CA1283015C

CA1283015C - Method and apparatus for thermal angioplasty

Info

Publication number: CA1283015C
Application number: CA000566427A
Authority: CA
Inventors: Bruno Strul; Tsvi Goldenberg
Original assignee: Advanced Interventional Systems Inc
Current assignee: Spectranetics LLC
Priority date: 1987-05-22
Filing date: 1988-05-10
Publication date: 1991-04-16
Anticipated expiration: 2008-05-10
Also published as: EP0292172B1; EP0292172A3; DE3850914D1; EP0292172A2; JPS6449573A; US4807620A

Abstract

ABSTRACT OF THE DISCLOSURE
An interventional therapeutic apparatus is described which includes a probe in the form of a transmission line such as coaxial cable. The probe is adapted to pass through the interior of a body cavity such as a blood vessel and includes an inductive load such as a ferrite bead at the remote end of the cable.
Radio frequency energy is applied to the cable and converted into heat by a ferrite bead. As a result of the heat conversion by the ferrite bead, the remote end of the coaxial cable is heated to a sufficient temperature to provide the effect desired, for example to melt or otherwise remove plaque deposits in blood vessels.

Description

. ,,- . ~.
. ., 8 A METHOD AND APPARATUS FOR T~ERMAL ANGIOPLASTY
9 . .
10 BACXGROUND OF THE INVENTION ..
11 1. Field of the Invention 12 This present invention relates to an apparatus 13 and method for applying heat to tissue within a body 14 cavi~y for therapeutic purposes~
16 2. Description of the Prior Art 17 As a natural consequence of aging and other 18 biochemical factors, athero~clerotic obstructions 19 consisting of fatty deposits, fibrous tis~ue and eventually calcium, tend to form on the vessel walls of 21 the human coronary, peripheral and cerebral va~culature.
~2 As this accumulation progresses, the lumen of the artery 23 is narrowed (or, sometimes completely blocked), 24 restricti~g or preventi~g adequate supply of oxysenated blood to supply the muscles of the heart or legs.
26 The state of inadequate oxyyenation, known as 27 "ischemia," when it occurs in the coronary arteries, 28 results in abnormalities of the biochemical, electrical 29 and mechanical functions of the heart. The clinical manifestation of this condition may include angina 31 pectoris, acute myocardial infarction or ventricular 32 arrhythmia that can lead to sudden death.

'' ~' ` ' .

' ~2~3~ ' .
.

1 In the peripheral arteries, the ischemia 2 condition commonly occur6 with exercise and iR termed 3 "intermittent claudication," i.e., the pain that occurs 4 in a muscle with inadequate! blood supply that is stressed by exercise. Pain may also occur at rest and 6 may be continuous in the toe~ and foot. Occasionally 7 numbness or paresthesias may b~ present. Ulceration and 8 gangrene of the toes and distal foot are common when the 9 disease reaches advanced stages. Amputation is L0 sometimes required.
11 Many therapeutic interventions have been 12 available to treat coronary, peripheral and cerebral 13 vascular disease. Drugs that dilate the vessel and 14 lower blood viscosity are prescribed to prevent or lessen an ischemic attack.
16 The introduction of polyvinyl balloon tipped 17 catheters marked the beginning of a new era in 18 revascularization techniques. These catheters are 19 threaded through the vasculature, via a percutaneous incision, to the site of occlusion. The balloon i~ then 21 inflated using an accessory device located outside the 22 body. As the balloon inflates, the lumen of the artery 23 is increased to allow for greater blood flow resulting 24 in the alleviation of symptoms.
Bypass surgery is frequently prescribed.
26 Grafting a healthy vessel around the diseased one 27 provides a new connection for blood flow to the por~ion 28 of the artery that is distal to the diseased section.
29 More recent treatment techniques have focused on the use of high energy laser light pulse~ conducted 31 through a ~iber optic bundle to remove atherosclerotic 32 depoqits.
33 The use of laser energy has also been used to 34 heat a metal tipped catheter which then melts or dissolves th~ plaque. Lasers, however, are co~plex, ~Z~33~

