detects thermal emissions picked up by the antenna catheter and which reflect the temperature of the fluid or tissue being examined. The receiver produces corresponding output signals to control a display which displays that temperature. Those same signals may also be used to control the transmitter to maintain a selected heating profile.

[0004] For such apparatus which detect thermal emissions in the microwave range which is of primary interest here, the receiver is usually a Dicke-type radiometer. Such a radiometer is a comparison radiometer system which utilizes a switch to alternately connect the antenna (the unknown temperature to be detected) and the reference temperature, which may be a stable noise source or a temperature sensor with in the catheter.

[0005] Every component of such a radiometer generates noise power that contributes to the overall noise of the system. Therefore, the total apparatus output contains not only noise received by the antenna, but also noise generated within the apparatus itself. Such variations within the apparatus can produce spurious output signal fluctuations that are sometimes greater than the signal level to be measured. To overcome these gain variations, Dicke developed the common load comparison radiometer. This configuration greatly reduces the effects of short-term gain fluctuations in the radiometer. More particularly, the switch provides a mechanism to allow both the reference and the unknown signals to pass through the apparatus essentially at the same time relative to the expected gain drift in the radiometer's amplifiers such that any drifting gain will be applied equally to both the antenna and reference signals.

[0006] Since the radiometer input is switched at a constant rate by the Dicke switch between the antenna and the constant-temperature reference load, the switched or modulated RF signal should, therefore, be inserted at a point prior to RF amplification in the radiometer and as close to the antenna as possible. Any component or transmission line located between the unknown temperature being detected and the Dicke switch can introduce an error. One such error source is the long coaxial cable which connects the antenna to the radiometer.

[0007] More particularly, the temperature of that cable contributes to the temperature measurement. The cable temperature is usually not known and varies along the length of the cable. That portion of the cable within the body will be at body temperature, whereas the segment of the cable outside the body will be at room temperature. All of these parameters may vary with the flexing and depth of insertion into the body of the probe. Also when the apparatus includes a transmitter, power is absorbed by the cable, causing the cable to be heated. If the loss in the cable is, say, 3 dB (which could easily be the case), one half of the antenna noise power comes from the desired tissue or fluid volume being examined and the rest results from in the cable.

[0008] Thus while errors common to both measurements, i.e. the unknown temperature versus the reference temperature, are cancelled in a Dicke-type radiometer, any changes or errors between the unknown and the Dicke switch affect only the unknown temperature measurement and are not common to both measurement paths.

[0009] Thus to achieve accurate measurement, it is highly desirable to minimize the losses between the antenna and the radiometer in order to improve the performance and reliability of the overall apparatus, without increasing the size, weight and cost thereof.

[0010] Accordingly the present invention aims to provide a minimally invasive antenna catheter apparatus for simultaneously controllably heating and sensing the temperature of fluid or tissue in a human or animal body.

[0011] Another object of the invention is to provide such apparatus which includes an antenna catheter which may be inserted into a patient and an external control unit and whose designs eliminate the need for a long, lossy coaxial cable between the catheter and the external unit.

[0012] A further object of the invention is to provide a minimally invasive temperature sensing apparatus comprising an antenna catheter and a receiver in the form of a radiometer wherein the radiometer is integrated right into the catheter.

[0013] Still another object of the invention is to provide an integrated antenna catheter including a built-in diplexer and microwave receiver which, when connected to an external control unit containing a transmitter, can simultaneously heat and detect the temperature of fluid or tissue adjacent to the catheter.

[0014] Other objects will, in part, be obvious and will, in part, appear hereinafter.

[0015] The invention accordingly comprises the features of construction, combination of elements and arrangement of parts which will be exemplified in the following detailed description and the scope of the invention will be indicated in the claims.

[0016] Briefly, the present apparatus comprises an antenna catheter for insertion into a human or animal patient. The catheter, sometimes also referred to as a probe, may be connected to an external control unit which may include a transmitter and a display. As usual, the apparatus also includes a receiver, preferably a Dicke-type radiometer. However, instead of incorporating the receiver into the external control unit as is done conventionally, all components of that receiver are integrated right into the catheter and preferably, but not necessarily, the transmitter transmits RF signals, i.e. below 300 MHz. Not only does this minimize the electrical distance between the antenna and the receiver, it also eliminates the need for long a coaxial cable which suffers signal losses and gives rise to measurement errors as described above. Therefore, the present apparatus has a higher signal-to-noise ratio and sensitivity than prior comparable apparatus of this type, yet is simpler and less expensive to make and maintain.

[0017] When the apparatus is intended to heat tissue as well as to sense the temperature thereof, a diplexer is also integrated right into the catheter as will be described in detail later so that the same catheter can simultaneously radiate energy received from an external transmitter at one frequency to heat tissue and detect thermal emissions from that tissue at a different frequency and which are indicative of tissue temperature.

[0026] Referring to FIG. 1 of the drawings, the apparatus comprises a minimally invasive flexible antenna catheter or probe 10 adapted to be threaded into a patient via a natural or incised passage in the patient's body, and an external control unit 12. The catheter is connected to unit 12 by a flexible cable 14 having an end connector 14a that connects to a mating connector 12a on unit 12. Typically, catheter 10 may be only 0.5 to 1.0 inch long and 0.08 to 0.10 inch in diameter.

[0027] Referring now to FIGS. 2 and 3, catheter 10 includes an inner conductor 16 and a coaxial tubular outer conductor 18. Conductor 16 is longer than conductor 18 and its front or leading end 16a is connected by solder, welding, etc. to the center of a conductive discoid toe plate 22 spaced in front of the outer conductor 18 which space is filled by a discoid dielectric spacer member 24. A hemispherical conductive shell 26 is mounted to the front face of toe plate 22. Together they form the conductive leading end or tip 10a of catheter 10. Shell 26 also defines a fluid-tight space 28 between the shell wall and the toe plate.

[0028] The rear or trailing end of outer conductor 18 is closed by a discoid heel cap 30 soldered or welded to conductor 18 and to the rear end 16b of inner conductor 16 which end extends into a small opening 32 at the center of heel cap 30. The rear end of center conductor 16 is also connected to the leading end of an inner conductor 33 of cable 14. Those two conductors meet in opening 32 with the cable end being anchored to heel cap 30.

[0029] As best seen in FIGS. 2 and 4, the segment of inner conductor 16 within the outer conductor 18 carries a dielectric sleeve 34 and is supported within conductor 18 by a conductive insert 36 which fits snugly in, and extends the length of, conductor 18. The conductor 16 and its sleeve 34 extend along an axial passage 38 in the insert. Insert 36 is in contact with both outer conductor 18 and heel cap 30.

[0030] As best seen in FIGS. 3 and 4, insert 36 is generally T-shaped in cross-section having a pair of arms 36a, 36a and a leg 36b which define, along with outer conductor 18, a pair of longitudinal passages 42, 42 which extend the length of the insert 36. These passages are able to accommodate a pair of tubes 44 whose forward ends are secured in a pair of holes 46, 46 in toe plate 22. The tubes extend along passages 42, 42 into through-holes 48, 48 in heel cap 30 where they communicate with passages 49, 49 in cable 14.

[0031] The tubes 44 provide a fluid path to and from the space 28 at the catheter tip 10a so that a fluid may be circulated through the catheter in order to cool the catheter tip or maintain it at a selected temperature. Actually, the insert 36 through which those tubes pass constitutes a heat sink. Therefore, the tubes may open into insert passages 42, 42 so that the entire catheter and its contents can be maintained at the temperature of that fluid. As shown in FIG. 1, the passages 49, 49 in cable 14 extend to connector 14a where they communicate with hoses 50 terminated by connectors 50a releasably coupled to control unit 12.

[0032] Of course, the insert passages 42, 42 and tubes 44, 44 could also be used for other purposes. For example a small hole 26 may be provided in shell 26 so that an irrigation fluid delivered by one or both tubes may be ejected from the catheter tip 10a. Also, the passages and/or tubes may accommodate small wires leading to a temperature sensor, ultrasonic transducer or other electrical device in the space 28 at the catheter tip, depending upon the particular application.

[0033] As best seen in FIGS. 2 and 3, a sheath 52 of a dielectric material e.g. PTFE, surrounds the outer conductor 18 of catheter 10. However, that sheath does not extend all the way to the front or leading end of the conductor, but rather terminates at a selected distance therefrom for reasons that will become apparent. The rear or trailing end of sheath 52 blends into cable 14 so that there is essentially no stepped boundary between the two, thereby enabling the catheter be threaded easily into a patient.