1 relatively large, expensive and very inefficient. The 2 uqe of lasers for removing plaque depoqits is still in 3 the *ormative stages.
4 In addition to the use o expandable balloons and lasers, it has been suggested in U.S. Patent ~o.
6 4,643,186 that the plaque deposits be radiated with 7 microwave energy via a transmission line inserted 8 through the affected artery. Such a technique has 9 several inherent problems. For example, the plaque will provide a variable load to the microwave source 11 depending upon the moisture content of the plaque. Such 12 a variable load mQkes it difficult to control the amount 13 of power applied to the plaque to ~aporize or otherwise 14 remove the same without creating a ri~k of damaging the vessel wall.
16 There is a need for a simple and effective 17 apparatus for reducing the occlusive effect of plaque 18 deposits within blood vessels.

20 SUMMA~Y OF THE INVENTION
.~
21 In accordance with the present invention, an 22 interventional therapeutic apparatus is provided for the 23 remote delivery of heat to body ti~sue. The apparatus 24 may be employed, or example, to reduce the occlusive effect of plaque deposits in blood vessels. The 26 apparatus includes a probe in the form of a transmission 27 line ~uch as a coaxial cable having proximal and distal 28 ends. The transmission line probe iq adapted to pa~s 29 through the interior of a body cavlty Ruch as a blood vessel. An inductive load such as a ferrite bead or 31 core is disposed at the distal end of the transmission 32 line, the load being arranged to convert radio frequency 33 (r.f.) signals transmitted through the line into heat, 34 with the conversion being optimal at a predetermined frequency. A variable frequency oscillator iB connected ' -` 11 - 1283015 1 to the proximal end of the transmission line for 2 appl~ing r.f. energy at a suitable fre~uency, e.g., 3 lOMHz to 3GH~, to frequerlcy of the r. f. energy i5 4 adjusted to substantially the predetermined frequency 80 that the inductive load is heated to a temperature 6 ~ufficient to aeliver the desired heat to the body 7 tissue undergoing treatment. For example, the distal 8 end of the line may be heated sufficiently to melt or 9 otherwise remove plaque with w~ich the distal end comes into contact. This enables the transmission line to be 11 pushed through restrictive plaque deposits and reduce 12 the occlusive effect thereof.
13 The magnitude of the power applied to ~he 14 proximal end of the line (i.e., incident power) and the magnitude of the power reflected back from the distal 16 end of the line (i.e., reflected power) may be ~easured 17 to enable an operator to adjust the level of pcwer 18 delivered to the load and thereby ~ontrol the 19 temperature at the distal end of the transmission line or probe. If desired signals representative of the 21 incident and applied power may be utilized in a closed 22 loop system to maintain the incident power at a preset 23 level (and thus the distal end of the line at a given 24 temperature~ and the reflected power at a minimum.
The features of the present inventions can 26 best be understood by reference to the following 27¦ descriptions taXen in conjunction with the accompanying ?81 drawings, wherein liXe numerals indicate like 29 1 componentS.
30 l 31 ¦ BRIEF DESCRIPTION OF THE DRAWING
32 Figure 1 is a perspective view of an apparatus 33 of the present invention shcwing the probe thereof 34 inserted into an artery in the leg of a patient.
Figure 2 is a blocX diagram of an electronic ~ - 1283015 : ~

1 ¦ circuit and a diagramatic vi~w o a section of a probe 2 ¦ in accordance with one embodiment of the inventio~.
3 ¦ Figure 3 is an enlargea crosq-sectional view 4 ¦ of the distal end o~ the probe of Figure 2.

5 ¦ Figure 4 illustrates the di~tal end of the 6¦ probe of Figure 2 inserted into an artery a~d abutting a 7¦ plaque formation which totally occlude~ the artery.

8¦ Figure 5 illustrates hcw the insertion of the 9¦ distal end of the probe throug~l the plaque deposit opens 10¦ a passage and reduces the occlusive effect of the 11¦ plaque.

12¦ Figure 6 is a blocX diagram of an alternative 13¦ electronic circuit for u~e in the invention.