[0034] Referring now to FIGS. 2 to 4, a filter circuit 54 and a microwave radiometer circuit 56, preferably in the form monolithic microwave integrated circuit chips (MMICS), are mounted to the top of insert 36 which thus functions as a carrier and heat-sink for those chips. Also, mounted directly to the inner conductor 16 just ahead of insert 36 is a coupling capacitor 58 which is recessed into the spacer member 24. One terminal of capacitor 58 is connected electrically to conductor 16 and the other is connected by way of a lead strip 60 to filter circuit 54 which is, in turn, connected to radiometer circuit 56. The output signal from radiometer circuit 56 as well as certain bias and control voltages are carried on conductor group 64 which extends along the top of insert 36 and exits the catheter through a hole 66 in heel cap 30. There, those conductors join corresponding conductors 68 (FIGS. 1 and 3) which extends along cable 14 to control unit 12. Also, a ground return conductor 69 from circuit 56 connects to a corresponding conductor 70 in cable 14.

[0035] Preferably radiometer circuit 56 operates at a frequency in the microwave range, e.g 1 GHz to 26 GHz, most preferably at a center frequency of 4 GHz. A conventional Dicke-type radiometer circuit is disclosed in my patent U.S. Pat. No. 4,557,272. Similar radiometer designs on a chip are available from Meridian Medical Systems, Inc., the assignee of this application.

[0036] Referring to FIG. 1, the control unit 12 includes a transmitter or generator 72 which delivers power to catheter 10 by way of cable 14. The output signal from transmitter 72 has a frequency that is different from and preferably much lower than the radiometer frequency. The illustrated transmitter has an output in the RF frequency range, i.e. below 300 MHz, most preferably 500 KHz, so that cable 14 does not have to be a coaxial cable thereby enabling it to be more flexible and less lossy than a coaxial cable. If that is not a problem for a given application, generator 72 could transmit at a higher frequency in the microwave range. Transmitter 72 is controlled by a processor 74 which receives instructions via control buttons 76a on an operator control panel 76 in unit 12.

[0037] The control unit 12 also includes an amplifier 78 which receives the temperature-indicating output signal from radiometer circuit 56 via conductors 68 in cable 14. Amplifier 78 conditions that signal and routes it to processor 74 which produces a corresponding control signal for controlling the output of transmitter 72. Processor 74 may also apply that signal to a display 82 which can display in real time the temperature of the fluid or tissue being probed by catheter 10.

[0038] Of course, display 82 can also display other parameters related to the proper operation of the apparatus such as transmitter output power, reflected power, catheter temperature, room temperature, elapsed time, etc.

[0039] Preferably also, control unit 12 includes a heating/cooling unit 84 controlled by processor 74 and connected via hoses 84a, 84a to fittings 84b, 84b to which connectors 50a, 50a are coupled. This enables unit 84 to circulate a fluid through catheter 10 to maintain the catheter at a selected temperature above or below room temperature or to follow a selected temperature regime, depending upon the particular application.

[0040] Referring now to FIG. 2, basically the inner conductor 16 in catheter 10 constitutes an RF transmission line terminated by the conductive rounded tip 10a. The transmission line is operated at the output signal frequency of transmitter 72, e.g. 500 KHz. When transmitter 72 is operative, the transmission line will radiate energy for heating only from the uninsulated segment of the catheter between the catheter tip 10a and the leading end of the dielectric sheath 52. Thus, that segment constitutes an RF heating or transmitting antenna T whose length is determined by the forward extent of sheath 52 on outer conductor 18. In other words, increasing the length of sheath 52 will reduce the exposed length of conductor 18, i.e. the surface that could contact tissue, and, in turn, reduce the antenna T length. Since the outer conductor 18 is at the same RF potential as conductor 16, it can provide an RF path between the antenna T and transmitter 72.

[0041] Referring to FIGS. 2 and 4, the conductive catheter tip 10a also comprises a temperature sensing microwave receiving antenna R which can pick up thermal emissions from tissue adjacent to the catheter 10. The segment of conductor 16 from the tip 10a to its junction with capacitor 58 comprises the microwave receiving path and this path continues along a lead strip 60 to filter circuit 54 and thence to radiometer circuit 56. It should be noted that while conductor 33 is basically an extension of conductor 16, it conducts only the RF signal via outer conductor 18, while conductor 16 conducts both the RF and microwave signals.

[0042] In some applications, it may be desirable to bridge the dielectric spacer member 24 with a small helical wire (2-4 turns) wound about member 24 in grooves therein as shown in phantom at 25 in FIG. 4 to connect tip 10a to conductor 18. This improves the RF heating pattern of antenna T without materially degrading the microwave antenna pattern of receiving antenna R.

[0043] To enable catheter 10 to simultaneously heat (transmit) and detect temperature (radiometrically sense), a passive diplexer D is integrated into catheter 10 in order to block the transmitter signals from the microwave receiving path and isolate the microwave signals from the transmitter. The diplexer D is formed by the a quarter-wave (λ_R/4) shorted stub 1 constituted by the segment of catheter 10 extending from capacitor 58 (or more specifically its connection to conductor 16 to the heel cap 30. This quarter wave stub S should be tuned to the frequency of the radiometer circuit 56, e.g. 4 GHz.

[0044] The tuned length of the stub S, i.e. the catheter segment between capacitor 58 and heel cap 30, is determined by the dielectric constant of the material in sleeve 34 as well as the radiometer frequency. For example, at a radiometer frequency of 4 GHz, when sleeve 34 is of PTFE (K=2.1), the stub length should be 0.5 inch. On the other hand, when a K=9 material is used, the stub length would be reduced to 0.25 inch. For an intermediate length, e.g. 0.38 inch, a K=3.8 material would be used.

[0045] FIG. 5 is a schematic diagram showing the components of the diplexer D including the quarter wave stub S, the coupling capacitor 58, the lead strip 60 as well as the components of the band pass filter circuit 54. As seen there, the present catheter 10 design introduces the stub S, tuned to the radiometer frequency, into the transmit path from transmitter 72 to tip 10a. The stub S and inductance L₁ in circuit 54 form a low pass filter that isolates the higher frequency (4 GHz) signal from the RF path to transmitter 72. On the other hand, the capacitor C and inductance L₂ together constitute a high pass filter that blocks the lower frequency (500 KHz) transmitter signal from the signal-receiving path to radiometer circuit 56. In some applications it may be desirable for the circuit 54 chip to include elements of the radiometer circuit 56. Placing parts of the radiometer on separate chips prevents feedback caused by having too much amplifier gain on a single chip.

[0046] In FIGS. 2 and 4 we show capacitor 58 mounted directly to the center conductor 16. However, it could be incorporated into filter circuit 54 in which case the lead strip 60 in FIG. 4 would extend from conductor 16 directly to circuit 54.

[0047] Refer now to FIG. 6 which shows the control unit 10 in somewhat greater detail. It includes various high pass (HP) and low pass (LP) filters which together form a backend diplexer to isolate the various signals being sent to and from catheter 10 all of which signals are transmitted along dedicated conductors in cable 14. Since the illustrated backend diplexer is only one of the many that could be incorporated into unit 10 and does not comprise the invention here, it will not be described in detail. Suffice it to say that the RF ground conductor 70 from catheter 10 leads to a grounding electrode 12a connected to unit 12. When the apparatus is in use, that electrode is placed in contact with the patient's skin to provide a RF ground return path between the catheter and the patient.

[0048] FIG. 7 is a graph showing the performance of the diplexer D. As seen there, it provides a good impedance match, i.e. greater than 20 dB, at a receive frequency of 4 GHz, while blocking or rejecting frequencies below 1 GHz including the 500 KHz transmitter frequency.

[0049] The above described construction enables essentially the same antenna components to transmit and receive signals simultaneously that can both heat tissue or fluid and detect the temperature of that tissue or fluid in real time, thus enabling the performance of various medical procedures discussed at the outset. The fact that the diplexer and radiometer are incorporated right into the catheter or probe 10 without materially increasing the size and/or stiffness of the catheter enables the apparatus to provide very precise and noise-free temperature measurements in a minimum amount of time.

[0050] It will also be seen from the foregoing that my catheter apparatus provides a particularly efficient and effective means for detecting the temperature of tissue or fluid in a human or animal body in order to monitor and/or heat such tissue in a controlled manner. Even though the catheter incorporates transmitting and receiving antennas, an entire microwave radiometer and a diplexer to electrically isolate the transmitter from the receiver, the catheter is still quite small and flexible so that it can be used in many minimally invasive medical applications.

[0051] It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained. Also, certain changes may be made in the above construction without departing from the scope of the invention. For example, in some applications, the transmitter 76 could transmit in the microwave region. In that event, however, cable 14 would be a coaxial cable which would make it less flexible and more lossy. Also, of course, if the catheter is to be used only for detection, the diplexer including stub S and circuit 54 could be omitted.

[0052] Therefore, 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.

[0053] 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.