14¦ Figure 7 is a schematic circuit diagram of an 15¦ attenua~or which may be used in the circuit of Figure 6.
161 .
l7¦ DESCRIPTION OF THE PREFERRED E~BODIMENT
l _ . . . . .
18¦ Referring now to the drawingR, and more 19¦ particularly to Figure 1, there is illustrated a housing 20¦ 10 containing an oscillator or r.f. power fiUpply, and 21¦ meter~ 12 and 14 for providing ~i~ual indi~ations of 22¦ certain parameters, i.e., incident and reflected power 23¦ as will be explained. Knob 16 allows manual control of 24¦ the magnitude of th~ r.f. power applied to the di~tal 25¦ end 20 of a coaxial transmission line or probe 22, and 26¦ knob 18 allows manual control of the frequency of the 27¦ r.f. signal or energy applied to the ~ine. The distal 28¦ end 24 of the probe, or line 22, is illustrated as being 29 ¦ inserted into an artery in a patient's leg 26.
30 ¦ Referring now to Figure 2, an r.f. power 31 ¦ -~upply, or oscillator unit 30, provide-s an r.f. signal 32 ¦ at its output 32 whlrh is connected to the pro~i~al end 33 ¦ 20 of the coaxial line 22. An r.f. p~wer ~upply 34 ¦ marketed under the Model No. M445 by Eaton Corp. with 35~ M 87 P1ug-ID Unit ~ay b- ergl~yed as uni= 30. ~De ':

~.. ,, ; . ~ .

~ ~ - iZ83015 1 output power and r.f. ~requency of ~he oscillator may be 2 controlled by manually operated potentiometers or the 3 like (via knobs 16 ~ 18), as i8 well known in the art, 4 and such controls are incorporated in the unit identified above. The frequency of the r.f~ energy is 6 greater than 1 megahertz (MHz). The ~requency of the 7 r.f. energy is preferrably within the range of lOMHz to 8 3 gigahertz (GHz) and a most preferred range i~ from 9 50OMHz to 1.2GHz.
A directional coupler 34 is inserted between 11 the oscillator 30 and the proximal end 20 of the 12 transmission line 20. The coupler 34 includes a power 13 input 36 connected to the output 32 of the oscillator 14 and power output 38 connected to the proximal end of the transmission line. The coupler 34 includes two signal 16 outputs 40 h 42 which are connected to the reflected 17 power meter 14 and the incident power meter 12, 18 respectively. The signal on output 42 is an analog 19 signal representing the magnitude of the power applied to the proximal end of the transmission line by the 21 oscillator (i.e., incident power) and the ~ignal on 22 output 40 is an analog signal representing the magnitude 23 of the power reflected back to the coupler 34 from the 24 distal end 24 of the line 22 (i.e., reflected power).
Meters 12 ~ 14, which may be of the D'Arsonval type, 26 display the level of incident and reflected power. The 27 coupler 34 may be of the type marXeted by the Bird 28 Electronics Corp. under the name Power Sensor and Model 29 ~o. 416~W.
Referring now to Figure 3, there is 31 illustrated a cross-sectional view of the distal end of 32 the transmission line 22. The line is in the form of a 33 flexi~le coaxial cable having a center conductor 46, an 34 annular insulating layer 48, an outer conducting shield 35 50, which ~ay be in the form of a wire mes~, and an .. ,.. .. . .,: , ~ 330~
. ,' l insulating covering material 52~ An inductive load such 2 as a ferrite bead or core 54 is disposed at the distal 3 end o the line between the central conductor 46 and the 4 outer conducting shield 50, as shown. A cap 56 made of a suitable ~etal such as stainless steel, a platinum, 6 silver or gold alloy closes the distal end 24 of the 7 line or probe 22. The cap 56 closes the transmission 8 line circuit at the distal end 24 by electrically 9 connecting the center conductor and the metal sheath 50.
The cap 56 is also placed in good heat-conducting 11 relationship with the ferrite bead 54 to transrer heat 12 therefrom to the plaque to be melted.
13 The inductive load or ferrite bead 54 acts as 14 a lossy inductor load to r.f. energy transmitted down the line 22 by converting the r.f. energy to heat.
16 The diameter of the coaxial line or probe 22 17 must be small enough to be inserted into the blood 18 vessels of interest, Preferably the diameter of the 19 probe is within the range of l to 2 millimeters.
In operation, the distal end of the probe or 21 line 22 is inserted into an artery (in which stenotic 2Z plaque is to be removed or reduced) and guided through 23 the artery by conventional fluoroscopy techniques until 24 the distal end of the probe abuts the plaque deposit.
The frequency of the oscillator 30 is then manually 26 tuned, e.g., by knob 18, until the reflected power is 27 approximately at a minimum. At this frequency (and 28 there may be more than one such frequency) the load is 29 matched to the oscillator and line and essentially all of the power applied to the line is converted into heat 31 within the ferrite bead or inductive load. The 32 temperature of the probe tip (distal end 24) may then be 33 controlled by adjusting the magnitude of the incident 35 ~ powe e.g., via hnob i6).

- lZ83015 1 The raflected power measurement at meter 14 2 provides an indication of the temperature of the prohe 3 tip since ferrite ~hanges its magnetic properties as a 4 function of temperature. When the Curie point is reached, the ferrite loses il:s magnetic properties and 6 ceases to act as an inductive load, ~hereby limiting the 7 maximum temperatures achievable at the probe tip.
8 The Curie temperature of the ~errite depends 9 upon the alloy used. For exarnple, a nickel-zinc ferrite alloy marketed as ferrite No. 61 by Fair-Rite Corp. of 11 New York reaches it~ Curie point at about 350C.
12 Referring ncw to Figures 4 and 5, the manner 13 in which the probe 22 may be used to reduce the 14 occlusive effect of athero clerotic plaque is illustrated. The distal end 24 of the probe 22 (closed 16 by the cap 56) when heated to a sufficient temperature 17 melts the plaque 57 which it comes in contact with and 18 causes a reshaping of the plaque so that a relatively 19 smooth and open passage through the artery wall 58 is provided a~ illustratea in Figure 5.
21 Another embodiment of ~he invention i5 22 illustrated in Figure 6 in which the incident and 23 reflected power signals ~re used in a closed loop to 24 maintain the magnitude of the reflected power at a minimum and the magnit~de of the incident power at a 26 preset level. The sys~em of Figure 5 includes a voltage 27 controlled oscillator 60 ~hich has a voltase control ~8 input 62 and an r.f. output 64. The r. f . output 64 is 29 connected to an input 66 of an attenuator or modulator 68. The attenuator 68 has a voltage control input 70 31 and an output 72 connected to the r.f. input 36 of the 32 directional coupler 34 via a power amplifier 74. The 33 incident and reflected pow~r ~ignal~ are applied via 34 analog to digital converters 76 and 76 to input ports 78 and 80 of a microprocessor 82. The microprocessor 82 ~ 33~)~S

1 processes the incident and reflected power signals and ;! applies output ~Lgnals to the control input~ 62 ~nd 70 3 o the VCO 60 and the attenuator 68 via digital to 4 analog converters 84 and 86, as shGwn~ The micro-processor is arranged (by appropriate programming) to 6 change the amplitude or voltage level of signal applied 7 to the input 62 of the, ~CO and thereby changing t,h~
8 frequency of the r.~. output at 64 as needed to minimize 9 the level or magnitude of the reflected power signal at input port 80. The microprocessor is also arranged (by 11 appropriate programming) to adjust the level of the 12 signal applied to control input 72 of the attenua~or 68 13 so that the incident power will be maintained at a level 14 preset into the microprocessor in a well-known manner.
The attenuator/m~dulator m~y be arranged to either 16 attenuate or modulate the applied r.f. siynal ~from the 17 oscillator) to thereby change the magnitude of the 18 incident power as is well known in the art.
19 Figure 7 illustrates one type of attenuator that may be emplayed a~ unit 68 in Figure 6. The 2~ attenuator of Figure 7 varies the amount of the 22 attenuation of the input r.f. signal and thus the 23 magnitude of the output signal (with a constant r.f~
24 input at 66) by changing the amount of bias voltage on a P/N diode 90. A pair of bypa~s capacitors 92 and 94 are 26 connected between the cathode of the diode and the r.f.
27 input and output, as shown. An inductor 96 i~ also 28 connected between the control input 70 and the ~athode 2~ of the diode. ~ne level of diode conduction ~resulting from the level of positive bias voltage ~upplied by the 31 microprocessor 82) determines how much the diode ~hunts 32 the load (connected to the output 72). When the diode 33 is reverse biased, the attenuation is effectively zero 34 and the load is m2tched allowing all of the r.f. power to be trans~tted to the load. A8 the diode turn~ on '. : ': ' ~ 330~

1 due to a positive bias voltage, it shunts the load and 2 part or all of the signal is reflected back to (and 3 absorbed by) the source or VCo 62.
4 In the operation of the circuit of Figure 6, the operator merely presets the desired incident power 6 into the microprocessor 82 and follows the procedure 7 outlined with respect to the circuitry of Figure 4, 8 except that the oscillator (and attenuator) is 9 automatically controlled.
There has thus been described a simple, 11 efficient and reliable apparatus for the delivery of 12 therapeutic heat to body tissue, for example to melt 13 stenotic plaque and reduce the occlusive effect thereof 14 in blood vessels.
The above description presents the best mode 16 contemplated in carrying Ollt our invention. Our 17 invention is, however, susceptible to modifIcations and 18 alternate constructions from the embodiments shown in 19 the drawings and described above. Consequently, it is not the intention to limit the invention to the 21 particular embodiments disclosed. Qn the contrary, the 22 invention is intended and shall cover all modifications, 23 si~es and alternate constructions fallins within the 24 spirit and scope of the invention, as expressed in the appended clalms when read in light of the description 26 ~ and drawin ~'

Claims

1. In an interventional therapeutic apparatus for remote delivery of heat to body tissue, the combination which comprises:
(a) an oscillator for providing an r.f. output signal within the range of 10 MHz to 2 GHz;
(b) a transmission line having a proximal end connected to the oscillator for receiving the output signal and a distal end, the transmission line being constructed and arranged to pass through the interior of a body cavity; and (c) an inductive load dispose at the distal end of the transmission line, the inductive load comprising a magnetic material and operating to convert the r.f. signal transmitted through the transmission line into heat, the conversion being optimized at a predetermined frequency, the frequency of the output signal from the oscillator being set at substantially said predetermined frequency, whereby the inductive load is heated to a temperature sufficient for therapeutic effects.

2. The apparatus of claim 1 adapted for thermal angioplasty wherein the transmission line is constructed and arranged to pass through a blood vessel so that the occlusive effect of plaque residing within the vessel may be reduced.

3. The apparatus of claim 1 further including means for measuring the power reflected from the distal end of transmission line back to the proximal end of the transmission line.

4. The apparatus of claim 3 further including means for measuring the incident power applied to the proximal end of the transmission line by the oscillator, and means for adjusting the magnitude of the incident power.

5. The apparatus of claim 4 wherein the inductive load is characterized by a temperature rise up to a maximum value which is proportional to the magnitude of the difference between the incident and reflected power, whereby the temperature of the load and distal end of the transmission line can be controlled by adjusting the magnitude of the incident power while maintaining the reflected power at a predetermined level.

6. The apparatus of claim 5 wherein the inductive load is further characterized by a loss of ability to convert r.f.
energy into heat at a preset maximum temperature.

7. The apparatus of claim 3 wherein the inductive load is ferrite.

8. The apparatus of claim 7 wherein the reflected power measuring means includes reflected power signal generating means for producing a reflected power signal representative of the magnitude of the power reflected from the distal end to the proximal end of the transmission line.

9. The apparatus of claim 8 wherein the oscillator further includes means for adjusting the frequency of said output signal and further including means responsive to the reflected power signal for controlling the frequency adjusting means to minimize the reflected power.

10. The apparatus of claim 9 further including incident power adjustment means for controlling the magnitude of the power delivered to the transmission line.

11. The apparatus of claim 10 further including means for providing an incident power signal representative of the magnitude of the power delivered to the transmission line by the oscillator and wherein the power adjustment means is responsive to the incidental power signal for maintaining the delivered power at a present level.

12. The apparatus of any one of claims 1 to 11 wherein the oscillator further includes means for adjusting the frequency of said output signal.

13. The apparatus of any one of claims 1 to 11 wherein the oscillator further includes means for adjusting the frequency of said output signal, and further including means for measuring the power reflected from the distal end of the transmission line back to the proximal end of the transmission line, the reflected power measuring means being arranged to produce a reflected power signal and wherein the frequency adjusting means is responsive to the reflected power signal and arranged to minimize the magnitude of the reflected power.

14. A delivery system for thermal angioplasty comprising:
(a) a coaxial transmission line having distal and proximal ends with a center conductor and an outer conducting shield and being arranged to pass through the interior of a blood vessel, the proximal end of the transmission line being adapted to be coupled to a source of r.f. energy;
(b) a core of magnetic material disposed between the center conductor and outer shield at the distal end of the transmission line, the magnetic material being arranged to convert r.f. energy into heat;
(c) a heat conductive cap closing the distal end of the transmission line and in heat conducting relationship to the core, whereby the application of r.f. energy to the proximal end of the transmission line will cause the core and cap to heat sufficiently to allow the distal end of the transmission line to be forced through plaque restricting deposits in the vessels and reduce the occlusive effect thereof.

15. The delivery system of claim 13 wherein the magnetic core is characterized by a temperature rise up to a predetermined maximum level which is proportional to the magnitude of the r.f. power applied to the proximal end of the transmission line with substantially minimum power being reflected back to the proximal end from the distal end of the line.

16. The delivery system of claim 15 wherein said magnetic material is ferrite.

17. The delivery system of claim 16 wherein the diameter of the distal end of the transmission line is approximately 2mm.

18. A device for reducing the occlusive effect of plaque in blood vessels in living tissue which comprises:
(a) a coaxial transmission line having proximal and distal ends and a center conductor and outer conducting shield, the line being adapted to pass through the interior of a blood vessel;
(b) a ferrite core disposed between the center conductor and outer shield at the distal end of the transmission line;
(c) an electrically and heat conducting cap closing the distal end of the transmission line in electrical contact with the center conductor and outer shield and in heat conducting relationship with the ferrite core for providing a closed electrical circuit at the distal end of the transmission line and for receiving and dissipating heat generated in the ferrite core; and (d) power supply means coupled to the proximal end of the transmission line for applying r.f. energy thereto, whereby the r.f. energy is converted into heat by the ferrite core which heat is transmitted to the cap thereby enabling the distal end of the transmission line to ablate plaque deposits in contact with the cap and reduce the occlusive effect of such plaque within the vessel in which the transmission line is inserted.

19. The device as defined in claim 18 including means to adjust the frequency of the r.f. energy output from the power supply means to minimize the magnitude of energy reflected back to the proximal end from the distal end of the line.

20. The device as claimed in claim 19 further including reflected power measuring means for measuring the magnitude of the reflected power.

21. The device as defined in claim 20 wherein the reflected power measuring means is further arranged to produce a reflected power signal representative of the magnitude of the reflected power and wherein the frequency adjusting means is responsive to the reflected power signal.

22. The device as defined in claim 21 further including means to adjust the magnitude of incident r.f. power applied to the proximal end of the transmission line and means to measure the magnitude of the incident r.f. power.

23. The device as defined in claim 22 wherein the incident power measuring means is further arranged to produce an incident power signal representative of the magnitude of the incident power and wherein the power adjustment means is responsive to the incident power signal and arranged to maintain the magnitude of the incident r.f. power at a preset level.

24. The device as defined in any one of claims 18 to 23 wherein the power supply means is further arranged to apply r.f.
energy to the transmission line within the frequency range of 1 MHz to 3 GHz.

25. The device as defined in any one of claims 18 to 23 wherein the power supply means is further arranged to apply r.f.
energy to the transmission line within the frequency range of 500 MHz to 1.2 GHz.